![1.4 Acknowledgements
Cover by Fran Caye
Parts of this book were edited by Adaobi Obi Tulton.
Thanks for answering questions, discussing language topics,
etc.:
Allen Wirfs-Brock (@awbjs)
Benedikt Meurer (@bmeurer)
Brian Terlson (@bterlson)
Daniel Ehrenberg (@littledan)
Jordan Harband (@ljharb)
Mathias Bynens (@mathias)
Myles Borins (@MylesBorins)
Rob Palmer (@robpalmer2)
Šime Vidas (@simevidas)
And many others
Thanks for reviewing:
Johannes Weber (@jowe)
[Generated: 2019-08-31 17:39]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-22-320.jpg)
![5 History and evolution of
JavaScript
5.1 How JavaScript was created
5.2 Standardizing JavaScript
5.3 Timeline of ECMAScript versions
5.4 Ecma Technical Committee 39 (TC39)
5.5 The TC39 process
5.5.1 Tip: Think in individual features and stages, not
ECMAScript versions
5.6 FAQ: TC39 process
5.6.1 How is [my favorite proposed feature] doing?
5.6.2 Is there an official list of ECMAScript features?
5.7 Evolving JavaScript: Don’t break the web](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-42-320.jpg)
![5.3 Timeline of ECMAScript
versions
This is a brief timeline of ECMAScript versions:
ECMAScript 1 (June 1997): First version of the standard.
ECMAScript 2 (June 1998): Small update to keep ECMA-262 in
sync with the ISO standard.
ECMAScript 3 (December 1999): Adds many core features –
“[…] regular expressions, better string handling, new control
statements [do-while, switch], try/catch exception handling,
[…]”
ECMAScript 4 (abandoned in July 2008): Would have been a
massive upgrade (with static typing, modules, namespaces, and
more), but ended up being too ambitious and dividing the
language’s stewards.
ECMAScript 5 (December 2009): Brought minor improvements
– a few standard library features and strict mode.
ECMAScript 5.1 (June 2011): Another small update to keep
Ecma and ISO standards in sync.
ECMAScript 6 (June 2015): A large update that fulfilled many of
the promises of ECMAScript 4. This version is the first one
whose official name – ECMAScript 2015 – is based on the year
of publication.
ECMAScript 2016 (June 2016): First yearly release. The shorter
release life cycle resulted in fewer new features compared to the
large ES6.](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-46-320.jpg)
![5.6 FAQ: TC39 process
5.6.1 How is [my favorite proposed
feature] doing?
If you are wondering what stages various proposed features are in,
consult the GitHub repository proposals.
5.6.2 Is there an official list of
ECMAScript features?
Yes, the TC39 repo lists finished proposals and mentions in which
ECMAScript versions they were introduced.](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-52-320.jpg)
![Objects:
Arrays (Arrays are also objects):
8.1.2 Modules
Each module is a single file. Consider, for example, the following two
files with modules in them:
// An arrow function whose body is an expression
(a, b) => a + b
// Creating a plain object via an object literal
const obj = {
first: 'Jane', // property
last: 'Doe', // property
getFullName() { // property (method)
return this.first + ' ' + this.last;
},
};
// Getting a property value
assert.equal(obj.first, 'Jane');
// Setting a property value
obj.first = 'Janey';
// Calling the method
assert.equal(obj.getFullName(), 'Janey Doe');
// Creating an Array via an Array literal
const arr = ['a', 'b', 'c'];
// Getting an Array element
assert.equal(arr[1], 'b');
// Setting an Array element
arr[1] = 'β';](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-72-320.jpg)
![Parsed as:
Example 2: Unintended division.
Parsed as:
Example 3: Unintended property access.
Executed as:
a = b + c(d + e).print();
a = b
/hi/g.exec(c).map(d)
a = b / hi / g.exec(c).map(d);
someFunction()
['ul', 'ol'].map(x => x + x)
const propKey = ('ul','ol'); // comma operator
assert.equal(propKey, 'ol');
someFunction()[propKey].map(x => x + x);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-90-320.jpg)
![9.2 The console.* API: printing data
and more
In browsers, the console is something you can bring up that is
normally hidden. For Node.js, the console is the terminal that
Node.js is currently running in.
The full console.* API is documented on MDN web docs and on the
Node.js website. It is not part of the JavaScript language standard,
but much functionality is supported by both browsers and Node.js.
In this chapter, we only look at the following two methods for
printing data (“printing” means displaying in the console):
console.log()
console.error()
9.2.1 Printing values: console.log()
(stdout)
There are two variants of this operation:
9.2.1.1 Printing multiple values
The first variant prints (text representations of) values on the
console:
console.log(...values: any[]): void
console.log(pattern: string, ...values: any[]): void](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-100-320.jpg)
![10.3 Normal comparison vs. deep
comparison
The strict equal() uses === to compare values. Therefore, an object is
only equal to itself – even if another object has the same content
(because === does not compare the contents of objects, only their
identities):
deepEqual() is a better choice for comparing objects:
This method works for Arrays, too:
assert.notEqual({foo: 1}, {foo: 1});
assert.deepEqual({foo: 1}, {foo: 1});
assert.notEqual(['a', 'b', 'c'], ['a', 'b', 'c']);
assert.deepEqual(['a', 'b', 'c'], ['a', 'b', 'c']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-107-320.jpg)
![10.4.2 Deep equality
function deepEqual(actual: any, expected: any, message?:
string): void
actual must be deeply equal to expected. If not, an
AssertionError is thrown.
function notDeepEqual(actual: any, expected: any, message?:
string): void
actual must not be deeply equal to expected. If it is, an
AssertionError is thrown.
10.4.3 Expecting exceptions
If you want to (or expect to) receive an exception, you need throws():
This function calls its first parameter, the function block, and only
succeeds if it throws an exception. Additional parameters can be
used to specify what that exception must look like.
function throws(block: Function, message?: string): void
String(err),
'AssertionError [ERR_ASSERTION]: x must be equal to 8');
}
assert.deepEqual([1,2,3], [1,2,3]);
assert.deepEqual([], []);
// To .equal(), an object is only equal to itself:
assert.notEqual([], []);
assert.notDeepEqual([1,2,3], [1,2]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-109-320.jpg)
![12.2.2 const and loops
You can use const with for-of loops, where a fresh binding is created
for each iteration:
In plain for loops, you must use let, however:
const arr = ['hello', 'world'];
for (const elem of arr) {
console.log(elem);
}
// Output:
// 'hello'
// 'world'
const arr = ['hello', 'world'];
for (let i=0; i<arr.length; i++) {
const elem = arr[i];
console.log(elem);
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-124-320.jpg)
![We can see that the function created in line A keeps its internal
number in the free variable startValue. This time, we don’t just read
from the birth scope, we use it to store data that we change and that
persists across function calls.
We can create more storage slots in the birth scope, via local
variables:
12.9.4 Use cases for closures
What are closures good for?
For starters, they are simply an implementation of static
scoping. As such, they provide context data for callbacks.
They can also be used by functions to store state that persists
across function calls. createInc() is an example of that.
};
}
const inc = createInc(5);
assert.equal(inc(2), 7);
function createInc(startValue) {
let index = -1;
return (step) => {
startValue += step;
index++;
return [index, startValue];
};
}
const inc = createInc(5);
assert.deepEqual(inc(2), [0, 7]);
assert.deepEqual(inc(2), [1, 9]);
assert.deepEqual(inc(2), [2, 11]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-146-320.jpg)
![To see what’s so special about this way of comparing, read on and
find out how objects are compared.
13.4.2 Objects
Objects are covered in detail in §28 “Single objects” and the
following chapter. Here, we mainly focus on how they differ from
primitive values.
Let’s first explore two common ways of creating objects:
Object literal:
The object literal starts and ends with curly braces {}. It creates
an object with two properties. The first property has the key
'first' (a string) and the value 'Jane'. The second property has
the key 'last' and the value 'Doe'. For more information on
object literals, consult §28.2.1 “Object literals: properties”.
Array literal:
The Array literal starts and ends with square brackets []. It
creates an Array with two elements: 'foo' and 'bar'. For more
assert.equal(123 === 123, true);
assert.equal('abc' === 'abc', true);
const obj = {
first: 'Jane',
last: 'Doe',
};
const arr = ['foo', 'bar'];](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-155-320.jpg)
![Primitive values are not instances of anything:
Exercise: instanceof
exercises/values/instanceof_exrc.mjs
> (function() {}) instanceof Function
true
> ({}) instanceof Object
true
> [] instanceof Array
true
> 123 instanceof Number
false
> '' instanceof String
false
> '' instanceof Object
false](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-160-320.jpg)
![14.1 Making sense of operators
JavaScript’s operators may seem quirky. With the following two
rules, they are easier to understand:
Operators coerce their operands to appropriate types
Most operators only work with primitive values
14.1.1 Operators coerce their operands to
appropriate types
If an operator gets operands that don’t have the proper types, it
rarely throws an exception. Instead, it coerces (automatically
converts) the operands so that it can work with them. Let’s look at
two examples.
First, the multiplication operator can only work with numbers.
Therefore, it converts strings to numbers before computing its result.
Second, the square brackets operator ([ ]) for accessing the
properties of an object can only handle strings and symbols. All other
values are coerced to string:
> '7' * '3'
21
const obj = {};
obj['true'] = 123;
// Coerce true to the string 'true'
assert.equal(obj[true], 123);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-167-320.jpg)
![14.1.2 Most operators only work with
primitive values
As mentioned before, most operators only work with primitive
values. If an operand is an object, it is usually coerced to a primitive
value – for example:
Why? The plus operator first coerces its operands to primitive
values:
Next, it concatenates the two strings:
> [1,2,3] + [4,5,6]
'1,2,34,5,6'
> String([1,2,3])
'1,2,3'
> String([4,5,6])
'4,5,6'
> '1,2,3' + '4,5,6'
'1,2,34,5,6'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-168-320.jpg)
![14.3 Assignment operators
14.3.1 The plain assignment operator
The plain assignment operator is used to change storage locations:
Initializers in variable declarations can also be viewed as a form of
assignment:
14.3.2 Compound assignment operators
Given an operator op, the following two ways of assigning are
equivalent:
myvar op= value
myvar = myvar op value
If, for example, op is +, then we get the operator += that works as
follows.
x = value; // assign to a previously declared variable
obj.propKey = value; // assign to a property
arr[index] = value; // assign to an Array element
const x = value;
let y = value;
let str = '';
str += '<b>';
str += 'Hello!';
str += '</b>';
assert.equal(str, '<b>Hello!</b>');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-170-320.jpg)
![14.4 Equality: == vs. ===
JavaScript has two kinds of equality operators: loose equality (==)
and strict equality (===). The recommendation is to always use the
latter.
Other names for == and ===
== is also called double equals. Its official name in the
language specification is abstract equality comparison.
=== is also called triple equals.
14.4.1 Loose equality (== and !=)
Loose equality is one of JavaScript’s quirks. It often coerces
operands. Some of those coercions make sense:
Others less so:
Objects are coerced to primitives if (and only if!) the other operand is
primitive:
> '123' == 123
true
> false == 0
true
> '' == 0
true
> [1, 2, 3] == '1,2,3'
true](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-172-320.jpg)
![If both operands are objects, they are only equal if they are the same
object:
Lastly, == considers undefined and null to be equal:
14.4.2 Strict equality (=== and !==)
Strict equality never coerces. Two values are only equal if they have
the same type. Let’s revisit our previous interaction with the ==
operator and see what the === operator does:
An object is only equal to another value if that value is the same
object:
> ['1', '2', '3'] == '1,2,3'
true
> [1, 2, 3] == ['1', '2', '3']
false
> [1, 2, 3] == [1, 2, 3]
false
> const arr = [1, 2, 3];
> arr == arr
true
> undefined == null
true
> false === 0
false
> '123' === 123
false
> [1, 2, 3] === '1,2,3'
false
> ['1', '2', '3'] === '1,2,3'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-173-320.jpg)
![The === operator does not consider undefined and null to be equal:
14.4.3 Recommendation: always use
strict equality
I recommend to always use ===. It makes your code easier to
understand and spares you from having to think about the quirks of
==.
Let’s look at two use cases for == and what I recommend to do
instead.
14.4.3.1 Use case for ==: comparing with a number or a
string
== lets you check if a value x is a number or that number as a string –
with a single comparison:
false
> [1, 2, 3] === ['1', '2', '3']
false
> [1, 2, 3] === [1, 2, 3]
false
> const arr = [1, 2, 3];
> arr === arr
true
> undefined === null
false
if (x == 123) {
// x is either 123 or '123'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-174-320.jpg)
![14.4.4 Even stricter than ===: Object.is()
Method Object.is() compares two values:
It is even stricter than ===. For example, it considers NaN, the error
value for computations involving numbers, to be equal to itself:
That is occasionally useful. For example, you can use it to implement
an improved version of the Array method .indexOf():
myIndexOf() finds NaN in an Array, while .indexOf() doesn’t:
The result -1 means that .indexOf() couldn’t find its argument in the
Array.
> Object.is(123, 123)
true
> Object.is(123, '123')
false
> Object.is(NaN, NaN)
true
> NaN === NaN
false
const myIndexOf = (arr, elem) => {
return arr.findIndex(x => Object.is(x, elem));
};
> myIndexOf([0,NaN,2], NaN)
1
> [0,NaN,2].indexOf(NaN)
-1](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-176-320.jpg)
![16.1 Converting to boolean
The meaning of “converting to [type]”
“Converting to [type]” is short for “Converting arbitrary values to
values of type [type]”.
These are three ways in which you can convert an arbitrary value x to
a boolean.
Boolean(x)
Most descriptive; recommended.
x ? true : false
Uses the conditional operator (explained later in this chapter).
!!x
Uses the logical Not operator (!). This operator coerces its
operand to boolean. It is applied a second time to get a non-
negated result.
Tbl. 4 describes how various values are converted to boolean.
Table 4: Converting values to booleans.
x Boolean(x)
undefined false
null false
boolean value x (no change)
number value 0 → false, NaN → false](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-187-320.jpg)
![All other values (including all objects) are truthy:
16.2.1 Checking for truthiness or
falsiness
The conditional operator that is used in the last line, is explained
later in this chapter.
Exercise: Truthiness
exercises/booleans/truthiness_exrc.mjs
> Boolean('abc')
true
> Boolean([])
true
> Boolean({})
true
if (x) {
// x is truthy
}
if (!x) {
// x is falsy
}
if (x) {
// x is truthy
} else {
// x is falsy
}
const result = x ? 'truthy' : 'falsy';](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-190-320.jpg)
![In other words, the following two expressions are roughly equivalent:
Examples:
16.5.3 Default values via logical Or (||)
Sometimes you receive a value and only want to use it if it isn’t either
null or undefined. Otherwise, you’d like to use a default value, as a
fallback. You can do that via the || operator:
The following code shows a real-world example:
If there are one or more matches for regex inside str then .match()
returns an Array. If there are no matches, it unfortunately returns
a || b
a ? a : b
> true || false
true
> true || 'abc'
true
> false || true
true
> false || 'abc'
'abc'
> 'abc' || 'def'
'abc'
const valueToUse = valueReceived || defaultValue;
function countMatches(regex, str) {
const matchResult = str.match(regex); // null or Array
return (matchResult || []).length;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-197-320.jpg)
![The decrementation operator -- works the same, but subtracts one
from its operand. The next two examples explain the difference
between the prefix and the suffix version.
Tbl. 7 summarizes the incrementation and decrementation
operators.
Table 7: Incrementation operators and decrementation
operators.
Operator Name Example
v++ Increment ES1 let v=0; [v++, v] → [0, 1]
++v Increment ES1 let v=0; [++v, v] → [1, 1]
v-- Decrement ES1 let v=1; [v--, v] → [1, 0]
--v Decrement ES1 let v=1; [--v, v] → [0, 0]
Next, we’ll look at examples of these operators in use.
Prefix ++ and prefix -- change their operands and then return them.
Suffix ++ and suffix -- return their operands and then change them.
let foo = 3;
assert.equal(++foo, 4);
assert.equal(foo, 4);
let bar = 3;
assert.equal(--bar, 2);
assert.equal(bar, 2);
let foo = 3;
assert.equal(foo++, 3);
assert.equal(foo, 4);
let bar = 3;](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-207-320.jpg)
![17.3.3.1 Operands: not just variables
You can also apply these operators to property values:
And to Array elements:
Exercise: Number operators
exercises/numbers-math/is_odd_test.mjs
assert.equal(bar--, 3);
assert.equal(bar, 2);
const obj = { a: 1 };
++obj.a;
assert.equal(obj.a, 2);
const arr = [ 4 ];
arr[0]++;
assert.deepEqual(arr, [5]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-208-320.jpg)
![NaN is the only JavaScript value that is not strictly equal to itself:
These are several ways of checking if a value x is NaN:
In the last line, we use the comparison quirk to detect NaN.
17.6.2 Finding NaN in Arrays
Some Array methods can’t find NaN:
Others can:
Alas, there is no simple rule of thumb. You have to check for each
method how it handles NaN.
const n = NaN;
assert.equal(n === n, false);
const x = NaN;
assert.equal(Number.isNaN(x), true); // preferred
assert.equal(Object.is(x, NaN), true);
assert.equal(x !== x, true);
> [NaN].indexOf(NaN)
-1
> [NaN].includes(NaN)
true
> [NaN].findIndex(x => Number.isNaN(x))
0
> [NaN].find(x => Number.isNaN(x))
NaN](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-213-320.jpg)
![17.7 Error value: Infinity
When is the error value Infinity returned?
Infinity is returned if a number is too large:
Infinity is returned if there is a division by zero:
17.7.1 Infinity as a default value
Infinity is larger than all other numbers (except NaN), making it a
good default value:
17.7.2 Checking for Infinity
> Math.pow(2, 1023)
8.98846567431158e+307
> Math.pow(2, 1024)
Infinity
> 5 / 0
Infinity
> -5 / 0
-Infinity
function findMinimum(numbers) {
let min = Infinity;
for (const n of numbers) {
if (n < min) min = n;
}
return min;
}
assert.equal(findMinimum([5, -1, 2]), -1);
assert.equal(findMinimum([]), Infinity);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-214-320.jpg)
![17.10 Background: floating point
precision
In JavaScript, computations with numbers don’t always produce
correct results – for example:
To understand why, we need to explore how JavaScript represents
floating point numbers internally. It uses three integers to do so,
which take up a total of 64 bits of storage (double precision):
Component Size Integer range
Sign 1 bit [0, 1]
Fraction 52 bits [0, 252−1]
Exponent 11 bits [−1023, 1024]
The floating point number represented by these integers is computed
as follows:
(–1)sign × 0b1.fraction × 2exponent
This representation can’t encode a zero because its second
component (involving the fraction) always has a leading 1. Therefore,
a zero is encoded via the special exponent −1023 and a fraction 0.
17.10.1 A simplified representation of
floating point numbers
> 0.1 + 0.2
0.30000000000000004](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-218-320.jpg)
![17.11.3 Safe integers
This is the range of integers that are safe in JavaScript (53 bits plus a
sign):
[–253–1, 253–1]
An integer is safe if it is represented by exactly one JavaScript
number. Given that JavaScript numbers are encoded as a fraction
multiplied by 2 to the power of an exponent, higher integers can also
be represented, but then there are gaps between them.
For example (18014398509481984 is 254):
The following properties of Number help determine if an integer is
safe:
> 18014398509481984
18014398509481984
> 18014398509481985
18014398509481984
> 18014398509481986
18014398509481984
> 18014398509481987
18014398509481988
assert.equal(Number.MAX_SAFE_INTEGER, (2 ** 53) - 1);
assert.equal(Number.MIN_SAFE_INTEGER, -Number.MAX_SAFE_INTEGER);
assert.equal(Number.isSafeInteger(5), true);
assert.equal(Number.isSafeInteger('5'), false);
assert.equal(Number.isSafeInteger(5.1), false);
assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER), true
assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER+1), fa](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-224-320.jpg)
![17.13 Quick reference: numbers
17.13.1 Global functions for numbers
JavaScript has the following four global functions for numbers:
isFinite()
isNaN()
parseFloat()
parseInt()
However, it is better to use the corresponding methods of Number
(Number.isFinite(), etc.), which have fewer pitfalls. They were
introduced with ES6 and are discussed below.
17.13.2 Static properties of Number
.EPSILON: number [ES6]
The difference between 1 and the next representable floating
point number. In general, a machine epsilon provides an upper
bound for rounding errors in floating point arithmetic.
Approximately: 2.2204460492503130808472633361816 ×
10-16
.MAX_SAFE_INTEGER: number [ES6]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-231-320.jpg)
![The largest integer that JavaScript can represent unambiguously
(253−1).
.MAX_VALUE: number [ES1]
The largest positive finite JavaScript number.
Approximately: 1.7976931348623157 × 10308
.MIN_SAFE_INTEGER: number [ES6]
The smallest integer that JavaScript can represent
unambiguously (−253+1).
.MIN_VALUE: number [ES1]
The smallest positive JavaScript number. Approximately 5 ×
10−324.
.NaN: number [ES1]
The same as the global variable NaN.
.NEGATIVE_INFINITY: number [ES1]
The same as -Number.POSITIVE_INFINITY.
.POSITIVE_INFINITY: number [ES1]
The same as the global variable Infinity.
17.13.3 Static methods of Number](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-232-320.jpg)
![.isFinite(num: number): boolean [ES6]
Returns true if num is an actual number (neither Infinity nor -
Infinity nor NaN).
.isInteger(num: number): boolean [ES6]
Returns true if num is a number and does not have a decimal
fraction.
.isNaN(num: number): boolean [ES6]
Returns true if num is the value NaN:
> Number.isFinite(Infinity)
false
> Number.isFinite(-Infinity)
false
> Number.isFinite(NaN)
false
> Number.isFinite(123)
true
> Number.isInteger(-17)
true
> Number.isInteger(33)
true
> Number.isInteger(33.1)
false
> Number.isInteger('33')
false
> Number.isInteger(NaN)
false
> Number.isInteger(Infinity)
false
> Number.isNaN(NaN)
true
> Number.isNaN(123)](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-233-320.jpg)
![.isSafeInteger(num: number): boolean [ES6]
Returns true if num is a number and unambiguously represents
an integer.
.parseFloat(str: string): number [ES6]
Coerces its parameter to string and parses it as a floating point
number. For converting strings to numbers, Number() (which
ignores leading and trailing whitespace) is usually a better
choice than Number.parseFloat() (which ignores leading
whitespace and illegal trailing characters and can hide
problems).
.parseInt(str: string, radix=10): number [ES6]
Coerces its parameter to string and parses it as an integer,
ignoring leading whitespace and illegal trailing characters:
The parameter radix specifies the base of the number to be
parsed:
false
> Number.isNaN('abc')
false
> Number.parseFloat(' 123.4#')
123.4
> Number(' 123.4#')
NaN
> Number.parseInt(' 123#')
123](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-234-320.jpg)
![Do not use this method to convert numbers to integers: coercing
to string is inefficient. And stopping before the first non-digit is
not a good algorithm for removing the fraction of a number.
Here is an example where it goes wrong:
It is better to use one of the rounding functions of Math to
convert a number to an integer:
17.13.4 Methods of Number.prototype
(Number.prototype is where the methods of numbers are stored.)
.toExponential(fractionDigits?: number): string [ES3]
Returns a string that represents the number via exponential
notation. With fractionDigits, you can specify, how many digits
should be shown of the number that is multiplied with the
exponent (the default is to show as many digits as necessary).
Example: number too small to get a positive exponent via
.toString().
> Number.parseInt('101', 2)
5
> Number.parseInt('FF', 16)
255
> Number.parseInt(1e21, 10) // wrong
1
> Math.trunc(1e21) // correct
1e+21](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-235-320.jpg)
![Example: fraction not small enough to get a negative exponent
via .toString().
.toFixed(fractionDigits=0): string [ES3]
Returns an exponent-free representation of the number,
rounded to fractionDigits digits.
If the number is 1021 or greater, even .toFixed() uses an
exponent:
.toPrecision(precision?: number): string [ES3]
> 1234..toString()
'1234'
> 1234..toExponential() // 3 fraction digits
'1.234e+3'
> 1234..toExponential(5)
'1.23400e+3'
> 1234..toExponential(1)
'1.2e+3'
> 0.003.toString()
'0.003'
> 0.003.toExponential()
'3e-3'
> 0.00000012.toString() // with exponent
'1.2e-7'
> 0.00000012.toFixed(10) // no exponent
'0.0000001200'
> 0.00000012.toFixed()
'0'
> (10 ** 21).toFixed()
'1e+21'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-236-320.jpg)
![Works like .toString(), but precision specifies how many digits
should be shown. If precision is missing, .toString() is used.
.toString(radix=10): string [ES1]
Returns a string representation of the number.
By default, you get a base 10 numeral as a result:
If you want the numeral to have a different base, you can specify
it via radix:
> 1234..toPrecision(3) // requires exponential notation
'1.23e+3'
> 1234..toPrecision(4)
'1234'
> 1234..toPrecision(5)
'1234.0'
> 1.234.toPrecision(3)
'1.23'
> 123.456.toString()
'123.456'
> 4..toString(2) // binary (base 2)
'100'
> 4.5.toString(2)
'100.1'
> 255..toString(16) // hexadecimal (base 16)
'ff'
> 255.66796875.toString(16)
'ff.ab'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-237-320.jpg)
![18.1 Data properties
Math.E: number [ES1]
Euler’s number, base of the natural logarithms, approximately
2.7182818284590452354.
Math.LN10: number [ES1]
The natural logarithm of 10, approximately
2.302585092994046.
Math.LN2: number [ES1]
The natural logarithm of 2, approximately
0.6931471805599453.
Math.LOG10E: number [ES1]
The logarithm of e to base 10, approximately
0.4342944819032518.
Math.LOG2E: number [ES1]
The logarithm of e to base 2, approximately
1.4426950408889634.
Math.PI: number [ES1]
The mathematical constant π, ratio of a circle’s circumference to
its diameter, approximately 3.1415926535897932.](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-240-320.jpg)
![Math.SQRT1_2: number [ES1]
The square root of 1/2, approximately 0.7071067811865476.
Math.SQRT2: number [ES1]
The square root of 2, approximately 1.4142135623730951.](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-241-320.jpg)
![18.2 Exponents, roots, logarithms
Math.cbrt(x: number): number [ES6]
Returns the cube root of x.
Math.exp(x: number): number [ES1]
Returns ex
(e being Euler’s number). The inverse of Math.log().
Math.expm1(x: number): number [ES6]
Returns Math.exp(x)-1. The inverse of Math.log1p(). Very small
numbers (fractions close to 0) are represented with a higher
precision. Therefore, this function returns more precise values
whenever .exp() returns values close to 1.
Math.log(x: number): number [ES1]
Returns the natural logarithm of x (to base e, Euler’s number).
The inverse of Math.exp().
> Math.cbrt(8)
2
> Math.exp(0)
1
> Math.exp(1) === Math.E
true
> Math.log(1)
0
> Math.log(Math.E)
1](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-242-320.jpg)
![Math.log1p(x: number): number [ES6]
Returns Math.log(1 + x). The inverse of Math.expm1(). Very
small numbers (fractions close to 0) are represented with a
higher precision. Therefore, you can provide this function with a
more precise argument whenever the argument for .log() is
close to 1.
Math.log10(x: number): number [ES6]
Returns the logarithm of x to base 10. The inverse of 10 ** x.
Math.log2(x: number): number [ES6]
Returns the logarithm of x to base 2. The inverse of 2 ** x.
Math.pow(x: number, y: number): number [ES1]
Returns xy
, x to the power of y. The same as x ** y.
> Math.log(Math.E ** 2)
2
> Math.log10(1)
0
> Math.log10(10)
1
> Math.log10(100)
2
> Math.log2(1)
0
> Math.log2(2)
1
> Math.log2(4)
2](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-243-320.jpg)
![Math.sqrt(x: number): number [ES1]
Returns the square root of x. The inverse of x ** 2.
> Math.pow(2, 3)
8
> Math.pow(25, 0.5)
5
> Math.sqrt(9)
3](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-244-320.jpg)
![18.3 Rounding
Rounding means converting an arbitrary number to an integer (a
number without a decimal fraction). The following functions
implement different approaches to rounding.
Math.ceil(x: number): number [ES1]
Returns the smallest (closest to −∞) integer i with x ≤ i.
Math.floor(x: number): number [ES1]
Returns the largest (closest to +∞) integer i with i ≤ x.
Math.round(x: number): number [ES1]
Returns the integer that is closest to x. If the decimal fraction of
x is .5 then .round() rounds up (to the integer closer to positive
infinity):
> Math.ceil(2.1)
3
> Math.ceil(2.9)
3
> Math.floor(2.1)
2
> Math.floor(2.9)
2
> Math.round(2.4)
2
> Math.round(2.5)
3](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-245-320.jpg)
![Math.trunc(x: number): number [ES6]
Removes the decimal fraction of x and returns the resulting
integer.
Tbl. 12 shows the results of the rounding functions for a few
representative inputs.
Table 12: Rounding functions of Math. Note how
things change with negative numbers because “larger”
always means “closer to positive infinity”.
-2.9 -2.5 -2.1 2.1 2.5 2.9
Math.floor -3 -3 -3 2 2 2
Math.ceil -2 -2 -2 3 3 3
Math.round -3 -2 -2 2 3 3
Math.trunc -2 -2 -2 2 2 2
> Math.trunc(2.1)
2
> Math.trunc(2.9)
2](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-246-320.jpg)
![18.4 Trigonometric Functions
All angles are specified in radians. Use the following two functions to
convert between degrees and radians.
Math.acos(x: number): number [ES1]
Returns the arc cosine (inverse cosine) of x.
Math.acosh(x: number): number [ES6]
Returns the inverse hyperbolic cosine of x.
Math.asin(x: number): number [ES1]
Returns the arc sine (inverse sine) of x.
function degreesToRadians(degrees) {
return degrees / 180 * Math.PI;
}
assert.equal(degreesToRadians(90), Math.PI/2);
function radiansToDegrees(radians) {
return radians / Math.PI * 180;
}
assert.equal(radiansToDegrees(Math.PI), 180);
> Math.acos(0)
1.5707963267948966
> Math.acos(1)
0
> Math.asin(0)
0
> Math.asin(1)
1.5707963267948966](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-247-320.jpg)
![Math.asinh(x: number): number [ES6]
Returns the inverse hyperbolic sine of x.
Math.atan(x: number): number [ES1]
Returns the arc tangent (inverse tangent) of x.
Math.atanh(x: number): number [ES6]
Returns the inverse hyperbolic tangent of x.
Math.atan2(y: number, x: number): number [ES1]
Returns the arc tangent of the quotient y/x.
Math.cos(x: number): number [ES1]
Returns the cosine of x.
Math.cosh(x: number): number [ES6]
Returns the hyperbolic cosine of x.
Math.hypot(...values: number[]): number [ES6]
Returns the square root of the sum of the squares of values
(Pythagoras’ theorem):
> Math.cos(0)
1
> Math.cos(Math.PI)
-1](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-248-320.jpg)
![Math.sin(x: number): number [ES1]
Returns the sine of x.
Math.sinh(x: number): number [ES6]
Returns the hyperbolic sine of x.
Math.tan(x: number): number [ES1]
Returns the tangent of x.
Math.tanh(x: number): number; [ES6]
Returns the hyperbolic tangent of x.
> Math.hypot(3, 4)
5
> Math.sin(0)
0
> Math.sin(Math.PI / 2)
1
> Math.tan(0)
0
> Math.tan(1)
1.5574077246549023](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-249-320.jpg)
![18.5 Various other functions
Math.abs(x: number): number [ES1]
Returns the absolute value of x.
Math.clz32(x: number): number [ES6]
Counts the leading zero bits in the 32-bit integer x. Used in DSP
algorithms.
Math.max(...values: number[]): number [ES1]
Converts values to numbers and returns the largest one.
Math.min(...values: number[]): number [ES1]
> Math.abs(3)
3
> Math.abs(-3)
3
> Math.abs(0)
0
> Math.clz32(0b01000000000000000000000000000000)
1
> Math.clz32(0b00100000000000000000000000000000)
2
> Math.clz32(2)
30
> Math.clz32(1)
31
> Math.max(3, -5, 24)
24](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-250-320.jpg)
![Converts values to numbers and returns the smallest one.
Math.random(): number [ES1]
Returns a pseudo-random number n where 0 ≤ n < 1.
Computing a random integer i where 0 ≤ i < max:
Math.sign(x: number): number [ES6]
Returns the sign of a number:
> Math.min(3, -5, 24)
-5
function getRandomInteger(max) {
return Math.floor(Math.random() * max);
}
> Math.sign(-8)
-1
> Math.sign(0)
0
> Math.sign(3)
1](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-251-320.jpg)
![20.2 Accessing characters and
code points
20.2.1 Accessing JavaScript characters
JavaScript has no extra data type for characters – characters are
always represented as strings.
20.2.2 Accessing Unicode code point
characters via for-of and spreading
Iterating over strings via for-of or spreading (...) visits Unicode
code point characters. Each code point character is encoded by 1–2
JavaScript characters. For more information, see §20.6 “Atoms of
text: Unicode characters, JavaScript characters, grapheme clusters”.
This is how you iterate over the code point characters of a string via
for-of:
const str = 'abc';
// Reading a character at a given index
assert.equal(str[1], 'b');
// Counting the characters in a string:
assert.equal(str.length, 3);
for (const ch of 'x🙂y') {
console.log(ch);
}
// Output:](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-267-320.jpg)
![And this is how you convert a string into an Array of code point
characters via spreading:
// 'x'
// '🙂'
// 'y'
assert.deepEqual([...'x🙂y'], ['x', '🙂', 'y']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-268-320.jpg)
![Plain objects have a default string representation that is not very
useful:
Arrays have a better string representation, but it still hides much
information:
Stringifying functions, returns their source code:
20.4.2 Customizing the stringification of
objects
You can override the built-in way of stringifying objects by
implementing the method toString():
> String({a: 1})
'[object Object]'
> String(['a', 'b'])
'a,b'
> String(['a', ['b']])
'a,b'
> String([1, 2])
'1,2'
> String(['1', '2'])
'1,2'
> String([true])
'true'
> String(['true'])
'true'
> String(true)
'true'
> String(function f() {return 4})
'function f() {return 4}'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-271-320.jpg)
![20.4.3 An alternate way of stringifying
values
The JSON data format is a text representation of JavaScript values.
Therefore, JSON.stringify() can also be used to convert values to
strings:
The caveat is that JSON only supports null, booleans, numbers,
strings, Arrays, and objects (which it always treats as if they were
created by object literals).
Tip: The third parameter lets you switch on multiline output and
specify how much to indent – for example:
This statement produces the following output:
{
"first": "Jane",
"last": "Doe"
}
const obj = {
toString() {
return 'hello';
}
};
assert.equal(String(obj), 'hello');
> JSON.stringify({a: 1})
'{"a":1}'
> JSON.stringify(['a', ['b']])
'["a",["b"]]'
console.log(JSON.stringify({first: 'Jane', last: 'Doe'}, null, 2](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-272-320.jpg)
![You can iterate over a string, which visits Unicode characters (not
JavaScript characters). Iteration is described later in this book. One
way of iterating is via a for-of loop:
Spreading (...) into Array literals is also based on iteration and
visits Unicode characters:
That makes it a good tool for counting Unicode characters:
20.6.2 Working with code units (char
codes)
Indices and lengths of strings are based on JavaScript characters (as
represented by UTF-16 code units).
To specify a code unit hexadecimally, you can use a code unit escape:
const str = '🙂a';
assert.equal(str.length, 3);
for (const codePointChar of str) {
console.log(codePointChar);
}
// Output:
// '🙂'
// 'a'
> [...'🙂a']
[ '🙂', 'a' ]
> [...'🙂a'].length
2
> '🙂a'.length
3](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-276-320.jpg)
![20.7 Quick reference: Strings
Strings are immutable; none of the string methods ever modify their
strings.
20.7.1 Converting to string
Tbl. 13 describes how various values are converted to strings.
Table 13: Converting values to strings.
x String(x)
undefined 'undefined'
null 'null'
Boolean value false → 'false', true → 'true'
Number value Example: 123 → '123'
String value x (input, unchanged)
An object Configurable via, e.g., toString()
20.7.2 Numeric values of characters
Char code: represents a JavaScript character numerically.
JavaScript’s name for Unicode code unit.
Size: 16 bits, unsigned
Convert number to character: String.fromCharCode() [ES1]
Convert character to number: string method .charCodeAt()
[ES1]
Code point: represents a Unicode character numerically.](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-278-320.jpg)
![Size: 21 bits, unsigned (17 planes, 16 bits each)
Convert number to character: String.fromCodePoint() [ES6]
Convert character to number: string method .codePointAt()
[ES6]
20.7.3 String operators
20.7.4 String.prototype: finding and
matching
(String.prototype is where the methods of strings are stored.)
.endsWith(searchString: string, endPos=this.length):
boolean [ES6]
Returns true if the string would end with searchString if its
length were endPos. Returns false otherwise.
.includes(searchString: string, startPos=0): boolean [ES6]
// Access characters via []
const str = 'abc';
assert.equal(str[1], 'b');
// Concatenate strings via +
assert.equal('a' + 'b' + 'c', 'abc');
assert.equal('take ' + 3 + ' oranges', 'take 3 oranges');
> 'foo.txt'.endsWith('.txt')
true
> 'abcde'.endsWith('cd', 4)
true](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-279-320.jpg)
![Returns true if the string contains the searchString and false
otherwise. The search starts at startPos.
.indexOf(searchString: string, minIndex=0): number [ES1]
Returns the lowest index at which searchString appears within
the string or -1, otherwise. Any returned index will
beminIndex` or higher.
.lastIndexOf(searchString: string, maxIndex=Infinity):
number [ES1]
Returns the highest index at which searchString appears within
the string or -1, otherwise. Any returned index will
bemaxIndex` or lower.
> 'abc'.includes('b')
true
> 'abc'.includes('b', 2)
false
> 'abab'.indexOf('a')
0
> 'abab'.indexOf('a', 1)
2
> 'abab'.indexOf('c')
-1
> 'abab'.lastIndexOf('ab', 2)
2
> 'abab'.lastIndexOf('ab', 1)
0
> 'abab'.lastIndexOf('ab')
2](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-280-320.jpg)
![[1 of 2] .match(regExp: string | RegExp): RegExpMatchArray |
null [ES3]
If regExp is a regular expression with flag /g not set, then
.match() returns the first match for regExp within the string. Or
null if there is no match. If regExp is a string, it is used to create
a regular expression (think parameter of new RegExp()) before
performing the previously mentioned steps.
The result has the following type:
Numbered capture groups become Array indices (which is why
this type extends Array). Named capture groups (ES2018)
become properties of .groups. In this mode, .match() works like
RegExp.prototype.exec().
Examples:
[2 of 2] .match(regExp: RegExp): string[] | null [ES3]
interface RegExpMatchArray extends Array<string> {
index: number;
input: string;
groups: undefined | {
[key: string]: string
};
}
> 'ababb'.match(/a(b+)/)
{ 0: 'ab', 1: 'b', index: 0, input: 'ababb', groups: undefin
> 'ababb'.match(/a(?<foo>b+)/)
{ 0: 'ab', 1: 'b', index: 0, input: 'ababb', groups: { foo:
> 'abab'.match(/x/)
null](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-281-320.jpg)
![If flag /g of regExp is set, .match() returns either an Array with
all matches or null if there was no match.
.search(regExp: string | RegExp): number [ES3]
Returns the index at which regExp occurs within the string. If
regExp is a string, it is used to create a regular expression (think
parameter of new RegExp()).
.startsWith(searchString: string, startPos=0): boolean [ES6]
Returns true if searchString occurs in the string at index
startPos. Returns false otherwise.
20.7.5 String.prototype: extracting
.slice(start=0, end=this.length): string [ES3]
> 'ababb'.match(/a(b+)/g)
[ 'ab', 'abb' ]
> 'ababb'.match(/a(?<foo>b+)/g)
[ 'ab', 'abb' ]
> 'abab'.match(/x/g)
null
> 'a2b'.search(/[0-9]/)
1
> 'a2b'.search('[0-9]')
1
> '.gitignore'.startsWith('.')
true
> 'abcde'.startsWith('bc', 1)
true](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-282-320.jpg)
![Returns the substring of the string that starts at (including)
index start and ends at (excluding) index end. If an index is
negative, it is added to .length before it is used (-1 becomes
this.length-1, etc.).
.split(separator: string | RegExp, limit?: number):
string[] [ES3]
Splits the string into an Array of substrings – the strings that
occur between the separators. The separator can be a string:
It can also be a regular expression:
The last invocation demonstrates that captures made by groups
in the regular expression become elements of the returned
Array.
Warning: .split('') splits a string into JavaScript
characters. That doesn’t work well when dealing with astral
> 'abc'.slice(1, 3)
'bc'
> 'abc'.slice(1)
'bc'
> 'abc'.slice(-2)
'bc'
> 'a | b | c'.split('|')
[ 'a ', ' b ', ' c' ]
> 'a : b : c'.split(/ *: */)
[ 'a', 'b', 'c' ]
> 'a : b : c'.split(/( *):( *)/)
[ 'a', ' ', ' ', 'b', ' ', ' ', 'c' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-283-320.jpg)
![Unicode characters (which are encoded as two JavaScript
characters). For example, emojis are astral:
Instead, it is better to use spreading:
.substring(start: number, end=this.length): string [ES1]
Use .slice() instead of this method. .substring() wasn’t
implemented consistently in older engines and doesn’t support
negative indices.
20.7.6 String.prototype: combining
.concat(...strings: string[]): string [ES3]
Returns the concatenation of the string and strings.
'a'.concat('b') is equivalent to 'a'+'b'. The latter is much
more popular.
.padEnd(len: number, fillString=' '): string [ES2017]
Appends (fragments of) fillString to the string until it has the
desired length len. If it already has or exceeds len, then it is
returned without any changes.
> '🙂X🙂'.split('')
[ 'uD83D', 'uDE42', 'X', 'uD83D', 'uDE42' ]
> [...'🙂X🙂']
[ '🙂', 'X', '🙂' ]
> 'ab'.concat('cd', 'ef', 'gh')
'abcdefgh'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-284-320.jpg)
![.padStart(len: number, fillString=' '): string [ES2017]
Prepends (fragments of) fillString to the string until it has the
desired length len. If it already has or exceeds len, then it is
returned without any changes.
.repeat(count=0): string [ES6]
Returns the string, concatenated count times.
20.7.7 String.prototype: transforming
.normalize(form: 'NFC'|'NFD'|'NFKC'|'NFKD' = 'NFC'): string
[ES6]
Normalizes the string according to the Unicode Normalization
Forms.
> '#'.padEnd(2)
'# '
> 'abc'.padEnd(2)
'abc'
> '#'.padEnd(5, 'abc')
'#abca'
> '#'.padStart(2)
' #'
> 'abc'.padStart(2)
'abc'
> '#'.padStart(5, 'abc')
'abca#'
> '*'.repeat()
''
> '*'.repeat(3)
'***'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-285-320.jpg)
![[1 of 2] .replace(searchValue: string | RegExp, replaceValue:
string): string [ES3]
Replace matches of searchValue with replaceValue. If
searchValue is a string, only the first verbatim occurrence is
replaced. If searchValue is a regular expression without flag /g,
only the first match is replaced. If searchValue is a regular
expression with /g then all matches are replaced.
Special characters in replaceValue are:
$$: becomes $
$n: becomes the capture of numbered group n (alas, $0
stands for the string '$0', it does not refer to the complete
match)
$&: becomes the complete match
$`: becomes everything before the match
$': becomes everything after the match
Examples:
> 'x.x.'.replace('.', '#')
'x#x.'
> 'x.x.'.replace(/./, '#')
'#.x.'
> 'x.x.'.replace(/./g, '#')
'####'
> 'a 2020-04 b'.replace(/([0-9]{4})-([0-9]{2})/, '|$2|')
'a |04| b'
> 'a 2020-04 b'.replace(/([0-9]{4})-([0-9]{2})/, '|$&|')
'a |2020-04| b'
> 'a 2020-04 b'.replace(/([0-9]{4})-([0-9]{2})/, '|$`|')
'a |a | b'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-286-320.jpg)
![Named capture groups (ES2018) are supported, too:
$<name> becomes the capture of named group name
Example:
[2 of 2] .replace(searchValue: string | RegExp, replacer:
(...args: any[]) => string): string [ES3]
If the second parameter is a function, occurrences are replaced
with the strings it returns. Its parameters args are:
matched: string. The complete match
g1: string|undefined. The capture of numbered group 1
g2: string|undefined. The capture of numbered group 2
(Etc.)
offset: number. Where was the match found in the input
string?
input: string. The whole input string
Named capture groups (ES2018) are supported, too. If there are
any, an argument is added at the end with an object whose
properties contain the captures:
assert.equal(
'a 2020-04 b'.replace(
/(?<year>[0-9]{4})-(?<month>[0-9]{2})/, '|$<month>|'),
'a |04| b');
const regexp = /([0-9]{4})-([0-9]{2})/;
const replacer = (all, year, month) => '|' + all + '|';
assert.equal(
'a 2020-04 b'.replace(regexp, replacer),
'a |2020-04| b');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-287-320.jpg)
![.toUpperCase(): string [ES1]
Returns a copy of the string in which all lowercase alphabetic
characters are converted to uppercase. How well that works for
various alphabets, depends on the JavaScript engine.
.toLowerCase(): string [ES1]
Returns a copy of the string in which all uppercase alphabetic
characters are converted to lowercase. How well that works for
various alphabets, depends on the JavaScript engine.
.trim(): string [ES5]
Returns a copy of the string in which all leading and trailing
whitespace (spaces, tabs, line terminators, etc.) is gone.
const regexp = /(?<year>[0-9]{4})-(?<month>[0-9]{2})/;
const replacer = (...args) => {
const groups=args.pop();
return '|' + groups.month + '|';
};
assert.equal(
'a 2020-04 b'.replace(regexp, replacer),
'a |04| b');
> '-a2b-'.toUpperCase()
'-A2B-'
> 'αβγ'.toUpperCase()
'ΑΒΓ'
> '-A2B-'.toLowerCase()
'-a2b-'
> 'ΑΒΓ'.toLowerCase()
'αβγ'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-288-320.jpg)
![.trimEnd(): string [ES2019]
Similar to .trim() but only the end of the string is trimmed:
.trimStart(): string [ES2019]
Similar to .trim() but only the beginning of the string is
trimmed:
20.7.8 Sources
TypeScript’s built-in typings
MDN web docs for JavaScript
ECMAScript language specification
Exercise: Using string methods
exercises/strings/remove_extension_test.mjs
Quiz
See quiz app.
> 'rn#t '.trim()
'#'
> ' abc '.trim()
'abc'
> ' abc '.trimEnd()
' abc'
> ' abc '.trimStart()
'abc '](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-289-320.jpg)
![Syntactically, a tagged template is a template literal that follows
a function (or rather, an expression that evaluates to a function).
That leads to the function being called. Its arguments are
derived from the contents of the template literal.
Note that getArgs() receives both the text of the literal and the
data interpolated via ${}.
const getArgs = (...args) => args;
assert.deepEqual(
getArgs`Count: ${5}!`,
[['Count: ', '!'], 5] );](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-292-320.jpg)
![21.3 Tagged templates
The expression in line A is a tagged template. It is equivalent to
invoking tagFunc() with the arguments listed in the Array in line B.
The function tagFunc before the first backtick is called a tag function.
Its arguments are:
Template strings (first argument): an Array with the text
fragments surrounding the interpolations ${}.
In the example: ['Setting ', ' is ', '!']
Substitutions (remaining arguments): the interpolated values.
In the example: 'dark mode' and true
The static (fixed) parts of the literal (the template strings) are kept
separate from the dynamic parts (the substitutions).
A tag function can return arbitrary values.
function tagFunc(...args) {
return args;
}
const setting = 'dark mode';
const value = true;
assert.deepEqual(
tagFunc`Setting ${setting} is ${value}!`, // (A)
[['Setting ', ' is ', '!'], 'dark mode', true] // (B)
);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-294-320.jpg)
![21.3.1 Cooked vs. raw template strings
(advanced)
So far, we have only seen the cooked interpretation of template
strings. But tag functions actually get two interpretations:
A cooked interpretation where backslashes have special
meaning. For example, t produces a tab character. This
interpretation of the template strings is stored as an Array in the
first argument.
A raw interpretation where backslashes do not have special
meaning. For example, t produces two characters – a backslash
and a t. This interpretation of the template strings is stored in
property .raw of the first argument (an Array).
The following tag function cookedRaw uses both interpretations:
function cookedRaw(templateStrings, ...substitutions) {
return {
cooked: [...templateStrings], // copy just the Array element
raw: templateStrings.raw,
substitutions,
};
}
assert.deepEqual(
cookedRaw`tab${'subst'}newline`,
{
cooked: ['tab', 'newline'],
raw: ['tab', 'newline'],
substitutions: ['subst'],
});](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-295-320.jpg)
![re-template-tag is a simple library for composing regular
expressions. Templates tagged with re produce regular expressions.
The main benefit is that you can interpolate regular expressions and
plain text via ${} (line A):
21.3.4 Tag function library: graphql-tag
The library graphql-tag lets you create GraphQL queries via tagged
templates:
Additionally, there are plugins for pre-compiling such queries in
Babel, TypeScript, etc.
const RE_YEAR = re`(?<year>[0-9]{4})`;
const RE_MONTH = re`(?<month>[0-9]{2})`;
const RE_DAY = re`(?<day>[0-9]{2})`;
const RE_DATE = re`/${RE_YEAR}-${RE_MONTH}-${RE_DAY}/u`; // (A)
const match = RE_DATE.exec('2017-01-27');
assert.equal(match.groups.year, '2017');
import gql from 'graphql-tag';
const query = gql`
{
user(id: 5) {
firstName
lastName
}
}
`;](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-297-320.jpg)
![21.7 Simple templating via
template literals
While template literals look like text templates, it is not immediately
obvious how to use them for (text) templating: A text template gets
its data from an object, while a template literal gets its data from
variables. The solution is to use a template literal in the body of a
function whose parameter receives the templating data – for
example:
21.7.1 A more complex example
As a more complex example, we’d like to take an Array of addresses
and produce an HTML table. This is the Array:
The function tmpl() that produces the HTML table looks as follows:
const tmpl = (data) => `Hello ${data.name}!`;
assert.equal(tmpl({name: 'Jane'}), 'Hello Jane!');
const addresses = [
{ first: '<Jane>', last: 'Bond' },
{ first: 'Lars', last: '<Croft>' },
];
onst tmpl = (addrs) => `
table>
${addrs.map(
(addr) => `
<tr>
<td>${escapeHtml(addr.first)}</td>
<td>${escapeHtml(addr.last)}</td>
</tr>](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-303-320.jpg)
![const obj = {
[sym]: 123,
};](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-307-320.jpg)
![As an example, let’s assume we are writing a library that treats
objects differently if they implement a special method. This is what
defining a property key for such a method and implementing it for an
object would look like:
The square brackets in line A enable us to specify that the method
must have the key specialMethod. More details are explained in
§28.5.2 “Computed property keys”.
const specialMethod = Symbol('specialMethod');
const obj = {
_id: 'kf12oi',
[specialMethod]() { // (A)
return this._id;
}
};
assert.equal(obj[specialMethod](), 'kf12oi');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-311-320.jpg)
 {
return x === null;
}
};
assert.equal(null instanceof PrimitiveNull, true);
> String({})
'[object Object]'
> String({ [Symbol.toStringTag]: 'is no money' })
'[object is no money]'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-312-320.jpg)
![22.3 Converting symbols
What happens if we convert a symbol sym to another primitive type?
Tbl. 14 has the answers.
Table 14: The results of converting symbols to other primitive types.
Convert
to
Explicit
conversion
Coercion (implicit
conv.)
boolean Boolean(sym) → OK !sym → OK
number Number(sym) →
TypeError
sym*2 → TypeError
string String(sym) → OK ''+sym → TypeError
sym.toString() → OK `${sym}` → TypeError
One key pitfall with symbols is how often exceptions are thrown
when converting them to something else. What is the thinking
behind that? First, conversion to number never makes sense and
should be warned about. Second, converting a symbol to a string is
indeed useful for diagnostic output. But it also makes sense to warn
about accidentally turning a symbol into a string (which is a different
kind of property key):
The downside is that the exceptions make working with symbols
more complicated. You have to explicitly convert symbols when
const obj = {};
const sym = Symbol();
assert.throws(
() => { obj['__'+sym+'__'] = true },
{ message: 'Cannot convert a Symbol value to a string' });](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-314-320.jpg)
![23 Control flow statements
23.1 Conditions of control flow statements
23.2 Controlling loops: break and continue
23.2.1 break
23.2.2 break plus label: leaving any labeled statement
23.2.3 continue
23.3 if statements
23.3.1 The syntax of if statements
23.4 switch statements
23.4.1 A first example of a switch statement
23.4.2 Don’t forget to return or break!
23.4.3 Empty case clauses
23.4.4 Checking for illegal values via a default clause
23.5 while loops
23.5.1 Examples of while loops
23.6 do-while loops
23.7 for loops
23.7.1 Examples of for loops
23.8 for-of loops
23.8.1 const: for-of vs. for
23.8.2 Iterating over iterables
23.8.3 Iterating over [index, element] pairs of Arrays
23.9 for-await-of loops
23.10 for-in loops (avoid)](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-316-320.jpg)
![23.2 Controlling loops: break and
continue
The two operators break and continue can be used to control loops
and other statements while you are inside them.
23.2.1 break
There are two versions of break: one with an operand and one
without an operand. The latter version works inside the following
statements: while, do-while, for, for-of, for-await-of, for-in and
switch. It immediately leaves the current statement:
23.2.2 break plus label: leaving any
labeled statement
break with an operand works everywhere. Its operand is a label.
Labels can be put in front of any statement, including blocks. break
foo leaves the statement whose label is foo:
for (const x of ['a', 'b', 'c']) {
console.log(x);
if (x === 'b') break;
console.log('---')
}
// Output:
// 'a'
// '---'
// 'b'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-319-320.jpg)
![In the following example, we use break with a label to leave a loop
differently when we succeeded (line A). Then we skip what comes
directly after the loop, which is where we end up if we failed.
23.2.3 continue
foo: { // label
if (condition) break foo; // labeled break
// ···
}
function findSuffix(stringArray, suffix) {
let result;
search_block: {
for (const str of stringArray) {
if (str.endsWith(suffix)) {
// Success:
result = str;
break search_block; // (A)
}
} // for
// Failure:
result = '(Untitled)';
} // search_block
return { suffix, result };
// Same as: {suffix: suffix, result: result}
}
assert.deepEqual(
findSuffix(['foo.txt', 'bar.html'], '.html'),
{ suffix: '.html', result: 'bar.html' }
);
assert.deepEqual(
findSuffix(['foo.txt', 'bar.html'], '.mjs'),
{ suffix: '.mjs', result: '(Untitled)' }
);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-320-320.jpg)
![continue only works inside while, do-while, for, for-of, for-await-
of, and for-in. It immediately leaves the current loop iteration and
continues with the next one – for example:
const lines = [
'Normal line',
'# Comment',
'Another normal line',
];
for (const line of lines) {
if (line.startsWith('#')) continue;
console.log(line);
}
// Output:
// 'Normal line'
// 'Another normal line'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-321-320.jpg)
![23.5 while loops
A while loop has the following syntax:
while («condition») {
«statements»
}
Before each loop iteration, while evaluates condition:
If the result is falsy, the loop is finished.
If the result is truthy, the while body is executed one more time.
23.5.1 Examples of while loops
The following code uses a while loop. In each loop iteration, it
removes the first element of arr via .shift() and logs it.
If the condition always evaluates to true, then while is an infinite
loop:
const arr = ['a', 'b', 'c'];
while (arr.length > 0) {
const elem = arr.shift(); // remove first element
console.log(elem);
}
// Output:
// 'a'
// 'b'
// 'c'
while (true) {
if (Math.random() === 0) break;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-328-320.jpg)
![This is how to log the contents of an Array via a for loop:
If you omit all three parts of the head, you get an infinite loop:
for (let i=0; i<3; i++) {
console.log(i);
}
// Output:
// 0
// 1
// 2
const arr = ['a', 'b', 'c'];
for (let i=0; i<arr.length; i++) {
console.log(arr[i]);
}
// Output:
// 'a'
// 'b'
// 'c'
for (;;) {
if (Math.random() === 0) break;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-331-320.jpg)
![23.8 for-of loops
A for-of loop iterates over an iterable – a data container that
supports the iteration protocol. Each iterated value is stored in a
variable, as specified in the head:
for («iteration_variable» of «iterable») {
«statements»
}
The iteration variable is usually created via a variable declaration:
But you can also use a (mutable) variable that already exists:
23.8.1 const: for-of vs. for
Note that in for-of loops you can use const. The iteration variable
can still be different for each iteration (it just can’t change during the
iteration). Think of it as a new const declaration being executed each
time in a fresh scope.
const iterable = ['hello', 'world'];
for (const elem of iterable) {
console.log(elem);
}
// Output:
// 'hello'
// 'world'
const iterable = ['hello', 'world'];
let elem;
for (elem of iterable) {
console.log(elem);
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-332-320.jpg)
![In contrast, in for loops you must declare variables via let or var if
their values change.
23.8.2 Iterating over iterables
As mentioned before, for-of works with any iterable object, not just
with Arrays – for example, with Sets:
23.8.3 Iterating over [index, element]
pairs of Arrays
Lastly, you can also use for-of to iterate over the [index, element]
entries of Arrays:
With [index, element], we are using destructuring to access Array
elements.
Exercise: for-of
exercises/control-flow/array_to_string_test.mjs
const set = new Set(['hello', 'world']);
for (const elem of set) {
console.log(elem);
}
const arr = ['a', 'b', 'c'];
for (const [index, elem] of arr.entries()) {
console.log(`${index} -> ${elem}`);
}
// Output:
// '0 -> a'
// '1 -> b'
// '2 -> c'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-333-320.jpg)
![23.10 for-in loops (avoid)
Recommendation: don’t use for-in loops
for-in has several pitfalls. Therefore, it is usually best to avoid it.
This is an example of using for-in properly, which involves
boilerplate code (line A):
We can implement the same functionality without for-in, which is
almost always better:
function getOwnPropertyNames(obj) {
const result = [];
for (const key in obj) {
if ({}.hasOwnProperty.call(obj, key)) { // (A)
result.push(key);
}
}
return result;
}
assert.deepEqual(
getOwnPropertyNames({ a: 1, b:2 }),
['a', 'b']);
assert.deepEqual(
getOwnPropertyNames(['a', 'b']),
['0', '1']); // strings!
function getOwnPropertyNames(obj) {
const result = [];
for (const key of Object.keys(obj)) {
result.push(key);
}
return result;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-335-320.jpg)
![24.1 Motivation: throwing and
catching exceptions
Consider the following code. It reads profiles stored in files into an
Array with instances of class Profile:
Let’s examine what happens in line B: An error occurred, but the best
place to handle the problem is not the current location, it’s line A.
There, we can skip the current file and move on to the next one.
Therefore:
function readProfiles(filePaths) {
const profiles = [];
for (const filePath of filePaths) {
try {
const profile = readOneProfile(filePath);
profiles.push(profile);
} catch (err) { // (A)
console.log('Error in: '+filePath, err);
}
}
}
function readOneProfile(filePath) {
const profile = new Profile();
const file = openFile(filePath);
// ··· (Read the data in `file` into `profile`)
return profile;
}
function openFile(filePath) {
if (!fs.existsSync(filePath)) {
throw new Error('Could not find file '+filePath); // (B)
}
// ··· (Open the file whose path is `filePath`)
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-338-320.jpg)
![The (roughly) equivalent arrow function looks as follows. Arrow
functions are expressions.
Here, the body of the arrow function is a block. But it can also be an
expression. The following arrow function works exactly like the
previous one.
If an arrow function has only a single parameter and that parameter
is an identifier (not a destructuring pattern) then you can omit the
parentheses around the parameter:
That is convenient when passing arrow functions as parameters to
other functions or methods:
This previous example demonstrates one benefit of arrow functions
– conciseness. If we perform the same task with a function
expression, our code is more verbose:
25.3.3.2 Arrow functions: lexical this
Ordinary functions can be both methods and real functions. Alas, the
two roles are in conflict:
const f = (x, y, z) => { return 123 };
const f = (x, y, z) => 123;
const id = x => x;
> [1,2,3].map(x => x+1)
[ 2, 3, 4 ]
[1,2,3].map(function (x) { return x+1 });](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-356-320.jpg)
![25.6 Parameter handling
Once again, I am only mentioning functions in this section, but
everything also applies to methods.
25.6.1 Terminology: parameters
vs. arguments
The term parameter and the term argument basically mean the
same thing. If you want to, you can make the following distinction:
Parameters are part of a function definition. They are also called
formal parameters and formal arguments.
Arguments are part of a function call. They are also called actual
parameters and actual arguments.
25.6.2 Terminology: callback
A callback or callback function is a function that is an argument of a
function or method call.
The following is an example of a callback:
const myArray = ['a', 'b'];
const callback = (x) => console.log(x);
myArray.forEach(callback);
// Output:
// 'a'
// 'b'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-363-320.jpg)
![JavaScript uses the term callback broadly
In other programming languages, the term callback often has a
narrower meaning: it refers to a pattern for delivering results
asynchronously, via a function-valued parameter. In this meaning,
the callback (or continuation) is invoked after a function has
completely finished its computation.
Callbacks as an asynchronous pattern, are described in the chapter
on asynchronous programming.
25.6.3 Too many or not enough
arguments
JavaScript does not complain if a function call provides a different
number of arguments than expected by the function definition:
Extra arguments are ignored.
Missing parameters are set to undefined.
For example:
function foo(x, y) {
return [x, y];
}
// Too many arguments:
assert.deepEqual(foo('a', 'b', 'c'), ['a', 'b']);
// The expected number of arguments:
assert.deepEqual(foo('a', 'b'), ['a', 'b']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-364-320.jpg)
![25.6.4 Parameter default values
Parameter default values specify the value to use if a parameter has
not been provided – for example:
undefined also triggers the default value:
25.6.5 Rest parameters
A rest parameter is declared by prefixing an identifier with three dots
(...). During a function or method call, it receives an Array with all
remaining arguments. If there are no extra arguments at the end, it is
an empty Array – for example:
// Not enough arguments:
assert.deepEqual(foo('a'), ['a', undefined]);
function f(x, y=0) {
return [x, y];
}
assert.deepEqual(f(1), [1, 0]);
assert.deepEqual(f(), [undefined, 0]);
assert.deepEqual(
f(undefined, undefined),
[undefined, 0]);
function f(x, ...y) {
return [x, y];
}
assert.deepEqual(
f('a', 'b', 'c'),
['a', ['b', 'c']]);
assert.deepEqual(](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-365-320.jpg)
![25.6.5.1 Enforcing a certain number of arguments via a
rest parameter
You can use a rest parameter to enforce a certain number of
arguments. Take, for example, the following function:
This is how we force callers to always provide two arguments:
In line A, we access the elements of args via destructuring.
25.6.6 Named parameters
When someone calls a function, the arguments provided by the caller
are assigned to the parameters received by the callee. Two common
ways of performing the mapping are:
1. Positional parameters: An argument is assigned to a parameter
if they have the same position. A function call with only
positional arguments looks as follows.
f(),
[undefined, []]);
function createPoint(x, y) {
return {x, y};
// same as {x: x, y: y}
}
function createPoint(...args) {
if (args.length !== 2) {
throw new Error('Please provide exactly 2 arguments!');
}
const [x, y] = args; // (A)
return {x, y};
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-366-320.jpg)
![iterable object and the iterated values all become arguments. In
other words, a single argument is expanded into multiple arguments
– for example:
Spreading and rest parameters use the same syntax (...), but they
serve opposite purposes:
Rest parameters are used when defining functions or methods.
They collect arguments into Arrays.
Spread arguments are used when calling functions or methods.
They turn iterable objects into arguments.
25.6.8.1 Example: spreading into Math.max()
Math.max() returns the largest one of its zero or more arguments.
Alas, it can’t be used for Arrays, but spreading gives us a way out:
function func(x, y) {
console.log(x);
console.log(y);
}
const someIterable = ['a', 'b'];
func(...someIterable);
// same as func('a', 'b')
// Output:
// 'a'
// 'b'
> Math.max(-1, 5, 11, 3)
11
> Math.max(...[-1, 5, 11, 3])
11
> Math.max(-1, ...[-5, 11], 3)
11](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-369-320.jpg)
![25.6.8.2 Example: spreading into Array.prototype.push()
Similarly, the Array method .push() destructively adds its zero or
more parameters to the end of its Array. JavaScript has no method
for destructively appending an Array to another one. Once again, we
are saved by spreading:
Exercises: Parameter handling
Positional parameters:
exercises/callables/positional_parameters_test.mjs
Named parameters:
exercises/callables/named_parameters_test.mjs
const arr1 = ['a', 'b'];
const arr2 = ['c', 'd'];
arr1.push(...arr2);
assert.deepEqual(arr1, ['a', 'b', 'c', 'd']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-370-320.jpg)
![Functions created via new Function() are sloppy.
25.7.3 Recommendations
Avoid dynamic evaluation of code as much as you can:
It’s a security risk because it may enable an attacker to execute
arbitrary code with the privileges of your code.
It may be switched off – for example, in browsers, via a Content
Security Policy.
Very often, JavaScript is dynamic enough so that you don’t need
eval() or similar. In the following example, what we are doing with
eval() (line A) can be achieved just as well without it (line B).
If you have to dynamically evaluate code:
Prefer new Function() over eval(): it always executes its code in
global context and a function provides a clean interface to the
evaluated code.
Prefer indirect eval over direct eval: evaluating code in global
context is safer.
Quiz
See quiz app.
const obj = {a: 1, b: 2};
const propKey = 'b';
assert.equal(eval('obj.' + propKey), 2); // (A)
assert.equal(obj[propKey], 2); // (B)](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-373-320.jpg)
![outer environments contains all variables that can currently be
accessed (minus shadowed variables).
When you make a function call, you create a new environment. The
outer environment of that environment is the environment in which
the function was created. To help set up the field outer of
environments created via function calls, each function has an
internal property named [[Scope]] that points to its “birth
environment”.
26.3.1 Executing the code
These are the pauses we are making while executing the code:
This is what happens:
Pause 1 – before calling f() (fig. 10).
Pause 2 – while executing f() (fig. 11).
Pause 3 – while executing square() (fig. 12).
After that, return statements pop execution entries off the stack.
function f(x) {
function square() {
const result = x * x;
// Pause 3
return result;
}
// Pause 2
return square();
}
// Pause 1
assert.equal(f(6), 36);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-380-320.jpg)
![0 f
Lexical environments
Execution contexts Functions
[[Scope]]
Figure 10: Nested scopes, pause 1 – before calling f(): The top-level
environment has a single entry, for f(). The birth environment of f()
is the top-level environment. Therefore, f’s [[Scope]] points to it.
1
0 f
square
6
x
outer
Lexical environments
Execution contexts Functions
[[Scope]]
[[Scope]]
f(6)
Figure 11: Nested scopes, pause 2 – while executing f(): There is now
an environment for the function call f(6). The outer environment of
that environment is the birth environment of f() (the top-level
environment at index 0). We can see that the field outer was set to
the value of f’s [[Scope]]. Furthermore, the [[Scope]] of the new
function square() is the environment that was just created.
2
1
0 f
square
6
x
outer
Lexical environments
Execution contexts Functions
[[Scope]]
[[Scope]]
result 36
outer
f(6)
square()
Figure 12: Nested scopes, pause 3 – while executing square(): The
previous pattern was repeated: the outer of the most recent
environment was set up via the [[Scope]] of the function that we just
called. The chain of scopes created via outer, contains all variables
that are active right now. For example, we can access result, square,](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-381-320.jpg)
![Only filling in some of the parameters of a function is called partial
application (the function has not been fully applied yet). Method
.bind() of functions performs partial application. In the previous
example, we can see that partial application is simple if a function is
curried.
26.4.0.1 Executing the code
As we are executing the following code, we are making three pauses:
This is what happens:
Pause 1 – during the execution of add(2) (fig. 13).
Pause 2 – after the execution of add(2) (fig. 14).
Pause 3 – while executing plus2(5) (fig. 15).
[[Scope]]
1
0
( uninit.)
plus2
add
2
x
outer
Lexical environments
Execution contexts Functions
[[Scope]]
add(2)
Figure 13: Closures, pause 1 – during the execution of add(2): We can
see that the function returned by add() already exists (see bottom
function add(x) {
return (y) => {
// Pause 3: plus2(5)
return x + y;
}; // Pause 1: add(2)
}
const plus2 = add(2);
// Pause 2
assert.equal(plus2(5), 7);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-384-320.jpg)
![right corner) and that it points to its birth environment via its
internal property [[Scope]]. Note that plus2 is still in its temporal
dead zone and uninitialized.
plus2
add
0
2
x
outer
Lexical environments
Execution contexts Functions
[[Scope]]
[[Scope]]
Kept alive by closure add(2)
Figure 14: Closures, pause 2 – after the execution of add(2): plus2
now points to the function returned by add(2). That function keeps
its birth environment (the environment of add(2)) alive via its
[[Scope]].
plus2
add
1
0
2
x
outer
Lexical environments
Execution contexts Functions
[[Scope]]
[[Scope]]
5
y
outer
add(2)
plus2(5)
Figure 15: Closures, pause 3 – while executing plus2(5): The
[[Scope]] of plus2 is used to set up the outer of the new
environment. That’s how the current function gets access to x.](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-385-320.jpg)
![CommonJS can be characterized as follows:
Designed for servers.
Modules are meant to be loaded synchronously (the importer
waits while the imported module is loaded and executed).
Compact syntax.
27.4.2 Client side: AMD (Asynchronous
Module Definition) modules
The AMD module format was created to be easier to use in browsers
than the CommonJS format. Its most popular implementation is
RequireJS. The following is an example of an AMD module.
// ···
}
function exportedFunc() {
importedFunc1();
importedFunc2();
internalFunc();
}
// Exports
module.exports = {
exportedFunc: exportedFunc,
};
define(['./other-module1.js', './other-module2.js'],
function (otherModule1, otherModule2) {
var importedFunc1 = otherModule1.importedFunc1;
var importedFunc2 = otherModule2.importedFunc2;
function internalFunc() {
// ···
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-394-320.jpg)
![Namespace imports are an alternative to named imports. If we
namespace-import a module, it becomes an object whose properties
are the named exports. This is what main.mjs looks like if we use a
namespace import:
27.6.4 Named exporting styles: inline
versus clause (advanced)
The named export style we have seen so far was inline: We exported
entities by prefixing them with the keyword export.
But we can also use separate export clauses. For example, this is
what lib/my-math.mjs looks like with an export clause:
With an export clause, we can rename before exporting and use
different names internally:
import * as myMath from './lib/my-math.mjs';
assert.equal(myMath.square(3), 9);
assert.deepEqual(
Object.keys(myMath), ['LIGHTSPEED', 'square']);
function times(a, b) {
return a * b;
}
function square(x) {
return times(x, x);
}
const LIGHTSPEED = 299792458;
export { square, LIGHTSPEED }; // semicolon!
function times(a, b) {
return a * b;](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-401-320.jpg)
![27.9 npm packages
The npm software registry is the dominant way of distributing
JavaScript libraries and apps for Node.js and web browsers. It is
managed via the npm package manager (short: npm). Software is
distributed as so-called packages. A package is a directory
containing arbitrary files and a file package.json at the top level that
describes the package. For example, when npm creates an empty
package inside a directory foo/, you get this package.json:
Some of these properties contain simple metadata:
name specifies the name of this package. Once it is uploaded to
the npm registry, it can be installed via npm install foo.
version is used for version management and follows semantic
versioning, with three numbers:
Major version: is incremented when incompatible API
changes are made.
{
"name": "foo",
"version": "1.0.0",
"description": "",
"main": "index.js",
"scripts": {
"test": "echo "Error: no test specified" && exit 1"
},
"keywords": [],
"author": "",
"license": "ISC"
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-410-320.jpg)
![28.2 Objects as records
Let’s first explore the role record of objects.
28.2.1 Object literals: properties
Object literals are one way of creating objects-as-records. They are a stand-out
feature of JavaScript: you can directly create objects – no need for classes! This is
an example:
In the example, we created an object via an object literal, which starts and ends
with curly braces {}. Inside it, we defined two properties (key-value entries):
The first property has the key first and the value 'Jane'.
The second property has the key last and the value 'Doe'.
We will later see other ways of specifying property keys, but with this way of
specifying them, they must follow the rules of JavaScript variable names. For
example, you can use first_name as a property key, but not first-name).
However, reserved words are allowed:
In order to check the effects of various operations on objects, we’ll occasionally
use Object.keys() in this part of the chapter. It lists property keys:
28.2.2 Object literals: property value shorthands
const jane = {
first: 'Jane',
last: 'Doe', // optional trailing comma
};
const obj = {
if: true,
const: true,
};
> Object.keys({a:1, b:2})
[ 'a', 'b' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-433-320.jpg)
![28.2.5 Object literals: methods
The following code shows how to create the method .says() via an object literal:
During the method call jane.says('hello'), jane is called the receiver of the
method call and assigned to the special variable this. That enables method
.says() to access the sibling property .first in line A.
28.2.6 Object literals: accessors
There are two kinds of accessors in JavaScript:
A getter is a method-like entity that is invoked by getting a property.
A setter is a method-like entity that is invoked by setting a property.
28.2.6.1 Getters
A getter is created by prefixing a method definition with the modifier get:
Object.keys(obj), []);
obj.unknownProperty = 'abc';
assert.deepEqual(
Object.keys(obj), ['unknownProperty']);
const jane = {
first: 'Jane', // data property
says(text) { // method
return `${this.first} says “${text}”`; // (A)
}, // comma as separator (optional at end)
};
assert.equal(jane.says('hello'), 'Jane says “hello”');
const jane = {
first: 'Jane',
last: 'Doe',
get full() {
return `${this.first} ${this.last}`;
},
};
assert.equal(jane.full, 'Jane Doe');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-435-320.jpg)
![28.2.6.2 Setters
A setter is created by prefixing a method definition with the modifier set:
Exercise: Creating an object via an object literal
exercises/single-objects/color_point_object_test.mjs
jane.first = 'John';
assert.equal(jane.full, 'John Doe');
const jane = {
first: 'Jane',
last: 'Doe',
set full(fullName) {
const parts = fullName.split(' ');
this.first = parts[0];
this.last = parts[1];
},
};
jane.full = 'Richard Roe';
assert.equal(jane.first, 'Richard');
assert.equal(jane.last, 'Roe');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-436-320.jpg)
![28.3 Spreading into object literals (...)
Inside a function call, spreading (...) turns the iterated values of an iterable
object into arguments.
Inside an object literal, a spread property adds the properties of another object
to the current one:
If property keys clash, the property that is mentioned last “wins”:
All values are spreadable, even undefined and null:
Property .length of strings and of Arrays is hidden from this kind of operation (it
is not enumerable; see §28.7.3 “Property attributes and property descriptors” for
more information).
28.3.1 Use case for spreading: copying objects
You can use spreading to create a copy of an object original:
> const obj = {foo: 1, bar: 2};
> {...obj, baz: 3}
{ foo: 1, bar: 2, baz: 3 }
> const obj = {foo: 1, bar: 2, baz: 3};
> {...obj, foo: true}
{ foo: true, bar: 2, baz: 3 }
> {foo: true, ...obj}
{ foo: 1, bar: 2, baz: 3 }
> {...undefined}
{}
> {...null}
{}
> {...123}
{}
> {...'abc'}
{ '0': 'a', '1': 'b', '2': 'c' }
> {...['a', 'b']}
{ '0': 'a', '1': 'b' }](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-437-320.jpg)
![In line A, we don’t extract the method .handleClick() properly. Instead, we
should do:
Exercise: Extracting a method
exercises/single-objects/method_extraction_exrc.mjs
28.4.5 this pitfall: accidentally shadowing this
Accidentally shadowing this is only an issue with ordinary
functions
Arrow functions don’t shadow this.
Consider the following problem: when you are inside an ordinary function, you
can’t access the this of the surrounding scope because the ordinary function has
its own this. In other words, a variable in an inner scope hides a variable in an
outer scope. That is called shadowing. The following code is an example:
In line A, we want to access the this of .prefixStringArray(). But we can’t since
the surrounding ordinary function has its own this that shadows (blocks access
}
handleClick(event) {
alert('Clicked ' + this.id);
}
}
elem.addEventListener('click', this.handleClick.bind(this));
const prefixer = {
prefix: '==> ',
prefixStringArray(stringArray) {
return stringArray.map(
function (x) {
return this.prefix + x; // (A)
});
},
};
assert.throws(
() => prefixer.prefixStringArray(['a', 'b']),
/^TypeError: Cannot read property 'prefix' of undefined$/);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-445-320.jpg)
![to) the this of the method. The value of the former this is undefined due to the
callback being function-called. That explains the error message.
The simplest way to fix this problem is via an arrow function, which doesn’t have
its own this and therefore doesn’t shadow anything:
We can also store this in a different variable (line A), so that it doesn’t get
shadowed:
Another option is to specify a fixed this for the callback via .bind() (line A):
Lastly, .map() lets us specify a value for this (line A) that it uses when invoking
the callback:
const prefixer = {
prefix: '==> ',
prefixStringArray(stringArray) {
return stringArray.map(
(x) => {
return this.prefix + x;
});
},
};
assert.deepEqual(
prefixer.prefixStringArray(['a', 'b']),
['==> a', '==> b']);
prefixStringArray(stringArray) {
const that = this; // (A)
return stringArray.map(
function (x) {
return that.prefix + x;
});
},
prefixStringArray(stringArray) {
return stringArray.map(
function (x) {
return this.prefix + x;
}.bind(this)); // (A)
},
prefixStringArray(stringArray) {
return stringArray.map(
function (x) {
return this.prefix + x;](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-446-320.jpg)
![Second, when getting or setting properties, we can use square brackets with
strings inside them:
You can also use these techniques for methods:
28.5.2 Computed property keys
So far, property keys were always fixed strings inside object literals. In this
section we learn how to dynamically compute property keys. That enables us to
use either arbitrary strings or symbols.
The syntax of dynamically computed property keys in object literals is inspired by
dynamically accessing properties. That is, we can use square brackets to wrap
expressions:
The main use case for computed keys is having symbols as property keys (line A).
};
// Get property
assert.equal(obj['Can be any string!'], 123);
// Set property
obj['Can be any string!'] = 'abc';
assert.equal(obj['Can be any string!'], 'abc');
const obj = {
'A nice method'() {
return 'Yes!';
},
};
assert.equal(obj['A nice method'](), 'Yes!');
const obj = {
['Hello world!']: true,
['f'+'o'+'o']: 123,
[Symbol.toStringTag]: 'Goodbye', // (A)
};
assert.equal(obj['Hello world!'], true);
assert.equal(obj.foo, 123);
assert.equal(obj[Symbol.toStringTag], 'Goodbye');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-450-320.jpg)
![Note that the square brackets operator for getting and setting properties works
with arbitrary expressions:
Methods can have computed property keys, too:
For the remainder of this chapter, we’ll mostly use fixed property keys again
(because they are syntactically more convenient). But all features are also
available for arbitrary strings and symbols.
Exercise: Non-destructively updating a property via spreading
(computed key)
exercises/single-objects/update_property_test.mjs
28.5.3 The in operator: is there a property with a
given key?
The in operator checks if an object has a property with a given key:
28.5.3.1 Checking if a property exists via truthiness
assert.equal(obj['f'+'o'+'o'], 123);
assert.equal(obj['==> foo'.slice(-3)], 123);
const methodKey = Symbol();
const obj = {
[methodKey]() {
return 'Yes!';
},
};
assert.equal(obj[methodKey](), 'Yes!');
const obj = {
foo: 'abc',
bar: false,
};
assert.equal('foo' in obj, true);
assert.equal('unknownKey' in obj, false);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-451-320.jpg)
![You can also use a truthiness check to determine if a property exists:
The previous checks work because obj.foo is truthy and because reading a
missing property returns undefined (which is falsy).
There is, however, one important caveat: truthiness checks fail if the property
exists, but has a falsy value (undefined, null, false, 0, "", etc.):
28.5.4 Deleting properties
You can delete properties via the delete operator:
28.5.5 Listing property keys
Table 18: Standard library methods for listing own (non-inherited) property keys.
All of them return Arrays with strings and/or symbols.
enumerable
non-
e.
string symbol
Object.keys() ✔ ✔
Object.getOwnPropertyNames() ✔ ✔ ✔
Object.getOwnPropertySymbols() ✔ ✔ ✔
assert.equal(
obj.foo ? 'exists' : 'does not exist',
'exists');
assert.equal(
obj.unknownKey ? 'exists' : 'does not exist',
'does not exist');
assert.equal(
obj.bar ? 'exists' : 'does not exist',
'does not exist'); // should be: 'exists'
const obj = {
foo: 123,
};
assert.deepEqual(Object.keys(obj), ['foo']);
delete obj.foo;
assert.deepEqual(Object.keys(obj), []);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-452-320.jpg)
![enumerable
non-
e.
string symbol
Reflect.ownKeys() ✔ ✔ ✔ ✔
Each of the methods in tbl. 18 returns an Array with the own property keys of the
parameter. In the names of the methods, you can see that the following
distinction is made:
A property key can be either a string or a symbol.
A property name is a property key whose value is a string.
A property symbol is a property key whose value is a symbol.
The next section describes the term enumerable and demonstrates each of the
methods.
28.5.5.1 Enumerability
Enumerability is an attribute of a property. Non-enumerable properties are
ignored by some operations – for example, by Object.keys() (see tbl. 18) and by
spread properties. By default, most properties are enumerable. The next example
shows how to change that. It also demonstrates the various ways of listing
property keys.
const enumerableSymbolKey = Symbol('enumerableSymbolKey');
const nonEnumSymbolKey = Symbol('nonEnumSymbolKey');
// We create enumerable properties via an object literal
const obj = {
enumerableStringKey: 1,
[enumerableSymbolKey]: 2,
}
// For non-enumerable properties, we need a more powerful tool
Object.defineProperties(obj, {
nonEnumStringKey: {
value: 3,
enumerable: false,
},
[nonEnumSymbolKey]: {
value: 4,](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-453-320.jpg)
![Object.defineProperties() is explained later in this chapter.
28.5.6 Listing property values via Object.values()
Object.values() lists the values of all enumerable properties of an object:
28.5.7 Listing property entries via
Object.entries()
Object.entries() lists key-value pairs of enumerable properties. Each pair is
encoded as a two-element Array:
enumerable: false,
},
});
assert.deepEqual(
Object.keys(obj),
[ 'enumerableStringKey' ]);
assert.deepEqual(
Object.getOwnPropertyNames(obj),
[ 'enumerableStringKey', 'nonEnumStringKey' ]);
assert.deepEqual(
Object.getOwnPropertySymbols(obj),
[ enumerableSymbolKey, nonEnumSymbolKey ]);
assert.deepEqual(
Reflect.ownKeys(obj),
[
'enumerableStringKey', 'nonEnumStringKey',
enumerableSymbolKey, nonEnumSymbolKey,
]);
const obj = {foo: 1, bar: 2};
assert.deepEqual(
Object.values(obj),
[1, 2]);
const obj = {foo: 1, bar: 2};
assert.deepEqual(
Object.entries(obj),
[
['foo', 1],
['bar', 2],
]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-454-320.jpg)
![Exercise: Object.entries()
exercises/single-objects/find_key_test.mjs
28.5.8 Properties are listed deterministically
Own (non-inherited) properties of objects are always listed in the following
order:
1. Properties with string keys that contain integer indices (that includes Array
indices):
In ascending numeric order
2. Remaining properties with string keys:
In the order in which they were added
3. Properties with symbol keys:
In the order in which they were added
The following example demonstrates how property keys are sorted according to
these rules:
The order of properties
The ECMAScript specification describes in more detail how properties are
ordered.
28.5.9 Assembling objects via Object.fromEntries()
Given an iterable over [key, value] pairs, Object.fromEntries() creates an object:
> Object.keys({b:0,a:0, 10:0,2:0})
[ '2', '10', 'b', 'a' ]
assert.deepEqual(
Object.fromEntries([['foo',1], ['bar',2]]),
{
foo: 1,
bar: 2,
}
);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-455-320.jpg)
![Object.fromEntries() does the opposite of Object.entries().
To demonstrate both, we’ll use them to implement two tool functions from the
library Underscore in the next subsubsections.
28.5.9.1 Example: pick(object, ...keys)
pick returns a copy of object that only has those properties whose keys are
mentioned as arguments:
We can implement pick() as follows:
28.5.9.2 Example: invert(object)
invert returns a copy of object where the keys and values of all properties are
swapped:
const address = {
street: 'Evergreen Terrace',
number: '742',
city: 'Springfield',
state: 'NT',
zip: '49007',
};
assert.deepEqual(
pick(address, 'street', 'number'),
{
street: 'Evergreen Terrace',
number: '742',
}
);
function pick(object, ...keys) {
const filteredEntries = Object.entries(object)
.filter(([key, _value]) => keys.includes(key));
return Object.fromEntries(filteredEntries);
}
assert.deepEqual(
invert({a: 1, b: 2, c: 3}),
{1: 'a', 2: 'b', 3: 'c'}
);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-456-320.jpg)
![We can implement invert() like this:
28.5.9.3 A simple implementation of Object.fromEntries()
The following function is a simplified version of Object.fromEntries():
28.5.9.4 A polyfill for Object.fromEntries()
The npm package object.fromentries is a polyfill for Object.entries(): it installs
its own implementation if that method doesn’t exist on the current platform.
Exercise: Object.entries() and Object.fromEntries()
exercises/single-objects/omit_properties_test.mjs
28.5.10 The pitfalls of using an object as a
dictionary
If you use plain objects (created via object literals) as dictionaries, you have to
look out for two pitfalls.
function invert(object) {
const mappedEntries = Object.entries(object)
.map(([key, value]) => [value, key]);
return Object.fromEntries(mappedEntries);
}
function fromEntries(iterable) {
const result = {};
for (const [key, value] of iterable) {
let coercedKey;
if (typeof key === 'string' || typeof key === 'symbol') {
coercedKey = key;
} else {
coercedKey = String(key);
}
result[coercedKey] = value;
}
return result;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-457-320.jpg)
![The first pitfall is that the in operator also finds inherited properties:
We want dict to be treated as empty, but the in operator detects the properties it
inherits from its prototype, Object.prototype.
The second pitfall is that you can’t use the property key __proto__ because it has
special powers (it sets the prototype of the object):
So how do we avoid these pitfalls?
Whenever you can, use Maps. They are the best solution for dictionaries.
If you can’t, use a library for objects-as-dictionaries that does everything
safely.
If you can’t, use an object without a prototype.
The following code demonstrates using objects without prototypes as
dictionaries:
We avoided both pitfalls: First, a property without a prototype does not inherit
any properties (line A). Second, in modern JavaScript, __proto__ is implemented
via Object.prototype. That means that it is switched off if Object.prototype is not
in the prototype chain.
Exercise: Using an object as a dictionary
const dict = {};
assert.equal('toString' in dict, true);
const dict = {};
dict['__proto__'] = 123;
// No property was added to dict:
assert.deepEqual(Object.keys(dict), []);
const dict = Object.create(null); // no prototype
assert.equal('toString' in dict, false); // (A)
dict['__proto__'] = 123;
assert.deepEqual(Object.keys(dict), ['__proto__']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-458-320.jpg)
![28.6 Standard methods (advanced)
Object.prototype defines several standard methods that can be overridden to
configure how an object is treated by the language. Two important ones are:
.toString()
.valueOf()
28.6.1 .toString()
.toString() determines how objects are converted to strings:
28.6.2 .valueOf()
.valueOf() determines how objects are converted to numbers:
> String({toString() { return 'Hello!' }})
'Hello!'
> String({})
'[object Object]'
> Number({valueOf() { return 123 }})
123
> Number({})
NaN](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-460-320.jpg)
![There is one caveat: Object.freeze(obj) freezes shallowly. That is, only the
properties of obj are frozen but not objects stored in properties.
28.7.3 Property attributes and property
descriptors
Just as objects are composed of properties, properties are composed of
attributes. The value of a property is only one of several attributes. Others
include:
writable: Is it possible to change the value of the property?
enumerable: Is the property considered by Object.keys(), spreading, etc.?
When you are using one of the operations for handling property attributes,
attributes are specified via property descriptors: objects where each property
represents one attribute. For example, this is how you read the attributes of a
property obj.foo:
And this is how you set the attributes of a property obj.bar:
const obj = { foo: 123 };
assert.deepEqual(
Object.getOwnPropertyDescriptor(obj, 'foo'),
{
value: 123,
writable: true,
enumerable: true,
configurable: true,
});
const obj = {
foo: 1,
bar: 2,
};
assert.deepEqual(Object.keys(obj), ['foo', 'bar']);
// Hide property `bar` from Object.keys()
Object.defineProperty(obj, 'bar', {
enumerable: false,
});](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-462-320.jpg)
![Enumerability is covered in greater detail earlier in this chapter. For more
information on property attributes and property descriptors, consult Speaking
JavaScript.
Quiz
See quiz app.
assert.deepEqual(Object.keys(obj), ['foo']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-463-320.jpg)
![__proto__
protoProp 'a'
proto
. . .
objProp
__proto__
'b'
obj
Figure 19: obj starts a chain of objects that continues with proto and
other objects.
Non-inherited properties are called own properties. obj has one own
property, .objProp.
29.1.1 JavaScript’s operations: all
properties vs. own properties
Some operations consider all properties (own and inherited) – for
example, getting properties:
Other operations only consider own properties – for example,
Object.keys():
> const obj = { foo: 1 };
> typeof obj.foo // own
'number'
> typeof obj.toString // inherited
'function'
> Object.keys(obj)
[ 'foo' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-467-320.jpg)
![Read on for another operation that also only considers own
properties: setting properties.
29.1.2 Pitfall: only the first member of a
prototype chain is mutated
One aspect of prototype chains that may be counter-intuitive is that
setting any property via an object – even an inherited one – only
changes that very object – never one of the prototypes.
Consider the following object obj:
In the next code snippet, we set the inherited property obj.protoProp
(line A). That “changes” it by creating an own property: When
reading obj.protoProp, the own property is found first and its value
overrides the value of the inherited property.
const proto = {
protoProp: 'a',
};
const obj = {
__proto__: proto,
objProp: 'b',
};
// In the beginning, obj has one own property
assert.deepEqual(Object.keys(obj), ['objProp']);
obj.protoProp = 'x'; // (A)
// We created a new own property:
assert.deepEqual(Object.keys(obj), ['objProp', 'protoProp']);
// The inherited property itself is unchanged:
assert.equal(proto.protoProp, 'a');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-468-320.jpg)
![We’ll explore the instanceof operator in more detail later, after we
have looked at subclassing.
29.2.6 Why I recommend classes
I recommend using classes for the following reasons:
Classes are a common standard for object creation and
inheritance that is now widely supported across frameworks
(React, Angular, Ember, etc.). This is an improvement to how
things were before, when almost every framework had its own
inheritance library.
They help tools such as IDEs and type checkers with their work
and enable new features there.
If you come from another language to JavaScript and are used to
classes, then you can get started more quickly.
JavaScript engines optimize them. That is, code that uses classes
is almost always faster than code that uses a custom inheritance
library.
You can subclass built-in constructor functions such as Error.
That doesn’t mean that classes are perfect:
> ({}) instanceof Person
false
> ({}) instanceof Object
true
> [] instanceof Array
true](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-480-320.jpg)
![29.3 Private data for classes
This section describes techniques for hiding some of the data of an
object from the outside. We discuss them in the context of classes,
but they also work for objects created directly, e.g., via object literals.
29.3.1 Private data: naming convention
The first technique makes a property private by prefixing its name
with an underscore. This doesn’t protect the property in any way; it
merely signals to the outside: “You don’t need to know about this
property.”
In the following code, the properties ._counter and ._action are
private.
class Countdown {
constructor(counter, action) {
this._counter = counter;
this._action = action;
}
dec() {
this._counter--;
if (this._counter === 0) {
this._action();
}
}
}
// The two properties aren’t really private:
assert.deepEqual(
Object.keys(new Countdown()),
['_counter', '_action']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-482-320.jpg)
![With this technique, you don’t get any protection and private names
can clash. On the plus side, it is easy to use.
29.3.2 Private data: WeakMaps
Another technique is to use WeakMaps. How exactly that works is
explained in the chapter on WeakMaps. This is a preview:
This technique offers you considerable protection from outside
access and there can’t be any name clashes. But it is also more
complicated to use.
29.3.3 More techniques for private data
const _counter = new WeakMap();
const _action = new WeakMap();
class Countdown {
constructor(counter, action) {
_counter.set(this, counter);
_action.set(this, action);
}
dec() {
let counter = _counter.get(this);
counter--;
_counter.set(this, counter);
if (counter === 0) {
_action.get(this)();
}
}
}
// The two pseudo-properties are truly private:
assert.deepEqual(
Object.keys(new Countdown()),
[]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-483-320.jpg)
![Let’s start by examining plain objects:
Object.prototype
{}
__proto__
null
__proto__
Figure 24: The prototype chain of an object created via an object
literal starts with that object, continues with Object.prototype, and
ends with null.
Fig. 24 shows a diagram for this prototype chain. We can see that {}
really is an instance of Object – Object.prototype is in its prototype
chain.
29.4.3.2 The prototype chain of []
What does the prototype chain of an Array look like?
> p({}) === Object.prototype
true
> p(p({})) === null
true
> p([]) === Array.prototype
true
> p(p([])) === Object.prototype
true
> p(p(p([]))) === null
true](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-489-320.jpg)
![Object.prototype
Array.prototype
[]
__proto__
__proto__
null
__proto__
Figure 25: The prototype chain of an Array has these members: the
Array instance, Array.prototype, Object.prototype, null.
This prototype chain (visualized in fig. 25) tells us that an Array
object is an instance of Array, which is a subclass of Object.
29.4.3.3 The prototype chain of function () {}
Lastly, the prototype chain of an ordinary function tells us that all
functions are objects:
29.4.3.4 Objects that aren’t instances of Object
An object is only an instance of Object if Object.prototype is in its
prototype chain. Most objects created via various literals are
instances of Object:
> p(function () {}) === Function.prototype
true
> p(p(function () {})) === Object.prototype
true](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-490-320.jpg)
![30.2 Core iteration constructs:
iterables and iterators
Two roles (described by interfaces) form the core of iteration
(fig. 28):
An iterable is an object whose contents can be traversed
sequentially.
An iterator is the pointer used for the traversal.
[Symbol.iterator]()
···
Iterable:
traversable data structure
next()
Iterator:
pointer for traversing iterable
returns
Figure 28: Iteration has two main interfaces: Iterable and Iterator.
The former has a method that returns the latter.
These are type definitions (in TypeScript’s notation) for the
interfaces of the iteration protocol:
interface Iterable<T> {
[Symbol.iterator]() : Iterator<T>;
}
interface Iterator<T> {
next() : IteratorResult<T>;
}
interface IteratorResult<T> {
value: T;
done: boolean;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-501-320.jpg)
![30.3 Iterating manually
This is an example of using the iteration protocol:
30.3.1 Iterating over an iterable via while
The following code demonstrates how to use a while loop to iterate
over an iterable:
const iterable = ['a', 'b'];
// The iterable is a factory for iterators:
const iterator = iterable[Symbol.iterator]();
// Call .next() until .done is true:
assert.deepEqual(
iterator.next(), { value: 'a', done: false });
assert.deepEqual(
iterator.next(), { value: 'b', done: false });
assert.deepEqual(
iterator.next(), { value: undefined, done: true });
function logAll(iterable) {
const iterator = iterable[Symbol.iterator]();
while (true) {
const {value, done} = iterator.next();
if (done) break;
console.log(value);
}
}
logAll(['a', 'b']);
// Output:
// 'a'
// 'b'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-503-320.jpg)
![30.4 Iteration in practice
We have seen how to use the iteration protocol manually, and it is
relatively cumbersome. But the protocol is not meant to be used
directly – it is meant to be used via higher-level language constructs
built on top of it. This section shows what that looks like.
30.4.1 Iterating over Arrays
JavaScript’s Arrays are iterable. That enables us to use the for-of
loop:
Destructuring via Array patterns (explained later) also uses iteration
under the hood:
30.4.2 Iterating over Sets
JavaScript’s Set data structure is iterable. That means for-of works:
const myArray = ['a', 'b', 'c'];
for (const x of myArray) {
console.log(x);
}
// Output:
// 'a'
// 'b'
// 'c'
const [first, second] = myArray;
assert.equal(first, 'a');
assert.equal(second, 'b');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-505-320.jpg)
![As does Array-destructuring:
const mySet = new Set().add('a').add('b').add('c');
for (const x of mySet) {
console.log(x);
}
// Output:
// 'a'
// 'b'
// 'c'
const [first, second] = mySet;
assert.equal(first, 'a');
assert.equal(second, 'b');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-506-320.jpg)
![30.5 Quick reference: synchronous
iteration
30.5.1 Iterable data sources
The following built-in data sources are iterable:
Arrays
Strings
Maps
Sets
(Browsers: DOM data structures)
To iterate over the properties of objects, you need helpers such as
Object.keys() and Object.entries(). That is necessary because
properties exist at a different level that is independent of the level of
data structures.
30.5.2 Iterating constructs
The following constructs are based on iteration:
Destructuring via an Array pattern:
The for-of loop:
const [x,y] = iterable;
for (const x of iterable) { /*···*/ }](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-507-320.jpg)
![Array.from():
Spreading (via ...) into function calls and Array literals:
new Map() and new Set():
Promise.all() and Promise.race():
yield*:
Quiz
See quiz app.
const arr = Array.from(iterable);
func(...iterable);
const arr = [...iterable];
const m = new Map(iterableOverKeyValuePairs);
const s = new Set(iterableOverElements);
const promise1 = Promise.all(iterableOverPromises);
const promise2 = Promise.race(iterableOverPromises);
function* generatorFunction() {
yield* iterable;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-508-320.jpg)
![31 Arrays (Array)
31.1 The two roles of Arrays in JavaScript
31.2 Basic Array operations
31.2.1 Creating, reading, writing Arrays
31.2.2 The .length of an Array
31.2.3 Clearing an Array
31.2.4 Spreading into Array literals
31.2.5 Arrays: listing indices and entries
31.2.6 Is a value an Array?
31.3 for-of and Arrays
31.3.1 for-of: iterating over elements
31.3.2 for-of: iterating over [index, element] pairs
31.4 Array-like objects
31.5 Converting iterable and Array-like values to Arrays
31.5.1 Converting iterables to Arrays via spreading (...)
31.5.2 Converting iterables and Array-like objects to Arrays
via Array.from() (advanced)
31.6 Creating and filling Arrays with arbitrary lengths
31.6.1 Do you need to create an empty Array that you’ll fill
completely later on?
31.6.2 Do you need to create an Array filled with a primitive
value?
31.6.3 Do you need to create an Array filled with objects?
31.6.4 Do you need to create a range of integers?](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-509-320.jpg)
![31.2 Basic Array operations
31.2.1 Creating, reading, writing Arrays
The best way to create an Array is via an Array literal:
The Array literal starts and ends with square brackets []. It creates
an Array with three elements: 'a', 'b', and 'c'.
To read an Array element, you put an index in square brackets
(indices start at zero):
To change an Array element, you assign to an Array with an index:
The range of Array indices is 32 bits (excluding the maximum
length): [0, 232−1)
31.2.2 The .length of an Array
Every Array has a property .length that can be used to both read and
change(!) the number of elements in an Array.
The length of an Array is always the highest index plus one:
const arr = ['a', 'b', 'c'];
assert.equal(arr[0], 'a');
arr[0] = 'x';
assert.deepEqual(arr, ['x', 'b', 'c']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-513-320.jpg)
![If you write to the Array at the index of the length, you append an
element:
Another way of (destructively) appending an element is via the Array
method .push():
If you set .length, you are pruning the Array by removing elements:
31.2.3 Clearing an Array
To clear (empty) an Array, you can either set its .length to zero:
or you can assign a new empty Array to the variable storing the
Array:
> const arr = ['a', 'b'];
> arr.length
2
> arr[arr.length] = 'c';
> arr
[ 'a', 'b', 'c' ]
> arr.length
3
> arr.push('d');
> arr
[ 'a', 'b', 'c', 'd' ]
> arr.length = 1;
> arr
[ 'a' ]
const arr = ['a', 'b', 'c'];
arr.length = 0;
assert.deepEqual(arr, []);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-514-320.jpg)
![The latter approach has the advantage of not affecting other
locations that point to the same Array. If, however, you do want to
reset a shared Array for everyone, then you need the former
approach.
Exercise: Removing empty lines via .push()
exercises/arrays/remove_empty_lines_push_test.mjs
31.2.4 Spreading into Array literals
Inside an Array literal, a spread element consists of three dots (...)
followed by an expression. It results in the expression being
evaluated and then iterated over. Each iterated value becomes an
additional Array element – for example:
That means that we can use spreading to create a copy of an Array:
Spreading is also convenient for concatenating Arrays (and other
iterables) into Arrays:
let arr = ['a', 'b', 'c'];
arr = [];
assert.deepEqual(arr, []);
> const iterable = ['b', 'c'];
> ['a', ...iterable, 'd']
[ 'a', 'b', 'c', 'd' ]
const original = ['a', 'b', 'c'];
const copy = [...original];
const arr1 = ['a', 'b'];
const arr2 = ['c', 'd'];](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-515-320.jpg)
![Due to spreading using iteration, it only works if the value is iterable:
Spreading into Array literals is shallow
Similar to spreading into object literals, spreading into Array
literals creates shallow copies. That is, nested Arrays are not
copied.
31.2.5 Arrays: listing indices and entries
Method .keys() lists the indices of an Array:
.keys() returns an iterable. In line A, we spread to obtain an Array.
Listing Array indices is different from listing properties. The former
produces numbers; the latter produces stringified numbers (in
addition to non-index property keys):
const concatenated = [...arr1, ...arr2, 'e'];
assert.deepEqual(
concatenated,
['a', 'b', 'c', 'd', 'e']);
> [...'abc'] // strings are iterable
[ 'a', 'b', 'c' ]
> [...123]
TypeError: number 123 is not iterable
> [...undefined]
TypeError: undefined is not iterable
const arr = ['a', 'b'];
assert.deepEqual(
[...arr.keys()], // (A)
[0, 1]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-516-320.jpg)
![Method .entries() lists the contents of an Array as [index, element]
pairs:
31.2.6 Is a value an Array?
Following are two ways of checking if a value is an Array:
instanceof is usually fine. You need Array.isArray() if a value may
come from another realm. Roughly, a realm is an instance of
JavaScript’s global scope. Some realms are isolated from each other
(e.g., Web Workers in browsers), but there are also realms between
which you can move data – for example, same-origin iframes in
browsers. x instanceof Array checks the prototype chain of x and
therefore returns false if x is an Array from another realm.
typeof categorizes Arrays as objects:
const arr = ['a', 'b'];
arr.prop = true;
assert.deepEqual(
Object.keys(arr),
['0', '1', 'prop']);
const arr = ['a', 'b'];
assert.deepEqual(
[...arr.entries()],
[[0, 'a'], [1, 'b']]);
> [] instanceof Array
true
> Array.isArray([])
true](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-517-320.jpg)
![> typeof []
'object'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-518-320.jpg)
![31.3 for-of and Arrays
We have already encountered the for-of loop. This section briefly
recaps how to use it for Arrays.
31.3.1 for-of: iterating over elements
The following for-of loop iterates over the elements of an Array.
31.3.2 for-of: iterating over [index,
element] pairs
The following for-of loop iterates over [index, element] pairs.
Destructuring (described later), gives us convenient syntax for
setting up index and element in the head of for-of.
for (const element of ['a', 'b']) {
console.log(element);
}
// Output:
// 'a'
// 'b'
for (const [index, element] of ['a', 'b'].entries()) {
console.log(index, element);
}
// Output:
// 0, 'a'
// 1, 'b'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-519-320.jpg)
![31.4 Array-like objects
Some operations that work with Arrays require only the bare
minimum: values must only be Array-like. An Array-like value is an
object with the following properties:
.length: holds the length of the Array-like object.
[0]: holds the element at index 0 (etc.). Note that if you use
numbers as property names, they are always coerced to strings.
Therefore, [0] retrieves the value of the property whose key is
'0'.
For example, Array.from() accepts Array-like objects and converts
them to Arrays:
The TypeScript interface for Array-like objects is:
Array-like objects are relatively rare in modern
JavaScript
// If you omit .length, it is interpreted as 0
assert.deepEqual(
Array.from({}),
[]);
assert.deepEqual(
Array.from({length:2, 0:'a', 1:'b'}),
[ 'a', 'b' ]);
interface ArrayLike<T> {
length: number;
[n: number]: T;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-520-320.jpg)
![31.5 Converting iterable and Array-
like values to Arrays
There are two common ways of converting iterable and Array-like
values to Arrays: spreading and Array.from().
31.5.1 Converting iterables to Arrays via
spreading (...)
Inside an Array literal, spreading via ... converts any iterable object
into a series of Array elements. For example:
The conversion works because the DOM collection is iterable.
31.5.2 Converting iterables and Array-
like objects to Arrays via Array.from()
(advanced)
Array.from() can be used in two modes.
// Get an Array-like collection from a web browser’s DOM
const domCollection = document.querySelectorAll('a');
// Alas, the collection is missing many Array methods
assert.equal('map' in domCollection, false);
// Solution: convert it to an Array
const arr = [...domCollection];
assert.deepEqual(
arr.map(x => x.href),
['https://2ality.com', 'https://exploringjs.com']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-522-320.jpg)
![31.5.2.1 Mode 1 of Array.from(): converting
The first mode has the following type signature:
Interface Iterable is shown in the chapter on synchronous iteration.
Interface ArrayLike appeared earlier in this chapter.
With a single parameter, Array.from() converts anything iterable or
Array-like to an Array:
31.5.2.2 Mode 2 of Array.from(): converting and mapping
The second mode of Array.from() involves two parameters:
In this mode, Array.from() does several things:
It iterates over iterable.
It calls mapFunc with each iterated value. The optional parameter
thisArg specifies a this for mapFunc.
It applies mapFunc to each iterated value.
It collects the results in a new Array and returns it.
.from<T>(iterable: Iterable<T> | ArrayLike<T>): T[]
> Array.from(new Set(['a', 'b']))
[ 'a', 'b' ]
> Array.from({length: 2, 0:'a', 1:'b'})
[ 'a', 'b' ]
.from<T, U>(
iterable: Iterable<T> | ArrayLike<T>,
mapFunc: (v: T, i: number) => U,
thisArg?: any)
: U[]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-523-320.jpg)
![In other words: we are going from an iterable with elements of type T
to an Array with elements of type U.
This is an example:
> Array.from(new Set(['a', 'b']), x => x + x)
[ 'aa', 'bb' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-524-320.jpg)
![31.6 Creating and filling Arrays
with arbitrary lengths
The best way of creating an Array is via an Array literal. However,
you can’t always use one: The Array may be too large, you may not
know its length during development, or you may want to keep its
length flexible. Then I recommend the following techniques for
creating, and possibly filling, Arrays.
31.6.1 Do you need to create an empty
Array that you’ll fill completely later on?
Note that the result has three holes (empty slots) – the last comma in
an Array literal is always ignored.
31.6.2 Do you need to create an Array
filled with a primitive value?
Caveat: If you use .fill() with an object, then each Array element
will refer to this object (sharing it).
> new Array(3)
[ , , ,]
> new Array(3).fill(0)
[0, 0, 0]
const arr = new Array(3).fill({});
arr[0].prop = true;
assert.deepEqual(](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-525-320.jpg)
![The next subsection explains how to fix this.
31.6.3 Do you need to create an Array
filled with objects?
31.6.4 Do you need to create a range of
integers?
Here is an alternative, slightly hacky technique for creating integer
ranges that start at zero:
arr, [
{prop: true},
{prop: true},
{prop: true},
]);
> Array.from({length: 3}, () => ({}))
[{}, {}, {}]
function createRange(start, end) {
return Array.from({length: end-start}, (_, i) => i+start);
}
assert.deepEqual(
createRange(2, 5),
[2, 3, 4]);
/** Returns an iterable */
function createRange(end) {
return new Array(end).keys();
}
assert.deepEqual(
[...createRange(4)],
[0, 1, 2, 3]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-526-320.jpg)
![31.7 Multidimensional Arrays
JavaScript does not have real multidimensional Arrays; you need to
resort to Arrays whose elements are Arrays:
function initMultiArray(...dimensions) {
function initMultiArrayRec(dimIndex) {
if (dimIndex >= dimensions.length) {
return 0;
} else {
const dim = dimensions[dimIndex];
const arr = [];
for (let i=0; i<dim; i++) {
arr.push(initMultiArrayRec(dimIndex+1));
}
return arr;
}
}
return initMultiArrayRec(0);
}
const arr = initMultiArray(4, 3, 2);
arr[3][2][1] = 'X'; // last in each dimension
assert.deepEqual(arr, [
[ [ 0, 0 ], [ 0, 0 ], [ 0, 0 ] ],
[ [ 0, 0 ], [ 0, 0 ], [ 0, 0 ] ],
[ [ 0, 0 ], [ 0, 0 ], [ 0, 0 ] ],
[ [ 0, 0 ], [ 0, 0 ], [ 0, 'X' ] ],
]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-528-320.jpg)
![31.8 More Array features
(advanced)
In this section, we look at phenomena you don’t encounter often
when working with Arrays.
31.8.1 Array indices are (slightly special)
property keys
You’d think that Array elements are special because you are
accessing them via numbers. But the square brackets operator [] for
doing so is the same operator that is used for accessing properties. It
coerces any value (that is not a symbol) to a string. Therefore, Array
elements are (almost) normal properties (line A) and it doesn’t
matter if you use numbers or strings as indices (lines B and C):
To make matters even more confusing, this is only how the language
specification defines things (the theory of JavaScript, if you will).
Most JavaScript engines optimize under the hood and do use actual
integers to access Array elements (the practice of JavaScript, if you
will).
const arr = ['a', 'b'];
arr.prop = 123;
assert.deepEqual(
Object.keys(arr),
['0', '1', 'prop']); // (A)
assert.equal(arr[0], 'a'); // (B)
assert.equal(arr['0'], 'a'); // (C)](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-529-320.jpg)
![Property keys (strings!) that are used for Array elements are called
indices. A string str is an index if converting it to a 32-bit unsigned
integer and back results in the original value. Written as a formula:
ToString(ToUint32(str)) === str
31.8.1.1 Listing indices
When listing property keys, indices are treated specially – they
always come first and are sorted like numbers ('2' comes before
'10'):
Note that .length, .entries() and .keys() treat Array indices as
numbers and ignore non-index properties:
We used a spread element (...) to convert the iterables returned by
.keys() and .entries() to Arrays.
31.8.2 Arrays are dictionaries and can
have holes
const arr = [];
arr.prop = true;
arr[1] = 'b';
arr[0] = 'a';
assert.deepEqual(
Object.keys(arr),
['0', '1', 'prop']);
assert.equal(arr.length, 2);
assert.deepEqual(
[...arr.keys()], [0, 1]);
assert.deepEqual(
[...arr.entries()], [[0, 'a'], [1, 'b']]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-530-320.jpg)
![We distinguish two kinds of Arrays in JavaScript:
An Array arr is dense if all indices i, with 0 ≤ i < arr.length,
exist. That is, the indices form a contiguous range.
An Array is sparse if the range of indices has holes in it. That is,
some indices are missing.
Arrays can be sparse in JavaScript because Arrays are actually
dictionaries from indices to values.
Recommendation: avoid holes
So far, we have only seen dense Arrays and it’s indeed
recommended to avoid holes: They make your code more
complicated and are not handled consistently by Array methods.
Additionally, JavaScript engines optimize dense Arrays, making
them faster.
31.8.2.1 Creating holes
You can create holes by skipping indices when assigning elements:
In line A, we are using Object.keys() because arr.keys() treats holes
as if they were undefined elements and does not reveal them.
const arr = [];
arr[0] = 'a';
arr[2] = 'c';
assert.deepEqual(Object.keys(arr), ['0', '2']); // (A)
assert.equal(0 in arr, true); // element
assert.equal(1 in arr, false); // hole](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-531-320.jpg)
![Another way of creating holes is to skip elements in Array literals:
You can also delete Array elements:
31.8.2.2 How do Array operations treat holes?
Alas, there are many different ways in which Array operations treat
holes.
Some Array operations remove holes:
Some Array operations ignore holes:
Some Array operations ignore but preserve holes:
Some Array operations treat holes as undefined elements:
const arr = ['a', , 'c'];
assert.deepEqual(Object.keys(arr), ['0', '2']);
const arr = ['a', 'b', 'c'];
assert.deepEqual(Object.keys(arr), ['0', '1', '2']);
delete arr[1];
assert.deepEqual(Object.keys(arr), ['0', '2']);
> ['a',,'b'].filter(x => true)
[ 'a', 'b' ]
> ['a', ,'a'].every(x => x === 'a')
true
> ['a',,'b'].map(x => 'c')
[ 'c', , 'c' ]
> Array.from(['a',,'b'], x => x)
[ 'a', undefined, 'b' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-532-320.jpg)
![Object.keys() works differently than .keys() (strings vs. numbers,
holes don’t have keys):
There is no rule to remember here. If it ever matters how an Array
operation treats holes, the best approach is to do a quick test in a
console.
> [...['a',,'b'].entries()]
[[0, 'a'], [1, undefined], [2, 'b']]
> [...['a',,'b'].keys()]
[ 0, 1, 2 ]
> Object.keys(['a',,'b'])
[ '0', '2' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-533-320.jpg)
![31.9 Adding and removing
elements (destructively and non-
destructively)
JavaScript’s Array is quite flexible and more like a combination of
array, stack, and queue. This section explores ways of adding and
removing Array elements. Most operations can be performed both
destructively (modifying the Array) and non-destructively
(producing a modified copy).
31.9.1 Prepending elements and Arrays
In the following code, we destructively prepend single elements to
arr1 and an Array to arr2:
Spreading lets us unshift an Array into arr2.
Non-destructive prepending is done via spread elements:
const arr1 = ['a', 'b'];
arr1.unshift('x', 'y'); // prepend single elements
assert.deepEqual(arr1, ['x', 'y', 'a', 'b']);
const arr2 = ['a', 'b'];
arr2.unshift(...['x', 'y']); // prepend Array
assert.deepEqual(arr2, ['x', 'y', 'a', 'b']);
const arr1 = ['a', 'b'];
assert.deepEqual(
['x', 'y', ...arr1], // prepend single elements
['x', 'y', 'a', 'b']);
assert.deepEqual(arr1, ['a', 'b']); // unchanged!](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-534-320.jpg)
![31.9.2 Appending elements and Arrays
In the following code, we destructively append single elements to
arr1 and an Array to arr2:
Spreading lets us push an Array into arr2.
Non-destructive appending is done via spread elements:
31.9.3 Removing elements
const arr2 = ['a', 'b'];
assert.deepEqual(
[...['x', 'y'], ...arr2], // prepend Array
['x', 'y', 'a', 'b']);
assert.deepEqual(arr2, ['a', 'b']); // unchanged!
const arr1 = ['a', 'b'];
arr1.push('x', 'y'); // append single elements
assert.deepEqual(arr1, ['a', 'b', 'x', 'y']);
const arr2 = ['a', 'b'];
arr2.push(...['x', 'y']); // append Array
assert.deepEqual(arr2, ['a', 'b', 'x', 'y']);
const arr1 = ['a', 'b'];
assert.deepEqual(
[...arr1, 'x', 'y'], // append single elements
['a', 'b', 'x', 'y']);
assert.deepEqual(arr1, ['a', 'b']); // unchanged!
const arr2 = ['a', 'b'];
assert.deepEqual(
[...arr2, ...['x', 'y']], // append Array
['a', 'b', 'x', 'y']);
assert.deepEqual(arr2, ['a', 'b']); // unchanged!](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-535-320.jpg)
![These are three destructive ways of removing Array elements:
.splice() is covered in more detail in the quick reference at the end
of this chapter.
Destructuring via a rest element lets you non-destructively remove
elements from the beginning of an Array (destructuring is covered
later).
Alas, a rest element must come last in an Array. Therefore, you can
only use it to extract suffixes.
Exercise: Implementing a queue via an Array
exercises/arrays/queue_via_array_test.mjs
// Destructively remove first element:
const arr1 = ['a', 'b', 'c'];
assert.equal(arr1.shift(), 'a');
assert.deepEqual(arr1, ['b', 'c']);
// Destructively remove last element:
const arr2 = ['a', 'b', 'c'];
assert.equal(arr2.pop(), 'c');
assert.deepEqual(arr2, ['a', 'b']);
// Remove one or more elements anywhere:
const arr3 = ['a', 'b', 'c', 'd'];
assert.deepEqual(arr3.splice(1, 2), ['b', 'c']);
assert.deepEqual(arr3, ['a', 'd']);
const arr1 = ['a', 'b', 'c'];
// Ignore first element, extract remaining elements
const [, ...arr2] = arr1;
assert.deepEqual(arr2, ['b', 'c']);
assert.deepEqual(arr1, ['a', 'b', 'c']); // unchanged!](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-536-320.jpg)
![search it for answers. This parameter lets you write reusable
callbacks for such algorithms.
What the callback is expected to return depends on the method it is
passed to. Possibilities include:
.map() fills its result with the values returned by its callback:
.find() returns the first Array element for which its callback
returns true:
Both of these methods are described in more detail later.
31.10.2 Searching elements: .find(),
.findIndex()
.find() returns the first element for which its callback returns a
truthy value (and undefined if it can’t find anything):
.findIndex() returns the index of the first element for which its
callback returns a truthy value (and -1 if it can’t find anything):
> ['a', 'b', 'c'].map(x => x + x)
[ 'aa', 'bb', 'cc' ]
> ['a', 'bb', 'ccc'].find(str => str.length >= 2)
'bb'
> [6, -5, 8].find(x => x < 0)
-5
> [6, 5, 8].find(x => x < 0)
undefined
> [6, -5, 8].findIndex(x => x < 0)
1](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-538-320.jpg)
![.findIndex() can be implemented as follows:
31.10.3 .map(): copy while giving elements
new values
.map() returns a modified copy of the receiver. The elements of the
copy are the results of applying map’s callback to the elements of the
receiver.
All of this is easier to understand via examples:
.map() can be implemented as follows:
> [6, 5, 8].findIndex(x => x < 0)
-1
function findIndex(arr, callback) {
for (const [i, x] of arr.entries()) {
if (callback(x, i, arr)) {
return i;
}
}
return -1;
}
> [1, 2, 3].map(x => x * 3)
[ 3, 6, 9 ]
> ['how', 'are', 'you'].map(str => str.toUpperCase())
[ 'HOW', 'ARE', 'YOU' ]
> [true, true, true].map((_x, index) => index)
[ 0, 1, 2 ]
function map(arr, mapFunc) {
const result = [];
for (const [i, x] of arr.entries()) {
result.push(mapFunc(x, i, arr));
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-539-320.jpg)
![Exercise: Numbering lines via .map()
exercises/arrays/number_lines_test.mjs
31.10.4 .flatMap(): mapping to zero or
more values
The type signature of Array<T>.prototype.flatMap() is:
Both .map() and .flatMap() take a function callback as a parameter
that controls how an input Array is translated to an output Array:
With .map(), each input Array element is translated to exactly
one output element. That is, callback returns a single value.
With .flatMap(), each input Array element is translated to zero
or more output elements. That is, callback returns an Array of
values (it can also return non-Array values, but that is rare).
This is .flatMap() in action:
return result;
}
.flatMap<U>(
callback: (value: T, index: number, array: T[]) => U|Array<U>,
thisValue?: any
): U[]
> ['a', 'b', 'c'].flatMap(x => [x,x])
[ 'a', 'a', 'b', 'b', 'c', 'c' ]
> ['a', 'b', 'c'].flatMap(x => [x])
[ 'a', 'b', 'c' ]
> ['a', 'b', 'c'].flatMap(x => [])
[]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-540-320.jpg)
![31.10.4.1 A simple implementation
You could implement .flatMap() as follows. Note: This
implementation is simpler than the built-in version, which, for
example, performs more checks.
What is .flatMap() good for? Let’s look at use cases!
31.10.4.2 Use case: filtering and mapping at the same time
The result of the Array method .map() always has the same length as
the Array it is invoked on. That is, its callback can’t skip Array
elements it isn’t interested in. The ability of .flatMap() to do so is
useful in the next example.
We will use the following function processArray() to create an Array
that we’ll then filter and map via .flatMap():
function flatMap(arr, mapFunc) {
const result = [];
for (const [index, elem] of arr.entries()) {
const x = mapFunc(elem, index, arr);
// We allow mapFunc() to return non-Arrays
if (Array.isArray(x)) {
result.push(...x);
} else {
result.push(x);
}
}
return result;
}
function processArray(arr, callback) {
return arr.map(x => {
try {
return { value: callback(x) };](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-541-320.jpg)
![Next, we create an Array results via processArray():
We can now use .flatMap() to extract just the values or just the
errors from results:
31.10.4.3 Use case: mapping to multiple values
The Array method .map() maps each input Array element to one
output element. But what if we want to map it to multiple output
elements?
} catch (e) {
return { error: e };
}
});
}
const results = processArray([1, -5, 6], throwIfNegative);
assert.deepEqual(results, [
{ value: 1 },
{ error: new Error('Illegal value: -5') },
{ value: 6 },
]);
function throwIfNegative(value) {
if (value < 0) {
throw new Error('Illegal value: '+value);
}
return value;
}
const values = results.flatMap(
result => result.value ? [result.value] : []);
assert.deepEqual(values, [1, 6]);
const errors = results.flatMap(
result => result.error ? [result.error] : []);
assert.deepEqual(errors, [new Error('Illegal value: -5')]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-542-320.jpg)
![That becomes necessary in the following example:
We want to convert an Array of strings to an Array of Unicode
characters (code points). The following function achieves that via
.flatMap():
Exercises: .flatMap()
exercises/arrays/convert_to_numbers_test.mjs
exercises/arrays/replace_objects_test.mjs
31.10.5 .filter(): only keep some of the
elements
The Array method .filter() returns an Array collecting all elements
for which the callback returns a truthy value.
For example:
.filter() can be implemented as follows:
> stringsToCodePoints(['many', 'a', 'moon'])
['m', 'a', 'n', 'y', 'a', 'm', 'o', 'o', 'n']
function stringsToCodePoints(strs) {
return strs.flatMap(str => [...str]);
}
> [-1, 2, 5, -7, 6].filter(x => x >= 0)
[ 2, 5, 6 ]
> ['a', 'b', 'c', 'd'].filter((_x,i) => (i%2)===0)
[ 'a', 'c' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-543-320.jpg)
![Exercise: Removing empty lines via .filter()
exercises/arrays/remove_empty_lines_filter_test.mjs
31.10.6 .reduce(): deriving a value from
an Array (advanced)
Method .reduce() is a powerful tool for computing a “summary” of
an Array arr. A summary can be any kind of value:
A number. For example, the sum of all elements of arr.
An Array. For example, a copy of arr, where each element is
twice the original element.
Etc.
reduce is also known as foldl (“fold left”) in functional programming
and popular there. One caveat is that it can make code difficult to
understand.
.reduce() has the following type signature (inside an Array<T>):
function filter(arr, filterFunc) {
const result = [];
for (const [i, x] of arr.entries()) {
if (filterFunc(x, i, arr)) {
result.push(x);
}
}
return result;
}
.reduce<U>(
callback: (accumulator: U, element: T, index: number, array: T
init?: U)
: U](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-544-320.jpg)
![T is the type of the Array elements, U is the type of the summary. The
two may or may not be different. accumulator is just another name
for “summary”.
To compute the summary of an Array arr, .reduce() feeds all Array
elements to its callback one at a time:
callback combines the previously computed summary (stored in its
parameter accumulator) with the current Array element and returns
the next accumulator. The result of .reduce() is the final accumulator
– the last result of callback after it has visited all elements.
In other words: callback does most of the work; .reduce() just
invokes it in a useful manner.
You could say that the callback folds Array elements into the
accumulator. That’s why this operation is called “fold” in functional
programming.
31.10.6.1 A first example
Let’s look at an example of .reduce() in action: function addAll()
computes the sum of all numbers in an Array arr.
const accumulator_0 = callback(init, arr[0]);
const accumulator_1 = callback(accumulator_0, arr[1]);
const accumulator_2 = callback(accumulator_1, arr[2]);
// Etc.
function addAll(arr) {
const startSum = 0;
const callback = (sum, element) => sum + element;](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-545-320.jpg)
![In this case, the accumulator holds the sum of all Array elements that
callback has already visited.
How was the result 6 derived from the Array in line A? Via the
following invocations of callback:
callback(0, 1) --> 1
callback(1, 2) --> 3
callback(3, 3) --> 6
Notes:
The first parameters are the current accumulators (starting with
parameter init of .reduce()).
The second parameters are the current Array elements.
The results are the next accumulators.
The last result of callback is also the result of .reduce().
Alternatively, we could have implemented addAll() via a for-of loop:
It’s hard to say which of the two implementations is “better”: the one
based on .reduce() is a little more concise, while the one based on
return arr.reduce(callback, startSum);
}
assert.equal(addAll([1, 2, 3]), 6); // (A)
assert.equal(addAll([7, -4, 2]), 5);
function addAll(arr) {
let sum = 0;
for (const element of arr) {
sum = sum + element;
}
return sum;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-546-320.jpg)
![for-of may be a little easier to understand – especially if you are not
familiar with functional programming.
31.10.6.2 Example: finding indices via .reduce()
The following function is an implementation of the Array method
.indexOf(). It returns the first index at which the given searchValue
appears inside the Array arr:
One limitation of .reduce() is that you can’t finish early (in a for-of
loop, you can break). Here, we always immediately return the result
once we have found it.
31.10.6.3 Example: doubling Array elements
Function double(arr) returns a copy of inArr whose elements are all
multiplied by 2:
const NOT_FOUND = -1;
function indexOf(arr, searchValue) {
return arr.reduce(
(result, elem, index) => {
if (result !== NOT_FOUND) {
// We have already found something: don’t change anythin
return result;
} else if (elem === searchValue) {
return index;
} else {
return NOT_FOUND;
}
},
NOT_FOUND);
}
assert.equal(indexOf(['a', 'b', 'c'], 'b'), 1);
assert.equal(indexOf(['a', 'b', 'c'], 'x'), -1);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-547-320.jpg)
![We modify the initial value [] by pushing into it. A non-destructive,
more functional version of double() looks as follows:
This version is more elegant but also slower and uses more memory.
Exercises: .reduce()
map() via .reduce():
exercises/arrays/map_via_reduce_test.mjs
filter() via .reduce():
exercises/arrays/filter_via_reduce_test.mjs
countMatches() via .reduce():
exercises/arrays/count_matches_via_reduce_test.mjs
function double(inArr) {
return inArr.reduce(
(outArr, element) => {
outArr.push(element * 2);
return outArr;
},
[]);
}
assert.deepEqual(
double([1, 2, 3]),
[2, 4, 6]);
function double(inArr) {
return inArr.reduce(
// Don’t change `outArr`, return a fresh Array
(outArr, element) => [...outArr, element * 2],
[]);
}
assert.deepEqual(
double([1, 2, 3]),
[2, 4, 6]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-548-320.jpg)
![31.11 .sort(): sorting Arrays
.sort() has the following type definition:
By default, .sort() sorts string representations of the elements.
These representations are compared via <. This operator compares
lexicographically (the first characters are most significant). You can
see that when sorting numbers:
When sorting human-language strings, you need to be aware that
they are compared according to their code unit values (char codes):
As you can see, all unaccented uppercase letters come before all
unaccented lowercase letters, which come before all accented letters.
Use Intl, the JavaScript internationalization API, if you want proper
sorting for human languages.
Note that .sort() sorts in place; it changes and returns its receiver:
sort(compareFunc?: (a: T, b: T) => number): this
> [200, 3, 10].sort()
[ 10, 200, 3 ]
> ['pie', 'cookie', 'éclair', 'Pie', 'Cookie', 'Éclair'].sort()
[ 'Cookie', 'Pie', 'cookie', 'pie', 'Éclair', 'éclair' ]
> const arr = ['a', 'c', 'b'];
> arr.sort() === arr
true
> arr
[ 'a', 'b', 'c' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-549-320.jpg)
![31.11.1 Customizing the sort order
You can customize the sort order via the parameter compareFunc,
which must return a number that is:
negative if a < b
zero if a === b
positive if a > b
Tip for remembering these rules
A negative number is less than zero (etc.).
31.11.2 Sorting numbers
You can use this helper function to sort numbers:
The following is a quick and dirty alternative.
function compareNumbers(a, b) {
if (a < b) {
return -1;
} else if (a === b) {
return 0;
} else {
return 1;
}
}
assert.deepEqual(
[200, 3, 10].sort(compareNumbers),
[3, 10, 200]);
> [200, 3, 10].sort((a,b) => a - b)
[ 3, 10, 200 ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-550-320.jpg)
![The downsides of this approach are:
It is cryptic.
There is a risk of numeric overflow or underflow, if a-b becomes
a large positive or negative number.
31.11.3 Sorting objects
You also need to use a compare function if you want to sort objects.
As an example, the following code shows how to sort objects by age.
Exercise: Sorting objects by name
exercises/arrays/sort_objects_test.mjs
const arr = [ {age: 200}, {age: 3}, {age: 10} ];
assert.deepEqual(
arr.sort((obj1, obj2) => obj1.age - obj2.age),
[{ age: 3 }, { age: 10 }, { age: 200 }] );](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-551-320.jpg)
![31.12 Quick reference: Array<T>
Legend:
R: method does not change the Array (non-destructive).
W: method changes the Array (destructive).
31.12.1 new Array()
new Array(n) creates an Array of length n that contains n holes:
new Array() creates an empty Array. However, I recommend to
always use [] instead.
31.12.2 Static methods of Array
Array.from<T>(iterable: Iterable<T> | ArrayLike<T>): T[]
[ES6]
Array.from<T,U>(iterable: Iterable<T> | ArrayLike<T>,
mapFunc: (v: T, k: number) => U, thisArg?: any): U[] [ES6]
Converts an iterable or an Array-like object to an Array.
Optionally, the input values can be translated via mapFunc before
they are added to the output Array.
// Trailing commas are always ignored.
// Therefore: number of commas = number of holes
assert.deepEqual(new Array(3), [,,,]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-552-320.jpg)
![Examples:
Array.of<T>(...items: T[]): T[] [ES6]
This static method is mainly useful for subclasses of Array,
where it serves as a custom Array literal:
31.12.3 Methods of Array<T>.prototype
.concat(...items: Array<T[] | T>): T[] [R, ES3]
Returns a new Array that is the concatenation of the receiver
and all items. Non-Array parameters (such as 'b' in the
following example) are treated as if they were Arrays with single
elements.
.copyWithin(target: number, start: number,
end=this.length): this [W, ES6]
Copies the elements whose indices range from (including) start
to (excluding) end to indices starting with target. Overlapping is
> Array.from(new Set(['a', 'b'])) // iterable
[ 'a', 'b' ]
> Array.from({length: 2, 0:'a', 1:'b'}) // Array-like object
[ 'a', 'b' ]
class MyArray extends Array {}
assert.equal(
MyArray.of('a', 'b') instanceof MyArray, true);
> ['a'].concat('b', ['c', 'd'])
[ 'a', 'b', 'c', 'd' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-553-320.jpg)
![handled correctly.
If start or end is negative, then .length is added to it.
.entries(): Iterable<[number, T]> [R, ES6]
Returns an iterable over [index, element] pairs.
.every(callback: (value: T, index: number, array: Array<T>)
=> boolean, thisArg?: any): boolean [R, ES5]
Returns true if callback returns a truthy value for every
element. Otherwise, it returns false. It stops as soon as it
receives a falsy value. This method corresponds to universal
quantification (“for all”, ∀) in mathematics.
Related method: .some() (“exists”).
.fill(value: T, start=0, end=this.length): this [W, ES6]
Assigns value to every index between (including) start and
(excluding) end.
> ['a', 'b', 'c', 'd'].copyWithin(0, 2, 4)
[ 'c', 'd', 'c', 'd' ]
> Array.from(['a', 'b'].entries())
[ [ 0, 'a' ], [ 1, 'b' ] ]
> [1, 2, 3].every(x => x > 0)
true
> [1, -2, 3].every(x => x > 0)
false
> [0, 1, 2].fill('a')
[ 'a', 'a', 'a' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-554-320.jpg)
![Caveat: Don’t use this method to fill an Array with an object obj;
then each element will refer to obj (sharing it). In this case, it’s
better to use Array.from().
.filter(callback: (value: T, index: number, array:
Array<T>) => any, thisArg?: any): T[] [R, ES5]
Returns an Array with only those elements for which callback
returns a truthy value.
.find(predicate: (value: T, index: number, obj: T[]) =>
boolean, thisArg?: any): T | undefined [R, ES6]
The result is the first element for which predicate returns a
truthy value. If there is no such element, the result is undefined.
.findIndex(predicate: (value: T, index: number, obj: T[])
=> boolean, thisArg?: any): number [R, ES6]
The result is the index of the first element for which predicate
returns a truthy value. If there is no such element, the result is
-1.
> [1, -2, 3].filter(x => x > 0)
[ 1, 3 ]
> [1, -2, 3].find(x => x < 0)
-2
> [1, 2, 3].find(x => x < 0)
undefined
> [1, -2, 3].findIndex(x => x < 0)
1
> [1, 2, 3].findIndex(x => x < 0)
-1](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-555-320.jpg)
![.flat(depth = 1): any[] [R, ES2019]
“Flattens” an Array: It descends into the Arrays that are nested
inside the input Array and creates a copy where all values it
finds at level depth or lower are moved to the top level.
.flatMap<U>(callback: (value: T, index: number, array: T[])
=> U|Array<U>, thisValue?: any): U[] [R, ES2019]
The result is produced by invoking callback() for each element
of the original Array and concatenating the Arrays it returns.
.forEach(callback: (value: T, index: number, array:
Array<T>) => void, thisArg?: any): void [R, ES5]
Calls callback for each element.
> [ 1,2, [3,4], [[5,6]] ].flat(0) // no change
[ 1, 2, [3,4], [[5,6]] ]
> [ 1,2, [3,4], [[5,6]] ].flat(1)
[1, 2, 3, 4, [5,6]]
> [ 1,2, [3,4], [[5,6]] ].flat(2)
[1, 2, 3, 4, 5, 6]
> ['a', 'b', 'c'].flatMap(x => [x,x])
[ 'a', 'a', 'b', 'b', 'c', 'c' ]
> ['a', 'b', 'c'].flatMap(x => [x])
[ 'a', 'b', 'c' ]
> ['a', 'b', 'c'].flatMap(x => [])
[]
['a', 'b'].forEach((x, i) => console.log(x, i))
// Output:](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-556-320.jpg)
![A for-of loop is usually a better choice: it’s faster, supports
break and can iterate over arbitrary iterables.
.includes(searchElement: T, fromIndex=0): boolean [R, ES2016]
Returns true if the receiver has an element whose value is
searchElement and false, otherwise. Searching starts at index
fromIndex.
.indexOf(searchElement: T, fromIndex=0): number [R, ES5]
Returns the index of the first element that is strictly equal to
searchElement. Returns -1 if there is no such element. Starts
searching at index fromIndex, visiting higher indices next.
.join(separator = ','): string [R, ES1]
Creates a string by concatenating string representations of all
elements, separating them with separator.
// 'a', 0
// 'b', 1
> [0, 1, 2].includes(1)
true
> [0, 1, 2].includes(5)
false
> ['a', 'b', 'a'].indexOf('a')
0
> ['a', 'b', 'a'].indexOf('a', 1)
2
> ['a', 'b', 'a'].indexOf('c')
-1](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-557-320.jpg)
![.keys(): Iterable<number> [R, ES6]
Returns an iterable over the keys of the receiver.
.lastIndexOf(searchElement: T, fromIndex=this.length-1):
number [R, ES5]
Returns the index of the last element that is strictly equal to
searchElement. Returns -1 if there is no such element. Starts
searching at index fromIndex, visiting lower indices next.
.map<U>(mapFunc: (value: T, index: number, array: Array<T>)
=> U, thisArg?: any): U[] [R, ES5]
Returns a new Array, in which every element is the result of
mapFunc being applied to the corresponding element of the
receiver.
> ['a', 'b', 'c'].join('##')
'a##b##c'
> ['a', 'b', 'c'].join()
'a,b,c'
> [...['a', 'b'].keys()]
[ 0, 1 ]
> ['a', 'b', 'a'].lastIndexOf('a')
2
> ['a', 'b', 'a'].lastIndexOf('a', 1)
0
> ['a', 'b', 'a'].lastIndexOf('c')
-1
> [1, 2, 3].map(x => x * 2)
[ 2, 4, 6 ]
> ['a', 'b', 'c'].map((x, i) => i)
[ 0, 1, 2 ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-558-320.jpg)
![.pop(): T | undefined [W, ES3]
Removes and returns the last element of the receiver. That is, it
treats the end of the receiver as a stack. The opposite of .push().
.push(...items: T[]): number [W, ES3]
Adds zero or more items to the end of the receiver. That is, it
treats the end of the receiver as a stack. The return value is the
length of the receiver after the change. The opposite of .pop().
.reduce<U>(callback: (accumulator: U, element: T, index:
number, array: T[]) => U, init?: U): U [R, ES5]
This method produces a summary of the receiver: it feeds all
Array elements to callback, which combines a current summary
(in parameter accumulator) with the current Array element and
returns the next accumulator:
> const arr = ['a', 'b', 'c'];
> arr.pop()
'c'
> arr
[ 'a', 'b' ]
> const arr = ['a', 'b'];
> arr.push('c', 'd')
4
> arr
[ 'a', 'b', 'c', 'd' ]
const accumulator_0 = callback(init, arr[0]);
const accumulator_1 = callback(accumulator_0, arr[1]);
const accumulator_2 = callback(accumulator_1, arr[2]);
// Etc.](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-559-320.jpg)
![The result of .reduce() is the last result of callback after it has
visited all Array elements.
If no init is provided, the Array element at index 0 is used and
the element at index 1 is visited first. Therefore, the Array must
have at least length 1.
.reduceRight<U>(callback: (accumulator: U, element: T,
index: number, array: T[]) => U, init?: U): U [R, ES5]
Works like .reduce(), but visits the Array elements backward,
starting with the last element.
.reverse(): this [W, ES1]
Rearranges the elements of the receiver so that they are in
reverse order and then returns the receiver.
.shift(): T | undefined [W, ES3]
> [1, 2, 3].reduce((accu, x) => accu + x, 0)
6
> [1, 2, 3].reduce((accu, x) => accu + String(x), '')
'123'
> [1, 2, 3].reduceRight((accu, x) => accu + String(x), '')
'321'
> const arr = ['a', 'b', 'c'];
> arr.reverse()
[ 'c', 'b', 'a' ]
> arr
[ 'c', 'b', 'a' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-560-320.jpg)
![Removes and returns the first element of the receiver. The
opposite of .unshift().
.slice(start=0, end=this.length): T[] [R, ES3]
Returns a new Array containing the elements of the receiver
whose indices are between (including) start and (excluding)
end.
Negative indices are allowed and added to .length:
.some(callback: (value: T, index: number, array: Array<T>)
=> boolean, thisArg?: any): boolean [R, ES5]
Returns true if callback returns a truthy value for at least one
element. Otherwise, it returns false. It stops as soon as it
receives a truthy value. This method corresponds to existential
quantification (“exists”, ∃) in mathematics.
> const arr = ['a', 'b', 'c'];
> arr.shift()
'a'
> arr
[ 'b', 'c' ]
> ['a', 'b', 'c', 'd'].slice(1, 3)
[ 'b', 'c' ]
> ['a', 'b'].slice() // shallow copy
[ 'a', 'b' ]
> ['a', 'b', 'c'].slice(-2)
[ 'b', 'c' ]
> [1, 2, 3].some(x => x < 0)
false](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-561-320.jpg)
![Related method: .every() (“for all”).
.sort(compareFunc?: (a: T, b: T) => number): this [W, ES1]
Sorts the receiver and returns it. By default, it sorts string
representations of the elements. It does so lexicographically and
according to the code unit values (char codes) of the characters:
You can customize the sort order via compareFunc, which returns
a number that is:
negative if a < b
zero if a === b
positive if a > b
Trick for sorting numbers (with a risk of numeric overflow or
underflow):
.sort() is stable
Since ECMAScript 2019, sorting is guaranteed to be stable: if
elements are considered equal by sorting, then sorting does
> [1, -2, 3].some(x => x < 0)
true
> ['pie', 'cookie', 'éclair', 'Pie', 'Cookie', 'Éclair'].sor
[ 'Cookie', 'Pie', 'cookie', 'pie', 'Éclair', 'éclair' ]
> [200, 3, 10].sort()
[ 10, 200, 3 ]
> [200, 3, 10].sort((a, b) => a - b)
[ 3, 10, 200 ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-562-320.jpg)
![not change the order of those elements (relative to each
other).
.splice(start: number, deleteCount=this.length-start,
...items: T[]): T[] [W, ES3]
At index start, it removes deleteCount elements and inserts the
items. It returns the deleted elements.
start can be negative and is added to .length if it is:
.toString(): string [R, ES1]
Converts all elements to strings via String(), concatenates them
while separating them with commas, and returns the result.
.unshift(...items: T[]): number [W, ES3]
Inserts the items at the beginning of the receiver and returns its
length after this modification.
> const arr = ['a', 'b', 'c', 'd'];
> arr.splice(1, 2, 'x', 'y')
[ 'b', 'c' ]
> arr
[ 'a', 'x', 'y', 'd' ]
> ['a', 'b', 'c'].splice(-2, 2)
[ 'b', 'c' ]
> [1, 2, 3].toString()
'1,2,3'
> ['1', '2', '3'].toString()
'1,2,3'
> [].toString()
''](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-563-320.jpg)
![.values(): Iterable<T> [R, ES6]
Returns an iterable over the values of the receiver.
31.12.4 Sources
TypeScript’s built-in typings
MDN web docs for JavaScript
ECMAScript language specification
Quiz
See quiz app.
> const arr = ['c', 'd'];
> arr.unshift('e', 'f')
4
> arr
[ 'e', 'f', 'c', 'd' ]
> [...['a', 'b'].values()]
[ 'a', 'b' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-564-320.jpg)
![32.1.3.2 The wrapped ArrayBuffer
32.1.3.3 Getting and setting elements
32.1.4 Using DataViews
This is how DataViews are used:
// Argument: Typed Array or Array-like object
const ta1 = new Uint8Array([0, 1, 2]);
const ta2 = Uint8Array.of(0, 1, 2);
const ta3 = new Uint8Array(3); // length of Typed Array
ta3[0] = 0;
ta3[1] = 1;
ta3[2] = 2;
assert.deepEqual(ta1, ta2);
assert.deepEqual(ta1, ta3);
const typedArray = new Int16Array(2); // 2 elements
assert.equal(typedArray.length, 2);
assert.deepEqual(
typedArray.buffer, new ArrayBuffer(4)); // 4 bytes
const typedArray = new Int16Array(2);
assert.equal(typedArray[1], 0); // initialized with 0
typedArray[1] = 72;
assert.equal(typedArray[1], 72);
const dataView = new DataView(new ArrayBuffer(4));
assert.equal(dataView.getInt16(0), 0);
assert.equal(dataView.getUint8(0), 0);
dataView.setUint8(0, 5);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-570-320.jpg)
![They are the types of the elements of Typed Arrays. For
example, all elements of a Int32Array have the type Int32. The
element type is the only aspect of Typed Arrays that differs.
They are the lenses through which an ArrayBuffer accesses its
DataView when you use methods such as .getInt32() and
.setInt32().
The element type Uint8C is special: it is not supported by DataView
and only exists to enable Uint8ClampedArray. This Typed Array is
used by the canvas element (where it replaces CanvasPixelArray) and
should otherwise be avoided. The only difference between Uint8C and
Uint8 is how overflow and underflow are handled (as explained in the
next subsection).
32.2.1 Handling overflow and underflow
Normally, when a value is out of the range of the element type,
modulo arithmetic is used to convert it to a value within range. For
signed and unsigned integers that means that:
The highest value plus one is converted to the lowest value (0 for
unsigned integers).
The lowest value minus one is converted to the highest value.
The following function helps illustrate how conversion works:
function setAndGet(typedArray, value) {
typedArray[0] = value;
return typedArray[0];
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-572-320.jpg)
![32.3 More information on Typed
Arrays
In this section, «ElementType»Array stands for Int8Array, Uint8Array,
etc. ElementType is Int8, Uint8, etc.
32.3.1 The static method
«ElementType»Array.from()
This method has the type signature:
.from() converts source into an instance of this (a Typed Array).
For example, normal Arrays are iterable and can be converted with
this method:
Typed Arrays are also iterable:
source can also be an Array-like object:
.from<S>(
source: Iterable<S>|ArrayLike<S>,
mapfn?: S => ElementType, thisArg?: any)
: «ElementType»Array
assert.deepEqual(
Uint16Array.from([0, 1, 2]),
Uint16Array.of(0, 1, 2));
assert.deepEqual(
Uint16Array.from(Uint8Array.of(0, 1, 2)),
Uint16Array.of(0, 1, 2));](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-576-320.jpg)
![ArrayBuffers and DataViews are not iterable.
32.3.3 Typed Arrays vs. normal Arrays
Typed Arrays are much like normal Arrays: they have a .length,
elements can be accessed via the bracket operator [], and they have
most of the standard Array methods. They differ from normal Arrays
in the following ways:
Typed Arrays have buffers. The elements of a Typed Array ta are
not stored in ta, they are stored in an associated ArrayBuffer
that can be accessed via ta.buffer:
Typed Arrays are initialized with zeros:
new Array(4) creates a normal Array without any elements.
It only has four holes (indices less than the .length that
have no associated elements).
new Uint8Array(4) creates a Typed Array whose four
elements are all 0.
All of the elements of a Typed Array have the same type:
// 1
// 2
const ta = new Uint16Array(2); // 2 elements
assert.deepEqual(
ta.buffer, new ArrayBuffer(4)); // 4 bytes
assert.deepEqual(new Uint8Array(4), Uint8Array.of(0, 0, 0, 0](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-579-320.jpg)
![Setting elements converts values to that type.
Getting elements returns numbers.
The .length of a Typed Array is derived from its ArrayBuffer and
never changes (unless you switch to a different ArrayBuffer).
Normal Arrays can have holes; Typed Arrays can’t.
32.3.4 Converting Typed Arrays to and
from normal Arrays
To convert a normal Array to a Typed Array, you pass it to a Typed
Array constructor (which accepts Array-like objects and Typed
Arrays) or to «ElementType»Array.from() (which accepts iterables
and Array-like objects). For example:
To convert a Typed Array to a normal Array, you can use spreading
or Array.from() (because Typed Arrays are iterable):
const ta = new Uint8Array(1);
ta[0] = 257;
assert.equal(ta[0], 1); // 257 % 256 (overflow)
ta[0] = '2';
assert.equal(ta[0], 2);
const ta = new Uint8Array(1);
assert.equal(ta[0], 0);
assert.equal(typeof ta[0], 'number');
const ta1 = new Uint8Array([0, 1, 2]);
const ta2 = Uint8Array.from([0, 1, 2]);
assert.deepEqual(ta1, ta2);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-580-320.jpg)
![32.3.5 Concatenating Typed Arrays
Typed Arrays don’t have a method .concat(), like normal Arrays do.
The workaround is to use their overloaded method .set():
It copies the existing typedArray or arrayLike into the receiver, at
index offset. TypedArray is a fictitious abstract superclass of all
concrete Typed Array classes.
The following function uses that method to copy zero or more Typed
Arrays (or Array-like objects) into an instance of resultConstructor:
assert.deepEqual(
[...Uint8Array.of(0, 1, 2)], [0, 1, 2] );
assert.deepEqual(
Array.from(Uint8Array.of(0, 1, 2)), [0, 1, 2] );
.set(typedArray: TypedArray, offset=0): void
.set(arrayLike: ArrayLike<number>, offset=0): void
function concatenate(resultConstructor, ...arrays) {
let totalLength = 0;
for (const arr of arrays) {
totalLength += arr.length;
}
const result = new resultConstructor(totalLength);
let offset = 0;
for (const arr of arrays) {
result.set(arr, offset);
offset += arr.length;
}
return result;
}
assert.deepEqual(
concatenate(Uint8Array, Uint8Array.of(1, 2), [3, 4]),
Uint8Array.of(1, 2, 3, 4));](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-581-320.jpg)
![32.4 Quick references: indices
vs. offsets
In preparation for the quick references on ArrayBuffers, Typed
Arrays, and DataViews, we need learn the differences between
indices and offsets:
Indices for the bracket operator [ ]: You can only use non-
negative indices (starting at 0).
In normal Arrays, writing to negative indices creates properties:
In Typed Arrays, writing to negative indices is ignored:
Indices for methods of ArrayBuffers, Typed Arrays, and
DataViews: Every index can be negative. If it is, it is added to the
length of the entity to produce the actual index. Therefore, -1
refers to the last element, -2 to the second-last, etc. Methods of
normal Arrays work the same way.
const arr = [6, 7];
arr[-1] = 5;
assert.deepEqual(
Object.keys(arr), ['0', '1', '-1']);
const tarr = Uint8Array.of(6, 7);
tarr[-1] = 5;
assert.deepEqual(
Object.keys(tarr), ['0', '1']);
const ui8 = Uint8Array.of(0, 1, 2);
assert.deepEqual(ui8.slice(-1), Uint8Array.of(2));](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-582-320.jpg)
![The optional mapfn lets you transform the elements of source
before they become elements of the result.
.of(...items: number[]): instanceof this
Creates a new instance of this whose elements are items
(coerced to the element type).
32.6.2 Properties of
TypedArray<T>.prototype
Indices accepted by Typed Array methods can be negative (they work
like traditional Array methods that way). Offsets must be non-
negative. For details, see §32.4 “Quick references: indices vs. offsets”.
32.6.2.1 Properties specific to Typed Arrays
The following properties are specific to Typed Arrays; normal Arrays
don’t have them:
get .buffer(): ArrayBuffer
assert.deepEqual(
Uint16Array.from([0, 1, 2]),
Uint16Array.of(0, 1, 2));
assert.deepEqual(
Int16Array.from(Int8Array.of(127, 126, 125), x => x * 2),
Int16Array.of(254, 252, 250));
assert.deepEqual(
Int16Array.of(-1234, 5, 67),
new Int16Array([-1234, 5, 67]) );](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-587-320.jpg)
![Returns the buffer backing this Typed Array.
get .length(): number
Returns the length in elements of this Typed Array’s buffer.
get .byteLength(): number
Returns the size in bytes of this Typed Array’s buffer.
get .byteOffset(): number
Returns the offset where this Typed Array “starts” inside its
ArrayBuffer.
.set(typedArray: TypedArray, offset=0): void
.set(arrayLike: ArrayLike<number>, offset=0): void
Copies all elements of the first parameter to this Typed Array.
The element at index 0 of the parameter is written to index
offset of this Typed Array (etc.). For more information on
Array-like objects, consult §31.4 “Array-like objects”.
.subarray(startIndex=0, endIndex=this.length):
TypedArray<T>
Returns a new Typed Array that has the same buffer as this
Typed Array, but a (generally) smaller range. If startIndex is
non-negative then the first element of the resulting Typed Array
is this[startIndex], the second this[startIndex+1] (etc.). If
startIndex in negative, it is converted appropriately.](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-588-320.jpg)
![32.6.2.2 Array methods
The following methods are basically the same as the methods of
normal Arrays:
.copyWithin(target: number, start: number,
end=this.length): this [W, ES6]
.entries(): Iterable<[number, T]> [R, ES6]
.every(callback: (value: T, index: number, array:
TypedArray<T>) => boolean, thisArg?: any): boolean [R, ES5]
.fill(value: T, start=0, end=this.length): this [W, ES6]
.filter(callback: (value: T, index: number, array:
TypedArray<T>) => any, thisArg?: any): T[] [R, ES5]
.find(predicate: (value: T, index: number, obj: T[]) =>
boolean, thisArg?: any): T | undefined [R, ES6]
.findIndex(predicate: (value: T, index: number, obj: T[])
=> boolean, thisArg?: any): number [R, ES6]
.forEach(callback: (value: T, index: number, array:
TypedArray<T>) => void, thisArg?: any): void [R, ES5]
.includes(searchElement: T, fromIndex=0): boolean [R, ES2016]
.indexOf(searchElement: T, fromIndex=0): number [R, ES5]
.join(separator = ','): string [R, ES1]
.keys(): Iterable<number> [R, ES6]
.lastIndexOf(searchElement: T, fromIndex=this.length-1):
number [R, ES5]
.map<U>(mapFunc: (value: T, index: number, array:
TypedArray<T>) => U, thisArg?: any): U[] [R, ES5]
.reduce<U>(callback: (accumulator: U, element: T, index:
number, array: T[]) => U, init?: U): U [R, ES5]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-589-320.jpg)
![.reduceRight<U>(callback: (accumulator: U, element: T,
index: number, array: T[]) => U, init?: U): U [R, ES5]
.reverse(): this [W, ES1]
.slice(start=0, end=this.length): T[] [R, ES3]
.some(callback: (value: T, index: number, array:
TypedArray<T>) => boolean, thisArg?: any): boolean [R, ES5]
.sort(compareFunc?: (a: T, b: T) => number): this [W, ES1]
.toString(): string [R, ES1]
.values(): Iterable<number> [R, ES6]
For details on how these methods work, please consult §31.12.3
“Methods of Array<T>.prototype”.
32.6.3 new «ElementType»Array()
Each Typed Array constructor has a name that follows the pattern
«ElementType»Array, where «ElementType» is one of the element types
in the table at the beginning. That means that there are nine
constructors for Typed Arrays:
Float32Array, Float64Array
Int8Array, Int16Array, Int32Array
Uint8Array, Uint8ClampedArray, Uint16Array, Uint32Array
Each constructor has four overloaded versions – it behaves
differently depending on how many arguments it receives and what
their types are:
new «ElementType»Array(buffer: ArrayBuffer, byteOffset=0,
length=0)](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-590-320.jpg)
![33.1 Using Maps
An instance of Map maps keys to values. A single key-value mapping
is called an entry.
33.1.1 Creating Maps
There are three common ways of creating Maps.
First, you can use the constructor without any parameters to create
an empty Map:
Second, you can pass an iterable (e.g., an Array) over key-value
“pairs” (Arrays with two elements) to the constructor:
Third, the .set() method adds entries to a Map and is chainable:
33.1.2 Copying Maps
const emptyMap = new Map();
assert.equal(emptyMap.size, 0);
const map = new Map([
[1, 'one'],
[2, 'two'],
[3, 'three'], // trailing comma is ignored
]);
const map = new Map()
.set(1, 'one')
.set(2, 'two')
.set(3, 'three');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-597-320.jpg)
![As we’ll see later, Maps are also iterables over key-value pairs.
Therefore, you can use the constructor to create a copy of a Map.
That copy is shallow: keys and values are the same; they are not
duplicated.
33.1.3 Working with single entries
.set() and .get() are for writing and reading values (given keys).
.has() checks if a Map has an entry with a given key. .delete()
removes entries.
const original = new Map()
.set(false, 'no')
.set(true, 'yes');
const copy = new Map(original);
assert.deepEqual(original, copy);
const map = new Map();
map.set('foo', 123);
assert.equal(map.get('foo'), 123);
// Unknown key:
assert.equal(map.get('bar'), undefined);
// Use the default value '' if an entry is missing:
assert.equal(map.get('bar') || '', '');
const map = new Map([['foo', 123]]);
assert.equal(map.has('foo'), true);
assert.equal(map.delete('foo'), true)
assert.equal(map.has('foo'), false)](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-598-320.jpg)
![.values() works like .keys(), but for values instead of keys.
33.1.6 Getting the entries of a Map
.entries() returns an iterable over the entries of a Map:
Spreading (...) converts the iterable returned by .entries() to an
Array:
Map instances are also iterables over entries. In the following code,
we use destructuring to access the keys and values of map:
assert.deepEqual(
[...map.keys()],
[false, true]);
const map = new Map()
.set(false, 'no')
.set(true, 'yes')
;
for (const entry of map.entries()) {
console.log(entry);
}
// Output:
// [false, 'no']
// [true, 'yes']
assert.deepEqual(
[...map.entries()],
[[false, 'no'], [true, 'yes']]);
for (const [key, value] of map) {
console.log(key, value);
}
// Output:
// false, 'no'
// true, 'yes'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-600-320.jpg)
![33.1.7 Listed in insertion order: entries,
keys, values
Maps record in which order entries were created and honor that
order when listing entries, keys, or values:
33.1.8 Converting between Maps and
Objects
As long as a Map only uses strings and symbols as keys, you can
convert it to an object (via Object.fromEntries()):
You can also convert an object to a Map with string or symbol keys
(via Object.entries()):
const map1 = new Map([
['a', 1],
['b', 2],
]);
assert.deepEqual(
[...map1.keys()], ['a', 'b']);
const map2 = new Map([
['b', 2],
['a', 1],
]);
assert.deepEqual(
[...map2.keys()], ['b', 'a']);
const map = new Map([
['a', 1],
['b', 2],
]);
const obj = Object.fromEntries(map);
assert.deepEqual(
obj, {a: 1, b: 2});](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-601-320.jpg)
![const obj = {
a: 1,
b: 2,
};
const map = new Map(Object.entries(obj));
assert.deepEqual(
map, new Map([['a', 1], ['b', 2]]));](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-602-320.jpg)
![33.2 Example: Counting characters
countChars() returns a Map that maps characters to numbers of
occurrences.
function countChars(chars) {
const charCounts = new Map();
for (let ch of chars) {
ch = ch.toLowerCase();
const prevCount = charCounts.get(ch) || 0;
charCounts.set(ch, prevCount+1);
}
return charCounts;
}
const result = countChars('AaBccc');
assert.deepEqual(
[...result],
[
['a', 2],
['b', 1],
['c', 3],
]
);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-603-320.jpg)
![33.4 Missing Map operations
33.4.1 Mapping and filtering Maps
You can .map() and .filter() an Array, but there are no such
operations for a Map. The solution is:
1. Convert the Map into an Array of [key, value] pairs.
2. Map or filter the Array.
3. Convert the result back to a Map.
I’ll use the following Map to demonstrate how that works.
Mapping originalMap:
Filtering originalMap:
const originalMap = new Map()
.set(1, 'a')
.set(2, 'b')
.set(3, 'c');
const mappedMap = new Map( // step 3
[...originalMap] // step 1
.map(([k, v]) => [k * 2, '_' + v]) // step 2
);
assert.deepEqual([...mappedMap],
[[2,'_a'], [4,'_b'], [6,'_c']]);
const filteredMap = new Map( // step 3
[...originalMap] // step 1
.filter(([k, v]) => k < 3) // step 2
);
assert.deepEqual([...filteredMap],
[[1,'a'], [2,'b']]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-606-320.jpg)
![Step 1 is performed by spreading (...) in the Array literal.
33.4.2 Combining Maps
There are no methods for combining Maps, which is why we must
use a workaround that is similar to the one from the previous
section.
Let’s combine the following two Maps:
To combine map1 and map2, we turn them into Arrays via spreading
(...) and concatenate those Arrays. Afterward, we convert the result
back to a Map. All of that is done in line A.
Exercise: Combining two Maps
const map1 = new Map()
.set(1, '1a')
.set(2, '1b')
.set(3, '1c')
;
const map2 = new Map()
.set(2, '2b')
.set(3, '2c')
.set(4, '2d')
;
const combinedMap = new Map([...map1, ...map2]); // (A)
assert.deepEqual(
[...combinedMap], // convert to Array for comparison
[ [ 1, '1a' ],
[ 2, '2b' ],
[ 3, '2c' ],
[ 4, '2d' ] ]
);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-607-320.jpg)
![33.5 Quick reference: Map<K,V>
Note: For the sake of conciseness, I’m pretending that all keys have
the same type K and that all values have the same type V.
33.5.1 Constructor
new Map<K, V>(entries?: Iterable<[K, V]>) [ES6]
If you don’t provide the parameter entries, then an empty Map
is created. If you do provide an iterable over [key, value] pairs,
then those pairs are added as entries to the Map. For example:
33.5.2 Map<K,V>.prototype: handling single
entries
.get(key: K): V [ES6]
Returns the value that key is mapped to in this Map. If there is
no key key in this Map, undefined is returned.
const map = new Map([
[ 1, 'one' ],
[ 2, 'two' ],
[ 3, 'three' ], // trailing comma is ignored
]);
const map = new Map([[1, 'one'], [2, 'two']]);
assert.equal(map.get(1), 'one');
assert.equal(map.get(5), undefined);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-609-320.jpg)
![.set(key: K, value: V): this [ES6]
Maps the given key to the given value. If there is already an
entry whose key is key, it is updated. Otherwise, a new entry is
created. This method returns this, which means that you can
chain it.
.has(key: K): boolean [ES6]
Returns whether the given key exists in this Map.
.delete(key: K): boolean [ES6]
If there is an entry whose key is key, it is removed and true is
returned. Otherwise, nothing happens and false is returned.
33.5.3 Map<K,V>.prototype: handling all
entries
const map = new Map([[1, 'one'], [2, 'two']]);
map.set(1, 'ONE!')
.set(3, 'THREE!');
assert.deepEqual(
[...map.entries()],
[[1, 'ONE!'], [2, 'two'], [3, 'THREE!']]);
const map = new Map([[1, 'one'], [2, 'two']]);
assert.equal(map.has(1), true); // key exists
assert.equal(map.has(5), false); // key does not exist
const map = new Map([[1, 'one'], [2, 'two']]);
assert.equal(map.delete(1), true);
assert.equal(map.delete(5), false); // nothing happens
assert.deepEqual(
[...map.entries()],
[[2, 'two']]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-610-320.jpg)
![get .size: number [ES6]
Returns how many entries this Map has.
.clear(): void [ES6]
Removes all entries from this Map.
33.5.4 Map<K,V>.prototype: iterating and
looping
Both iterating and looping happen in the order in which entries were
added to a Map.
.entries(): Iterable<[K,V]> [ES6]
Returns an iterable with one [key, value] pair for each entry in
this Map. The pairs are Arrays of length 2.
const map = new Map([[1, 'one'], [2, 'two']]);
assert.equal(map.size, 2);
const map = new Map([[1, 'one'], [2, 'two']]);
assert.equal(map.size, 2);
map.clear();
assert.equal(map.size, 0);
const map = new Map([[1, 'one'], [2, 'two']]);
for (const entry of map.entries()) {
console.log(entry);
}
// Output:
// [1, 'one']
// [2, 'two']](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-611-320.jpg)
![.forEach(callback: (value: V, key: K, theMap: Map<K,V>) =>
void, thisArg?: any): void [ES6]
The first parameter is a callback that is invoked once for each
entry in this Map. If thisArg is provided, this is set to it for each
invocation. Otherwise, this is set to undefined.
.keys(): Iterable<K> [ES6]
Returns an iterable over all keys in this Map.
.values(): Iterable<V> [ES6]
Returns an iterable over all values in this Map.
[Symbol.iterator](): Iterable<[K,V]> [ES6]
const map = new Map([[1, 'one'], [2, 'two']]);
map.forEach((value, key) => console.log(value, key));
// Output:
// 'one', 1
// 'two', 2
const map = new Map([[1, 'one'], [2, 'two']]);
for (const key of map.keys()) {
console.log(key);
}
// Output:
// 1
// 2
const map = new Map([[1, 'one'], [2, 'two']]);
for (const value of map.values()) {
console.log(value);
}
// Output:
// 'one'
// 'two'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-612-320.jpg)
![The default way of iterating over Maps. Same as .entries().
33.5.5 Sources of this section
TypeScript’s built-in typings
const map = new Map([[1, 'one'], [2, 'two']]);
for (const [key, value] of map) {
console.log(key, value);
}
// Output:
// 1, 'one'
// 2, 'two'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-613-320.jpg)
![34.3 Examples
34.3.1 Caching computed results via
WeakMaps
With WeakMaps, you can associate previously computed results with
objects without having to worry about memory management. The
following function countOwnKeys() is an example: it caches previous
results in the WeakMap cache.
If we use this function with an object obj, you can see that the result
is only computed for the first invocation, while a cached value is used
for the second invocation:
34.3.2 Keeping private data in
WeakMaps
const cache = new WeakMap();
function countOwnKeys(obj) {
if (cache.has(obj)) {
return [cache.get(obj), 'cached'];
} else {
const count = Object.keys(obj).length;
cache.set(obj, count);
return [count, 'computed'];
}
}
> const obj = { foo: 1, bar: 2};
> countOwnKeys(obj)
[2, 'computed']
> countOwnKeys(obj)
[2, 'cached']](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-620-320.jpg)
![In the following code, the WeakMaps _counter and _action are used
to store the values of virtual properties of instances of Countdown:
This is how Countdown is used:
Exercise: WeakMaps for private data
exercises/weakmaps/weakmaps_private_data_test.mjs
const _counter = new WeakMap();
const _action = new WeakMap();
class Countdown {
constructor(counter, action) {
_counter.set(this, counter);
_action.set(this, action);
}
dec() {
let counter = _counter.get(this);
counter--;
_counter.set(this, counter);
if (counter === 0) {
_action.get(this)();
}
}
}
// The two pseudo-properties are truly private:
assert.deepEqual(
Object.keys(new Countdown()),
[]);
let invoked = false;
const cd = new Countdown(3, () => invoked = true);
cd.dec(); assert.equal(invoked, false);
cd.dec(); assert.equal(invoked, false);
cd.dec(); assert.equal(invoked, true);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-621-320.jpg)
![34.4 WeakMap API
The constructor and the four methods of WeakMap work the same as
their Map equivalents:
new WeakMap<K, V>(entries?: Iterable<[K, V]>) [ES6]
.delete(key: K) : boolean [ES6]
.get(key: K) : V [ES6]
.has(key: K) : boolean [ES6]
.set(key: K, value: V) : this [ES6]
Quiz
See quiz app.](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-622-320.jpg)
![35.1 Using Sets
35.1.1 Creating Sets
There are three common ways of creating Sets.
First, you can use the constructor without any parameters to create
an empty Set:
Second, you can pass an iterable (e.g., an Array) to the constructor.
The iterated values become elements of the new Set:
Third, the .add() method adds elements to a Set and is chainable:
35.1.2 Adding, removing, checking
membership
.add() adds an element to a Set.
.has() checks if an element is a member of a Set.
const emptySet = new Set();
assert.equal(emptySet.size, 0);
const set = new Set(['red', 'green', 'blue']);
const set = new Set()
.add('red')
.add('green')
.add('blue');
const set = new Set();
set.add('red');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-625-320.jpg)
![.delete() removes an element from a Set.
35.1.3 Determining the size of a Set and
clearing it
.size contains the number of elements in a Set.
.clear() removes all elements of a Set.
35.1.4 Iterating over Sets
Sets are iterable and the for-of loop works as you’d expect:
assert.equal(set.has('red'), true);
assert.equal(set.delete('red'), true); // there was a deletion
assert.equal(set.has('red'), false);
const set = new Set()
.add('foo')
.add('bar');
assert.equal(set.size, 2)
set.clear();
assert.equal(set.size, 0)
const set = new Set(['red', 'green', 'blue']);
for (const x of set) {
console.log(x);
}
// Output:
// 'red'
// 'green'
// 'blue'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-626-320.jpg)
![As you can see, Sets preserve insertion order. That is, elements are
always iterated over in the order in which they were added.
Given that Sets are iterable, you can use spreading (...) to convert
them to Arrays:
const set = new Set(['red', 'green', 'blue']);
const arr = [...set]; // ['red', 'green', 'blue']](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-627-320.jpg)
![35.2 Examples of using Sets
35.2.1 Removing duplicates from an
Array
Converting an Array to a Set and back, removes duplicates from the
Array:
35.2.2 Creating a set of Unicode
characters (code points)
Strings are iterable and can therefore be used as parameters for new
Set():
assert.deepEqual(
[...new Set([1, 2, 1, 2, 3, 3, 3])],
[1, 2, 3]);
assert.deepEqual(
new Set('abc'),
new Set(['a', 'b', 'c']));](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-628-320.jpg)
![35.3 What Set elements are
considered equal?
As with Map keys, Set elements are compared similarly to ===, with
the exception of NaN being equal to itself.
As with ===, two different objects are never considered equal (and
there is no way to change that at the moment):
> const set = new Set([NaN, NaN, NaN]);
> set.size
1
> set.has(NaN)
true
> const set = new Set();
> set.add({});
> set.size
1
> set.add({});
> set.size
2](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-629-320.jpg)
![35.4 Missing Set operations
Sets are missing several common operations. Such an operation can
usually be implemented by:
Converting the input Sets to Arrays by spreading into Array
literals.
Performing the operation on Arrays.
Converting the result to a Set and returning it.
35.4.1 Union (a ∪ b)
Computing the union of two Sets a and b means creating a Set that
contains the elements of both a and b.
35.4.2 Intersection (a ∩ b)
Computing the intersection of two Sets a and b means creating a Set
that contains those elements of a that are also in b.
const a = new Set([1,2,3]);
const b = new Set([4,3,2]);
// Use spreading to concatenate two iterables
const union = new Set([...a, ...b]);
assert.deepEqual([...union], [1, 2, 3, 4]);
const a = new Set([1,2,3]);
const b = new Set([4,3,2]);
const intersection = new Set(
[...a].filter(x => b.has(x)));](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-630-320.jpg)
![35.4.3 Difference (a b)
Computing the difference between two Sets a and b means creating a
Set that contains those elements of a that are not in b. This operation
is also sometimes called minus (−).
35.4.4 Mapping over Sets
Sets don’t have a method .map(). But we can borrow the one that
Arrays have:
35.4.5 Filtering Sets
We can’t directly .filter() Sets, so we need to use the corresponding
Array method:
assert.deepEqual([...intersection], [2, 3]);
const a = new Set([1,2,3]);
const b = new Set([4,3,2]);
const difference = new Set(
[...a].filter(x => !b.has(x)));
assert.deepEqual([...difference], [1]);
const set = new Set([1, 2, 3]);
const mappedSet = new Set([...set].map(x => x * 2));
// Convert mappedSet to an Array to check what’s inside it
assert.deepEqual([...mappedSet], [2, 4, 6]);
const set = new Set([1, 2, 3, 4, 5]);
const filteredSet = new Set([...set].filter(x => (x % 2) === 0))](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-631-320.jpg)
![assert.deepEqual([...filteredSet], [2, 4]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-632-320.jpg)
![35.5 Quick reference: Set<T>
35.5.1 Constructor
new Set<T>(values?: Iterable<T>) [ES6]
If you don’t provide the parameter values, then an empty Set is
created. If you do, then the iterated values are added as
elements to the Set. For example:
35.5.2 Set<T>.prototype: single Set
elements
.add(value: T): this [ES6]
Adds value to this Set. This method returns this, which means
that it can be chained.
.delete(value: T): boolean [ES6]
Removes value from this Set. Returns true if something was
deleted and false, otherwise.
const set = new Set(['red', 'green', 'blue']);
const set = new Set(['red']);
set.add('green').add('blue');
assert.deepEqual([...set], ['red', 'green', 'blue']);
const set = new Set(['red', 'green', 'blue']);
assert.equal(set.delete('red'), true); // there was a deleti
assert.deepEqual([...set], ['green', 'blue']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-633-320.jpg)
![.has(value: T): boolean [ES6]
Checks whether value is in this Set.
35.5.3 Set<T>.prototype: all Set elements
get .size: number [ES6]
Returns how many elements there are in this Set.
.clear(): void [ES6]
Removes all elements from this Set.
35.5.4 Set<T>.prototype: iterating and
looping
.values(): Iterable<T> [ES6]
Returns an iterable over all elements of this Set.
const set = new Set(['red', 'green']);
assert.equal(set.has('red'), true);
assert.equal(set.has('blue'), false);
const set = new Set(['red', 'green', 'blue']);
assert.equal(set.size, 3);
const set = new Set(['red', 'green', 'blue']);
assert.equal(set.size, 3);
set.clear();
assert.equal(set.size, 0);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-634-320.jpg)
![[Symbol.iterator](): Iterable<T> [ES6]
Default way of iterating over Sets. Same as .values().
.forEach(callback: (value: T, key: T, theSet: Set<T>) =>
void, thisArg?: any): void [ES6]
Feeds each element of this Set to callback(). value and key both
contain the current element. This redundancy was introduced so
that this callback has the same type signature as the callback of
Map.prototype.forEach().
You can specify the this of callback via thisArg. If you omit it,
this is undefined.
const set = new Set(['red', 'green']);
for (const x of set.values()) {
console.log(x);
}
// Output:
// 'red'
// 'green'
const set = new Set(['red', 'green']);
for (const x of set) {
console.log(x);
}
// Output:
// 'red'
// 'green'
const set = new Set(['red', 'green']);
set.forEach(x => console.log(x));
// Output:
// 'red'
// 'green'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-635-320.jpg)
![35.5.5 Symmetry with Map
The following two methods mainly exist so that Sets and Maps have
similar interfaces. Each Set element is handled as if it were a Map
entry whose key and value are both the element.
Set.prototype.entries(): Iterable<[T,T]> [ES6]
Set.prototype.keys(): Iterable<T> [ES6]
.entries() enables you to convert a Set to a Map:
const set = new Set(['a', 'b', 'c']);
const map = new Map(set.entries());
assert.deepEqual(
[...map.entries()],
[['a','a'], ['b','b'], ['c','c']]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-636-320.jpg)
![36.2 WeakSet API
The constructor and the three methods of WeakSet work the same as
their Set equivalents:
new WeakSet<T>(values?: Iterable<T>) [ES6]
.add(value: T): this [ES6]
.delete(value: T): boolean [ES6]
.has(value: T): boolean [ES6]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-640-320.jpg)
![37.1 A first taste of destructuring
With normal assignment, you extract one piece of data at a time – for
example:
With destructuring, you can extract multiple pieces of data at the
same time via patterns in locations that receive data. The left-hand
side of = in the previous code is one such location. In the following
code, the square brackets in line A are a destructuring pattern:
This code does the same as the previous code.
Note that the pattern is “smaller” than the data: we are only
extracting what we need.
const arr = ['a', 'b', 'c'];
const x = arr[0]; // extract
const y = arr[1]; // extract
const arr = ['a', 'b', 'c'];
const [x, y] = arr; // (A)
assert.equal(x, 'a');
assert.equal(y, 'b');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-643-320.jpg)
![37.3 Where can we destructure?
Destructuring patterns can be used at “data sink locations” such as:
Variable declarations:
Assignments:
Parameter definitions:
Note that variable declarations include const and let declarations in
for-of loops:
In the next two sections, we’ll look deeper into the two kinds of
destructuring: object-destructuring and Array-destructuring.
const [a] = ['x'];
assert.equal(a, 'x');
let [b] = ['y'];
assert.equal(b, 'y');
let b;
[b] = ['z'];
assert.equal(b, 'z');
const f = ([x]) => x;
assert.equal(f(['a']), 'a');
const arr = ['a', 'b'];
for (const [index, element] of arr.entries()) {
console.log(index, element);
}
// Output:
// 0, 'a'
// 1, 'b'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-646-320.jpg)
![37.4 Object-destructuring
Object-destructuring lets you batch-extract values of properties via
patterns that look like object literals:
You can think of the pattern as a transparent sheet that you place
over the data: the pattern key 'street' has a match in the data.
Therefore, the data value 'Evergreen Terrace' is assigned to the
pattern variable s.
You can also object-destructure primitive values:
And you can object-destructure Arrays:
Why does that work? Array indices are also properties.
const address = {
street: 'Evergreen Terrace',
number: '742',
city: 'Springfield',
state: 'NT',
zip: '49007',
};
const { street: s, city: c } = address;
assert.equal(s, 'Evergreen Terrace');
assert.equal(c, 'Springfield');
const {length: len} = 'abc';
assert.equal(len, 3);
const {0:x, 2:y} = ['a', 'b', 'c'];
assert.equal(x, 'a');
assert.equal(y, 'c');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-647-320.jpg)
![37.5 Array-destructuring
Array-destructuring lets you batch-extract values of Array elements
via patterns that look like Array literals:
You can skip elements by mentioning holes inside Array patterns:
The first element of the Array pattern in line A is a hole, which is why
the Array element at index 0 is ignored.
37.5.1 Array-destructuring works with
any iterable
Array-destructuring can be applied to any value that is iterable, not
just to Arrays:
const [x, y] = ['a', 'b'];
assert.equal(x, 'a');
assert.equal(y, 'b');
const [, x, y] = ['a', 'b', 'c']; // (A)
assert.equal(x, 'b');
assert.equal(y, 'c');
// Sets are iterable
const mySet = new Set().add('a').add('b').add('c');
const [first, second] = mySet;
assert.equal(first, 'a');
assert.equal(second, 'b');
// Strings are iterable
const [a, b] = 'xyz';
assert.equal(a, 'x');
assert.equal(b, 'y');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-650-320.jpg)
![37.5.2 Rest elements
In Array literals, you can have spread elements. In Array patterns,
you can have rest elements (which must come last):
A rest element variable, such as remaining (line A), is assigned an
Array with all elements of the destructured value that were not
mentioned yet.
const [x, y, ...remaining] = ['a', 'b', 'c', 'd']; // (A)
assert.equal(x, 'a');
assert.equal(y, 'b');
assert.deepEqual(remaining, ['c', 'd']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-651-320.jpg)
![37.6 Examples of destructuring
37.6.1 Array-destructuring: swapping
variable values
You can use Array-destructuring to swap the values of two variables
without needing a temporary variable:
37.6.2 Array-destructuring: operations
that return Arrays
Array-destructuring is useful when operations return Arrays, as does,
for example, the regular expression method .exec():
let x = 'a';
let y = 'b';
[x,y] = [y,x]; // swap
assert.equal(x, 'b');
assert.equal(y, 'a');
// Skip the element at index 0 (the whole match):
const [, year, month, day] =
/^([0-9]{4})-([0-9]{2})-([0-9]{2})$/
.exec('2999-12-31');
assert.equal(year, '2999');
assert.equal(month, '12');
assert.equal(day, '31');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-652-320.jpg)
![37.6.3 Object-destructuring: multiple
return values
Destructuring is very useful if a function returns multiple values –
either packaged as an Array or packaged as an object.
Consider a function findElement() that finds elements in an Array:
Its second parameter is a function that receives the value and index
of an element and returns a boolean indicating if this is the element
the caller is looking for.
We are now faced with a dilemma: Should findElement() return the
value of the element it found or the index? One solution would be to
create two separate functions, but that would result in duplicated
code because both functions would be very similar.
The following implementation avoids duplication by returning an
object that contains both index and value of the element that is
found:
findElement(array, (value, index) => «boolean expression»)
function findElement(arr, predicate) {
for (let index=0; index < arr.length; index++) {
const value = arr[index];
if (predicate(value)) {
// We found something:
return { value, index };
}
}
// We didn’t find anything:
return { value: undefined, index: -1 };
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-653-320.jpg)
![Destructuring helps us with processing the result of findElement():
As we are working with property keys, the order in which we mention
value and index doesn’t matter:
The kicker is that destructuring also serves us well if we are only
interested in one of the two results:
All of these conveniences combined make this way of handling
multiple return values quite versatile.
const arr = [7, 8, 6];
const {value, index} = findElement(arr, x => x % 2 === 0);
assert.equal(value, 8);
assert.equal(index, 1);
const {index, value} = findElement(arr, x => x % 2 === 0);
const arr = [7, 8, 6];
const {value} = findElement(arr, x => x % 2 === 0);
assert.equal(value, 8);
const {index} = findElement(arr, x => x % 2 === 0);
assert.equal(index, 1);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-654-320.jpg)
![37.7 What happens if a pattern
part does not match anything?
What happens if there is no match for part of a pattern? The same
thing that happens if you use non-batch operators: you get
undefined.
37.7.1 Object-destructuring and missing
properties
If a property in an object pattern has no match on the right-hand
side, you get undefined:
37.7.2 Array-destructuring and missing
elements
If an element in an Array pattern has no match on the right-hand
side, you get undefined:
const {prop: p} = {};
assert.equal(p, undefined);
const [x] = [];
assert.equal(x, undefined);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-655-320.jpg)
![can’t Array-destructure non-iterable objects either:
Quiz: basic
See quiz app.
assert.throws(
() => { const [x] = {}; },
{
name: 'TypeError',
message: '{} is not iterable',
}
);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-657-320.jpg)
![37.10 Default values
Normally, if a pattern has no match, the corresponding variable is set
to undefined:
If you want a different value to be used, you need to specify a default
value (via =):
In line A, we specify the default value for p to be 123. That default is
used because the data that we are destructuring has no property
named prop.
37.10.1 Default values in Array-
destructuring
Here, we have two default values that are assigned to the variables x
and y because the corresponding elements don’t exist in the Array
that is destructured.
The default value for the first element of the Array pattern is 1; the
default value for the second element is 2.
const {prop: p} = {};
assert.equal(p, undefined);
const {prop: p = 123} = {}; // (A)
assert.equal(p, 123);
const [x=1, y=2] = [];
assert.equal(x, 1);
assert.equal(y, 2);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-659-320.jpg)
![37.11 Parameter definitions are
similar to destructuring
Considering what we have learned in this chapter, parameter
definitions have much in common with an Array pattern (rest
elements, default values, etc.). In fact, the following two function
declarations are equivalent:
function f1(«pattern1», «pattern2») {
// ···
}
function f2(...args) {
const [«pattern1», «pattern2»] = args;
// ···
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-661-320.jpg)
![37.12 Nested destructuring
Until now, we have only used variables as assignment targets (data
sinks) inside destructuring patterns. But you can also use patterns as
assignment targets, which enables you to nest patterns to arbitrary
depths:
Inside the Array pattern in line A, there is a nested object pattern at
index 1.
Nested patterns can become difficult to understand, so they are best
used in moderation.
Quiz: advanced
See quiz app.
const arr = [
{ first: 'Jane', last: 'Bond' },
{ first: 'Lars', last: 'Croft' },
];
const [, {first}] = arr;
assert.equal(first, 'Lars');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-662-320.jpg)
![38.1.1 Generator functions return
iterables and fill them via yield
If you call a generator function, it returns an iterable (actually, an
iterator that is also iterable). The generator fills that iterable via the
yield operator:
38.1.2 yield pauses a generator function
Using a generator function involves the following steps:
Function-calling it returns an iterator iter (that is also an
iterable).
Iterating over iter repeatedly invokes iter.next(). Each time,
we jump into the body of the generator function until there is a
yield that returns a value.
function* genFunc1() {
yield 'a';
yield 'b';
}
const iterable = genFunc1();
// Convert the iterable to an Array, to check what’s inside:
assert.deepEqual([...iterable], ['a', 'b']);
// You can also use a for-of loop
for (const x of genFunc1()) {
console.log(x);
}
// Output:
// 'a'
// 'b'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-665-320.jpg)
![only a single numbered line. In turn, it asks genLines() for only a
single unnumbered line.
This incrementalism continues to work if, for example, genLines()
reads its lines from a large text file: If we ask numberLines() for a
numbered line, we get one as soon as genLines() has read its first
line from the text file.
Without generators, genLines() would first read all lines and return
them. Then numberLines() would number all lines and return them.
We therefore have to wait much longer until we get the first
numbered line.
Exercise: Turning a normal function into a generator
exercises/sync-generators/fib_seq_test.mjs
38.1.4 Example: Mapping over iterables
The following function mapIter() is similar to the Array method
.map(), but it returns an iterable, not an Array, and produces its
results on demand.
function* mapIter(iterable, func) {
let index = 0;
for (const x of iterable) {
yield func(x, index);
index++;
}
}
const iterable = mapIter(['a', 'b'], x => x + x);
assert.deepEqual([...iterable], ['aa', 'bb']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-669-320.jpg)
![38.2 Calling generators from
generators (advanced)
38.2.1 Calling generators via yield*
yield only works directly inside generators – so far we haven’t seen a
way of delegating yielding to another function or method.
Let’s first examine what does not work: in the following example,
we’d like foo() to call bar(), so that the latter yields two values for
the former. Alas, a naive approach fails:
Why doesn’t this work? The function call bar() returns an iterable,
which we ignore.
What we want is for foo() to yield everything that is yielded by bar().
That’s what the yield* operator does:
function* bar() {
yield 'a';
yield 'b';
}
function* foo() {
// Nothing happens if we call `bar()`:
bar();
}
assert.deepEqual(
[...foo()], []);
function* bar() {
yield 'a';
yield 'b';
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-671-320.jpg)
![In other words, the previous foo() is roughly equivalent to:
Note that yield* works with any iterable:
38.2.2 Example: Iterating over a tree
yield* lets us make recursive calls in generators, which is useful
when iterating over recursive data structures such as trees. Take, for
example, the following data structure for binary trees.
function* foo() {
yield* bar();
}
assert.deepEqual(
[...foo()], ['a', 'b']);
function* foo() {
for (const x of bar()) {
yield x;
}
}
function* gen() {
yield* [1, 2];
}
assert.deepEqual(
[...gen()], [1, 2]);
class BinaryTree {
constructor(value, left=null, right=null) {
this.value = value;
this.left = left;
this.right = right;
}
/** Prefix iteration: parent before children */
* [Symbol.iterator]() {
yield this.value;](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-672-320.jpg)
![Method [Symbol.iterator]() adds support for the iteration protocol,
which means that we can use a for-of loop to iterate over an instance
of BinaryTree:
Exercise: Iterating over a nested Array
exercises/sync-generators/iter_nested_arrays_test.mjs
if (this.left) {
// Same as yield* this.left[Symbol.iterator]()
yield* this.left;
}
if (this.right) {
yield* this.right;
}
}
}
const tree = new BinaryTree('a',
new BinaryTree('b',
new BinaryTree('c'),
new BinaryTree('d')),
new BinaryTree('e'));
for (const x of tree) {
console.log(x);
}
// Output:
// 'a'
// 'b'
// 'c'
// 'd'
// 'e'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-673-320.jpg)
![38.3 Background: external
iteration vs. internal iteration
In preparation for the next section, we need to learn about two
different styles of iterating over the values “inside” an object:
External iteration (pull): Your code asks the object for the values
via an iteration protocol. For example, the for-of loop is based
on JavaScript’s iteration protocol:
Internal iteration (push): You pass a callback function to a
method of the object and the method feeds the values to the
callback. For example, Arrays have the method .forEach():
The next section has examples for both styles of iteration.
for (const x of ['a', 'b']) {
console.log(x);
}
// Output:
// 'a'
// 'b'
['a', 'b'].forEach((x) => {
console.log(x);
});
// Output:
// 'a'
// 'b'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-674-320.jpg)
![How can we reuse this traversal and do something other than logging
the paths?
38.4.2 Internal iteration (push)
One way of reusing traversal code is via internal iteration: Each
traversed value is passsed to a callback (line A).
38.4.3 External iteration (pull)
// Output:
// 'mydir/a.txt'
// 'mydir/b.txt'
// 'mydir/subdir'
// 'mydir/subdir/c.txt'
function visitPaths(dir, callback) {
for (const fileName of fs.readdirSync(dir)) {
const filePath = path.resolve(dir, fileName);
callback(filePath); // (A)
const stats = fs.statSync(filePath);
if (stats.isDirectory()) {
visitPaths(filePath, callback);
}
}
}
const paths = [];
visitPaths('mydir', p => paths.push(p));
assert.deepEqual(
paths,
[
'mydir/a.txt',
'mydir/b.txt',
'mydir/subdir',
'mydir/subdir/c.txt',
]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-676-320.jpg)
![Another way of reusing traversal code is via external iteration: We
can write a generator that yields all traversed values (line A).
function* iterPaths(dir) {
for (const fileName of fs.readdirSync(dir)) {
const filePath = path.resolve(dir, fileName);
yield filePath; // (A)
const stats = fs.statSync(filePath);
if (stats.isDirectory()) {
yield* iterPaths(filePath);
}
}
}
const paths = [...iterPaths('mydir')];](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-677-320.jpg)
![40.5 Promise.all(): concurrency and
Arrays of Promises
40.5.1 Sequential execution
vs. concurrent execution
Consider the following code:
Using .then() in this manner executes Promise-based functions
sequentially: only after the result of asyncFunc1() is settled will
asyncFunc2() be executed.
The static method Promise.all() helps execute Promise-based
functions more concurrently:
Its type signature is:
const asyncFunc1 = () => Promise.resolve('one');
const asyncFunc2 = () => Promise.resolve('two');
asyncFunc1()
.then(result1 => {
assert.equal(result1, 'one');
return asyncFunc2();
})
.then(result2 => {
assert.equal(result2, 'two');
});
Promise.all([asyncFunc1(), asyncFunc2()])
.then(arr => {
assert.deepEqual(arr, ['one', 'two']);
});](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-725-320.jpg)
![The parameter promises is an iterable of Promises. The result is a
single Promise that is settled as follows:
If and when all input Promises are fulfilled, the output Promise
is fulfilled with an Array of the fulfillment values.
As soon as at least one input Promise is rejected, the output
Promise is rejected with the rejection value of that input
Promise.
In other words: We go from an iterable of Promises to a Promise for
an Array.
40.5.2 Concurrency tip: focus on when
operations start
Tip for determining how “concurrent” asynchronous code is: Focus
on when asynchronous operations start, not on how their Promises
are handled.
For example, each of the following functions executes asyncFunc1()
and asyncFunc2() concurrently because they are started at nearly the
same time.
Promise.all<T>(promises: Iterable<Promise<T>>): Promise<T[]>
function concurrentAll() {
return Promise.all([asyncFunc1(), asyncFunc2()]);
}
function concurrentThen() {
const p1 = asyncFunc1();
const p2 = asyncFunc2();](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-726-320.jpg)
![On the other hand, both of the following functions execute
asyncFunc1() and asyncFunc2() sequentially: asyncFunc2() is only
invoked after the Promise of asyncFunc1() is fulfilled.
40.5.3 Promise.all() is fork-join
Promise.all() is loosely related to the concurrency pattern “fork
join” – for example:
httpGet() is the promisified version of XMLHttpRequest that we
implemented earlier.
return p1.then(r1 => p2.then(r2 => [r1, r2]));
}
function sequentialThen() {
return asyncFunc1()
.then(r1 => asyncFunc2()
.then(r2 => [r1, r2]));
}
function sequentialAll() {
const p1 = asyncFunc1();
const p2 = p1.then(() => asyncFunc2());
return Promise.all([p1, p2]);
}
Promise.all([
// Fork async computations
httpGet('http://example.com/file1.txt'),
httpGet('http://example.com/file2.txt'),
])
// Join async computations
.then(([text1, text2]) => {
assert.equal(text1, 'Content of file1.txtn');
assert.equal(text2, 'Content of file2.txtn');
});](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-727-320.jpg)
![40.5.4 Asynchronous .map() via
Promise.all()
Array transformation methods such as .map(), .filter(), etc., are
made for synchronous computations – for example:
What happens if the callback of .map() is a Promise-based function
(a function that maps normal values to Promises)? Then the result of
.map() is an Array of Promises. Alas, that is not data that normal
code can work with. Thankfully, we can fix that via Promise.all(): It
converts an Array of Promises into a Promise that is fulfilled with an
Array of normal values.
40.5.4.1 A more realistic example
The following code is a more realistic example: in the section on
fork-join, there was an example where we downloaded two resources
identified by two fixed URLs. Let’s turn that code fragment into a
function timesTwoSync(x) {
return 2 * x;
}
const arr = [1, 2, 3];
const result = arr.map(timesTwoSync);
assert.deepEqual(result, [2, 4, 6]);
function timesTwoAsync(x) {
return new Promise(resolve => resolve(x * 2));
}
const arr = [1, 2, 3];
const promiseArr = arr.map(timesTwoAsync);
Promise.all(promiseArr)
.then(result => {
assert.deepEqual(result, [2, 4, 6]);
});](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-728-320.jpg)
![function that accepts an Array of URLs and downloads the
corresponding resources:
Exercise: Promise.all() and listing files
exercises/promises/list_files_async_test.mjs
function downloadTexts(urls) {
const promisedTexts = urls.map(httpGet);
return Promise.all(promisedTexts);
}
downloadTexts([
'http://example.com/file1.txt',
'http://example.com/file2.txt',
])
.then(texts => {
assert.deepEqual(
texts, [
'Content of file1.txtn',
'Content of file2.txtn',
]);
});](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-729-320.jpg)
![That is, paused('second') is only started after paused('first') is
completely finished.
41.6.2 await: running asynchronous
functions concurrently
If we want to run multiple functions concurrently, we can use the
tool method Promise.all():
Here, both asynchronous functions are started at the same time.
Once both are settled, await gives us either an Array of fulfillment
values or – if at least one Promise is rejected – an exception.
Recall from §40.5.2 “Concurrency tip: focus on when operations
start” that what counts is when we start a Promise-based
computation; not how we process its result. Therefore, the following
code is as “concurrent” as the previous one:
// 'START second'
// 'END second'
async function concurrentPromiseAll() {
const result = await Promise.all([
paused('first'), paused('second')
]);
assert.deepEqual(result, ['first', 'second']);
}
// Output:
// 'START first'
// 'START second'
// 'END first'
// 'END second'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-750-320.jpg)
![42.1 Basic asynchronous iteration
42.1.1 Protocol: async iteration
To understand how asynchronous iteration works, let’s first revisit
synchronous iteration. It comprises the following interfaces:
An Iterable is a data structure whose contents can be accessed
via iteration. It is a factory for iterators.
An Iterator is a factory for iteration results that we retrieve by
calling the method .next().
Each IterationResult contains the iterated .value and a boolean
.done that is true after the last element and false before.
For the protocol for asynchronous iteration, we only want to change
one thing: the values produced by .next() should be delivered
asynchronously. There are two conceivable options:
The .value could contain a Promise<T>.
.next() could return Promise<IteratorResult<T>>.
interface Iterable<T> {
[Symbol.iterator]() : Iterator<T>;
}
interface Iterator<T> {
next() : IteratorResult<T>;
}
interface IteratorResult<T> {
value: T;
done: boolean;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-755-320.jpg)
![In other words, the question is whether to wrap just values or whole
iterator results in Promises.
It has to be the latter because when .next() returns a result, it starts
an asynchronous computation. Whether or not that computation
produces a value or signals the end of the iteration can only be
determined after it is finished. Therefore, both .done and .value need
to be wrapped in a Promise.
The interfaces for async iteration look as follows.
The only difference to the synchronous interfaces is the return type
of .next() (line A).
42.1.2 Using async iteration directly
The following code uses the asynchronous iteration protocol directly:
interface AsyncIterable<T> {
[Symbol.asyncIterator]() : AsyncIterator<T>;
}
interface AsyncIterator<T> {
next() : Promise<IteratorResult<T>>; // (A)
}
interface IteratorResult<T> {
value: T;
done: boolean;
}
const asyncIterable = syncToAsyncIterable(['a', 'b']); // (A)
const asyncIterator = asyncIterable[Symbol.asyncIterator]();
// Call .next() until .done is true:
asyncIterator.next() // (B)
.then(iteratorResult => {](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-756-320.jpg)
![In line A, we create an asynchronous iterable over the value 'a' and
'b'. We’ll see an implementation of syncToAsyncIterable() later.
We call .next() in line B, line C and line D. Each time, we use
.next() to unwrap the Promise and assert.deepEqual() to check the
unwrapped value.
We can simplify this code if we use an async function. Now we
unwrap Promises via await and the code looks almost like we are
doing synchronous iteration:
assert.deepEqual(
iteratorResult,
{ value: 'a', done: false });
return asyncIterator.next(); // (C)
})
.then(iteratorResult => {
assert.deepEqual(
iteratorResult,
{ value: 'b', done: false });
return asyncIterator.next(); // (D)
})
.then(iteratorResult => {
assert.deepEqual(
iteratorResult,
{ value: undefined, done: true });
})
;
async function f() {
const asyncIterable = syncToAsyncIterable(['a', 'b']);
const asyncIterator = asyncIterable[Symbol.asyncIterator]();
// Call .next() until .done is true:
assert.deepEqual(
await asyncIterator.next(),
{ value: 'a', done: false });
assert.deepEqual(
await asyncIterator.next(),](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-757-320.jpg)
![42.1.3 Using async iteration via for-await-
of
The asynchronous iteration protocol is not meant to be used directly.
One of the language constructs that supports it is the for-await-of
loop, which is an asynchronous version of the for-of loop. It can be
used in async functions and async generators (which are introduced
later in this chapter). This is an example of for-await-of in use:
for-await-of is relatively flexible. In addition to asynchronous
iterables, it also supports synchronous iterables:
And it supports synchronous iterables over values that are wrapped
in Promises:
{ value: 'b', done: false });
assert.deepEqual(
await asyncIterator.next(),
{ value: undefined, done: true });
}
for await (const x of syncToAsyncIterable(['a', 'b'])) {
console.log(x);
}
// Output:
// 'a'
// 'b'
for await (const x of ['a', 'b']) {
console.log(x);
}
// Output:
// 'a'
// 'b'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-758-320.jpg)
![Exercise: Convert an async iterable to an Array
Warning: We’ll soon see the solution for this exercise in this
chapter.
exercises/async-
iteration/async_iterable_to_array_test.mjs
const arr = [Promise.resolve('a'), Promise.resolve('b')];
for await (const x of arr) {
console.log(x);
}
// Output:
// 'a'
// 'b'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-759-320.jpg)
![42.2.1 Example: creating an async
iterable via an async generator
Let’s look at an example. The following code creates an async iterable
with three numbers:
Does the result of yield123() conform to the async iteration
protocol?
// Output
yield someValue;
yield* otherAsyncGen();
}
async function* yield123() {
for (let i=1; i<=3; i++) {
yield i;
}
}
(async () => {
const asyncIterable = yield123();
const asyncIterator = asyncIterable[Symbol.asyncIterator]();
assert.deepEqual(
await asyncIterator.next(),
{ value: 1, done: false });
assert.deepEqual(
await asyncIterator.next(),
{ value: 2, done: false });
assert.deepEqual(
await asyncIterator.next(),
{ value: 3, done: false });
assert.deepEqual(
await asyncIterator.next(),
{ value: undefined, done: true });
})();](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-761-320.jpg)
![We wrapped the code in an immediately invoked async arrow
function.
42.2.2 Example: converting a sync
iterable to an async iterable
The following asynchronous generator converts a synchronous
iterable to an asynchronous iterable. It implements the function
syncToAsyncIterable() that we have used previously.
Note: The input is synchronous in this case (no await is needed).
42.2.3 Example: converting an async
iterable to an Array
The following function is a solution to a previous exercise. It converts
an async iterable to an Array (think spreading, but for async iterables
instead of sync iterables).
async function* syncToAsyncIterable(syncIterable) {
for (const elem of syncIterable) {
yield elem;
}
}
async function asyncIterableToArray(asyncIterable) {
const result = [];
for await (const value of asyncIterable) {
result.push(value);
}
return result;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-762-320.jpg)
![Note that we can’t use an async generator in this case: We get our
input via for-await-of and return an Array wrapped in a Promise.
The latter requirement rules out async generators.
This is a test for asyncIterableToArray():
Note the await in line A, which is needed to unwrap the Promise
returned by asyncIterableToArray(). In order for await to work, this
code fragment must be run inside an async function.
42.2.4 Example: transforming an async
iterable
Let’s implement an async generator that produces a new async
iterable by transforming an existing async iterable.
To test this function, we use asyncIterableToArray() from the
previous section.
async function* createAsyncIterable() {
yield 'a';
yield 'b';
}
const asyncIterable = createAsyncIterable();
assert.deepEqual(
await asyncIterableToArray(asyncIterable), // (A)
['a', 'b']
);
async function* timesTwo(asyncNumbers) {
for await (const x of asyncNumbers) {
yield x * 2;
}
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-763-320.jpg)
![Exercise: Async generators
Warning: We’ll soon see the solution for this exercise in this
chapter.
exercises/async-iteration/number_lines_test.mjs
42.2.5 Example: mapping over
asynchronous iterables
As a reminder, this is how to map over synchronous iterables:
The asynchronous version looks as follows:
async function* createAsyncIterable() {
for (let i=1; i<=3; i++) {
yield i;
}
}
assert.deepEqual(
await asyncIterableToArray(timesTwo(createAsyncIterable())),
[2, 4, 6]
);
function* mapSync(iterable, func) {
let index = 0;
for (const x of iterable) {
yield func(x, index);
index++;
}
}
const syncIterable = mapSync(['a', 'b', 'c'], s => s.repeat(3));
assert.deepEqual(
[...syncIterable],
['aaa', 'bbb', 'ccc']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-764-320.jpg)
![Note how similar the sync implementation and the async
implementation are. The only two differences are the async in line A
and the await in line B. That is comparable to going from a
synchronous function to an asynchronous function – we only need to
add the keyword async and the occasional await.
To test mapAsync(), we use the helper function
asyncIterableToArray() (shown earlier in this chapter):
Once again, we await to unwrap a Promise (line A) and this code
fragment must run inside an async function.
Exercise: filterAsyncIter()
exercises/async-iteration/filter_async_iter_test.mjs
async function* mapAsync(asyncIterable, func) { // (A)
let index = 0;
for await (const x of asyncIterable) { // (B)
yield func(x, index);
index++;
}
}
async function* createAsyncIterable() {
yield 'a';
yield 'b';
}
const mapped = mapAsync(
createAsyncIterable(), s => s.repeat(3));
assert.deepEqual(
await asyncIterableToArray(mapped), // (A)
['aaa', 'bbb']);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-765-320.jpg)
![Now that we have an asynchronous iterable over lines, we can use
the solution of a previous exercise, numberLines(), to number those
lines:
async function* chunkIterable() {
yield 'FirstnSec';
yield 'ondnThirdnF';
yield 'ourth';
}
const linesIterable = chunksToLines(chunkIterable());
assert.deepEqual(
await asyncIterableToArray(linesIterable),
[
'Firstn',
'Secondn',
'Thirdn',
'Fourth',
]);
async function* numberLines(linesAsync) {
let lineNumber = 1;
for await (const line of linesAsync) {
yield lineNumber + ': ' + line;
lineNumber++;
}
}
const numberedLines = numberLines(chunksToLines(chunkIterable())
assert.deepEqual(
await asyncIterableToArray(numberedLines),
[
'1: Firstn',
'2: Secondn',
'3: Thirdn',
'4: Fourth',
]);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-768-320.jpg)
![43 Regular expressions
(RegExp)
43.1 Creating regular expressions
43.1.1 Literal vs. constructor
43.1.2 Cloning and non-destructively modifying regular
expressions
43.2 Syntax
43.2.1 Syntax characters
43.2.2 Basic atoms
43.2.3 Unicode property escapes [ES2018]
43.2.4 Character classes
43.2.5 Groups
43.2.6 Quantifiers
43.2.7 Assertions
43.2.8 Disjunction (|)
43.3 Flags
43.3.1 Flag: Unicode mode via /u
43.4 Properties of regular expression objects
43.4.1 Flags as properties
43.4.2 Other properties
43.5 Methods for working with regular expressions
43.5.1 In general, regular expressions match anywhere in a
string
43.5.2 regExp.test(str): is there a match? [ES3]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-769-320.jpg)
![43.5.3 str.search(regExp): at what index is the match?
[ES3]
43.5.4 regExp.exec(str): capturing groups [ES3]
43.5.5 str.match(regExp): return all matching substrings
[ES3]
43.5.6 str.replace(searchValue, replacementValue) [ES3]
43.5.7 Other methods for working with regular expressions
43.6 Flag /g and its pitfalls
43.6.1 Pitfall: You can’t inline a regular expression with flag
/g
43.6.2 Pitfall: Removing /g can break code
43.6.3 Pitfall: Adding /g can break code
43.6.4 Pitfall: Code can break if .lastIndex isn’t zero
43.6.5 Dealing with /g and .lastIndex
43.7 Techniques for working with regular expressions
43.7.1 Escaping arbitrary text for regular expressions
43.7.2 Matching everything or nothing
Availability of features
Unless stated otherwise, each regular expression feature has been
available since ES3.](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-770-320.jpg)
![43.1 Creating regular expressions
43.1.1 Literal vs. constructor
The two main ways of creating regular expressions are:
Literal: compiled statically (at load time).
Constructor: compiled dynamically (at runtime).
Both regular expressions have the same two parts:
The body abc – the actual regular expression.
The flags u and i. Flags configure how the pattern is interpreted.
For example, i enables case-insensitive matching. A list of
available flags is given later in this chapter.
43.1.2 Cloning and non-destructively
modifying regular expressions
There are two variants of the constructor RegExp():
new RegExp(pattern : string, flags = '') [ES3]
A new regular expression is created as specified via pattern. If
flags is missing, the empty string '' is used.
/abc/ui
new RegExp('abc', 'ui')](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-771-320.jpg)
![new RegExp(regExp : RegExp, flags = regExp.flags) [ES6]
regExp is cloned. If flags is provided, then it determines the
flags of the clone.
The second variant is useful for cloning regular expressions,
optionally while modifying them. Flags are immutable and this is the
only way of changing them – for example:
function copyAndAddFlags(regExp, flagsToAdd='') {
// The constructor doesn’t allow duplicate flags;
// make sure there aren’t any:
const newFlags = [...new Set(regExp.flags + flagsToAdd)].join(
return new RegExp(regExp, newFlags);
}
assert.equal(/abc/i.flags, 'i');
assert.equal(copyAndAddFlags(/abc/i, 'g').flags, 'gi');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-772-320.jpg)
![43.2 Syntax
43.2.1 Syntax characters
At the top level of a regular expression, the following syntax
characters are special. They are escaped by prefixing a backslash ().
^ $ . * + ? ( ) [ ] { } |
In regular expression literals, you must escape slashs:
In the argument of new RegExp(), you don’t have to escape slashes:
43.2.2 Basic atoms
Atoms are the basic building blocks of regular expressions.
Pattern characters are all characters except syntax characters
(^, $, etc.). Pattern characters match themselves. Examples: A b
%
. matches any character. You can use the flag /s (dotall) to
control if the dot matches line terminators or not (more below).
Character escapes (each escape matches a single fixed
character):
Control escapes (for a few control characters):
> ///.test('/')
true
> new RegExp('/').test('/')
true](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-773-320.jpg)
![f: form feed (FF)
n: line feed (LF)
r: carriage return (CR)
t: character tabulation
v: line tabulation
Arbitrary control characters: cA (Ctrl-A), …, cZ (Ctrl-Z)
Unicode code units: u00E4
Unicode code points (require flag /u): u{1F44D}
Character class escapes (each escape matches one out of a set of
characters):
d: digits (same as [0-9])
D: non-digits
w: “word” characters (same as [A-Za-z0-9_], related to
identifiers in programming languages)
W: non-word characters
s: whitespace (space, tab, line terminators, etc.)
S: non-whitespace
Unicode property escapes [ES2018]: p{White_Space},
P{White_Space}, etc.
Require flag /u.
Described in the next subsection.
43.2.3 Unicode property escapes
[ES2018]
43.2.3.1 Unicode character properties](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-774-320.jpg)
![A character class wraps class ranges in square brackets. The class
ranges specify a set of characters:
[«class ranges»] matches any character in the set.
[^«class ranges»] matches any character not in the set.
Rules for class ranges:
Non-syntax characters stand for themselves: [abc]
Only the following four characters are special and must be
escaped via slashes:
^ - ]
^ only has to be escaped if it comes first.
- need not be escaped if it comes first or last.
Character escapes (n, u{1F44D}, etc.) have the usual meaning.
Watch out: b stands for backspace. Elsewhere in a regular
expression, it matches word boundaries.
Character class escapes (d, p{White_Space}, etc.) have the
usual meaning.
Ranges of characters are specified via dashes: [a-z]
43.2.5 Groups
Positional capture group: (#+)
Backreference: 1, 2, etc.
Named capture group [ES2018]: (?<hashes>#+)](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-778-320.jpg)
![Backreference: k<hashes>
Noncapturing group: (?:#+)
43.2.6 Quantifiers
By default, all of the following quantifiers are greedy (they match as
many characters as possible):
?: match never or once
*: match zero or more times
+: match one or more times
{n}: match n times
{n,}: match n or more times
{n,m}: match at least n times, at most m times.
To make them reluctant (so that they match as few characters as
possible), put question marks (?) after them:
43.2.7 Assertions
^ matches only at the beginning of the input
$ matches only at the end of the input
b matches only at a word boundary
B matches only when not at a word boundary
43.2.7.1 Lookahead
> /".*"/.exec('"abc"def"')[0] // greedy
'"abc"def"'
> /".*?"/.exec('"abc"def"')[0] // reluctant
'"abc"'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-779-320.jpg)
![Positive lookahead: (?=«pattern») matches if pattern matches
what comes next.
Example: sequences of lowercase letters that are followed by an X.
Note that the X itself is not part of the matched substring.
Negative lookahead: (?!«pattern») matches if pattern does not
match what comes next.
Example: sequences of lowercase letters that are not followed by an
X.
43.2.7.2 Lookbehind [ES2018]
Positive lookbehind: (?<=«pattern») matches if pattern matches
what came before.
Example: sequences of lowercase letters that are preceded by an X.
Negative lookbehind: (?<!«pattern») matches if pattern does not
match what came before.
Example: sequences of lowercase letters that are not preceded by an
X.
> 'abcX def'.match(/[a-z]+(?=X)/g)
[ 'abc' ]
> 'abcX def'.match(/[a-z]+(?!X)/g)
[ 'ab', 'def' ]
> 'Xabc def'.match(/(?<=X)[a-z]+/g)
[ 'abc' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-780-320.jpg)
![Example: replace “.js” with “.html”, but not in “Node.js”.
43.2.8 Disjunction (|)
Caveat: this operator has low precedence. Use groups if necessary:
^aa|zz$ matches all strings that start with aa and/or end with zz.
Note that | has a lower precedence than ^ and $.
^(aa|zz)$ matches the two strings 'aa' and 'zz'.
^a(a|z)z$ matches the two strings 'aaz' and 'azz'.
> 'Xabc def'.match(/(?<!X)[a-z]+/g)
[ 'bc', 'def' ]
> 'Node.js: index.js and main.js'.replace(/(?<!Node).js/g, '.ht
'Node.js: index.html and main.html'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-781-320.jpg)
![for a regular expression in an input string. With /g, they
consider all matches.
/i (.ignoreCase): switches on case-insensitive matching:
/m (.multiline): If this flag is on, ^ matches the beginning of
each line and $ matches the end of each line. If it is off, ^
matches the beginning of the whole input string and $ matches
the end of the whole input string.
/u (.unicode): This flag switches on the Unicode mode for a
regular expression. That mode is explained in the next
subsection.
/y (.sticky): This flag mainly makes sense in conjunction with
/g. When both are switched on, any match must directly follow
the previous one (that is, it must start at index .lastIndex of the
regular expression object). Therefore, the first match must be at
index 0.
> /a/.test('A')
false
> /a/i.test('A')
true
> 'a1na2na3'.match(/^a./gm)
[ 'a1', 'a2', 'a3' ]
> 'a1na2na3'.match(/^a./g)
[ 'a1' ]
> 'a1a2 a3'.match(/a./gy)
[ 'a1', 'a2' ]
> '_a1a2 a3'.match(/a./gy) // first match must be at index 0
null](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-783-320.jpg)
![The main use case for /y is tokenization (during parsing).
/s (.dotall): By default, the dot does not match line
terminators. With this flag, it does:
Workaround if /s isn’t supported: Use [^] instead of a dot.
43.3.1 Flag: Unicode mode via /u
The flag /u switches on a special Unicode mode for regular
expressions. That mode enables several features:
In patterns, you can use Unicode code point escapes such as
u{1F42A} to specify characters. Code unit escapes such as
u03B1 only have a range of four hexadecimal digits (which
corresponds to the basic multilingual plane).
In patterns, you can use Unicode property escapes such as
p{White_Space}.
Many escapes are now forbidden. For example: a - :
> 'a1a2 a3'.match(/a./g)
[ 'a1', 'a2', 'a3' ]
> '_a1a2 a3'.match(/a./g)
[ 'a1', 'a2', 'a3' ]
> /./.test('n')
false
> /./s.test('n')
true
> /[^]/.test('n')
true](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-784-320.jpg)
![43.3.1.1 Consequence: you can put Unicode characters in
character classes
With /u, the two code units of 🙂 are treated as a single character:
Without /u, 🙂 is treated as two characters:
Note that ^ and $ demand that the input string have a single
character. That’s why the first result is false.
43.3.1.2 Consequence: the dot operator (.) matches
Unicode characters, not JavaScript characters
With /u, the dot operator matches Unicode characters:
.match() plus /g returns an Array with all the matches of a regular
expression.
Without /u, the dot operator matches JavaScript characters:
> /^[🙂]$/u.test('🙂')
true
> /^[uD83DuDE42]$/.test('uD83DuDE42')
false
> /^[uD83DuDE42]$/.test('uDE42')
true
> '🙂'.match(/./gu).length
1
> 'uD83DuDE80'.match(/./g).length
2](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-786-320.jpg)
![43.4.2 Other properties
Each regular expression also has the following properties:
.source [ES3]: The regular expression pattern
.flags [ES6]: The flags of the regular expression
.lastIndex [ES3]: Used when flag /g is switched on. Consult §43.6
“Flag /g and its pitfalls” for details.
> /abc/ig.source
'abc'
> /abc/ig.flags
'gi'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-789-320.jpg)
![43.5 Methods for working with
regular expressions
43.5.1 In general, regular expressions
match anywhere in a string
Note that, in general, regular expressions match anywhere in a
string:
You can change that by using assertions such as ^ or by using the flag
/y:
43.5.2 regExp.test(str): is there a match?
[ES3]
The regular expression method .test() returns true if regExp
matches str:
> /a/.test('__a__')
true
> /^a/.test('__a__')
false
> /^a/.test('a__')
true
> /bc/.test('ABCD')
false
> /bc/i.test('ABCD')
true
> /.mjs$/.test('main.mjs')
true](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-790-320.jpg)
![With .test() you should normally avoid the /g flag. If you use it, you
generally don’t get the same result every time you call the method:
The results are due to /a/ having two matches in the string. After all
of those were found, .test() returns false.
43.5.3 str.search(regExp): at what index is
the match? [ES3]
The string method .search() returns the first index of str at which
there is a match for regExp:
43.5.4 regExp.exec(str): capturing groups
[ES3]
43.5.4.1 Getting a match object for the first match
Without the flag /g, .exec() returns the captures of the first match
for regExp in str:
> const r = /a/g;
> r.test('aab')
true
> r.test('aab')
true
> r.test('aab')
false
> '_abc_'.search(/abc/)
1
> 'main.mjs'.search(/.mjs$/)
4](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-791-320.jpg)
![The result is a match object with the following properties:
[0]: the complete substring matched by the regular expression
[1]: capture of positional group 1 (etc.)
.index: where did the match occur?
.input: the string that was matched against
.groups: captures of named groups
43.5.4.2 Named capture groups [ES2018]
The previous example contained a single positional group. The
following example demonstrates named groups:
In the result of .exec(), you can see that a named group is also a
positional group – its capture exists twice:
assert.deepEqual(
/(a+)b/.exec('ab aab'),
{
0: 'ab',
1: 'a',
index: 0,
input: 'ab aab',
groups: undefined,
}
);
assert.deepEqual(
/(?<as>a+)b/.exec('ab aab'),
{
0: 'ab',
1: 'a',
index: 0,
input: 'ab aab',
groups: { as: 'a' },
}
);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-792-320.jpg)
![Once as a positional capture (property '1').
Once as a named capture (property groups.as).
43.5.4.3 Looping over multiple matches
If you want to retrieve all matches of a regular expression (not just
the first one), you need to switch on the flag /g. Then you can call
.exec() multiple times and get one match each time. After the last
match, .exec() returns null.
Therefore, you can loop over all matches as follows:
Sharing regular expressions with /g has a few pitfalls, which are
explained later.
> const regExp = /(a+)b/g;
> regExp.exec('ab aab')
{ 0: 'ab', 1: 'a', index: 0, input: 'ab aab', groups: undefined
> regExp.exec('ab aab')
{ 0: 'aab', 1: 'aa', index: 3, input: 'ab aab', groups: undefine
> regExp.exec('ab aab')
null
const regExp = /(a+)b/g;
const str = 'ab aab';
let match;
// Check for null via truthiness
// Alternative: while ((match = regExp.exec(str)) !== null)
while (match = regExp.exec(str)) {
console.log(match[1]);
}
// Output:
// 'a'
// 'aa'](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-793-320.jpg)
![Exercise: Extract quoted text via .exec()
exercises/regexps/extract_quoted_test.mjs
43.5.5 str.match(regExp): return all
matching substrings [ES3]
Without /g, .match() works like .exec() – it returns a single match
object.
With /g, .match() returns all substrings of str that match regExp:
If there is no match, .match() returns null:
You can use the Or operator to protect yourself against null:
43.5.6 str.replace(searchValue,
replacementValue) [ES3]
.replace() is overloaded – it works differently, depending on the
types of its parameters:
If searchValue is:
> 'ab aab'.match(/(a+)b/g)
[ 'ab', 'aab' ]
> 'xyz'.match(/(a+)b/g)
null
const numberOfMatches = (str.match(regExp) || []).length;](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-794-320.jpg)
![Text Result
$` text before match
$' text after match
$n capture of positional group n (n > 0)
$<name> capture of named group name [ES2018]
Example: Inserting the text before, inside, and after the matched
substring.
Example: Inserting the captures of positional groups.
Example: Inserting the captures of named groups.
43.5.6.2 replacementValue is a function
If the replacement value is a function, you can compute each
replacement. In the following example, we multiply each non-
negative integer that we find by two.
> 'a1 a2'.replace(/a/g, "($`|$&|$')")
'(|a|1 a2)1 (a1 |a|2)2'
> const regExp = /^([A-Za-z]+): (.*)$/ug;
> 'first: Jane'.replace(regExp, 'KEY: $1, VALUE: $2')
'KEY: first, VALUE: Jane'
> const regExp = /^(?<key>[A-Za-z]+): (?<value>.*)$/ug;
> 'first: Jane'.replace(regExp, 'KEY: $<key>, VALUE: $<value>')
'KEY: first, VALUE: Jane'
assert.equal(
'3 cats and 4 dogs'.replace(/[0-9]+/g, (all) => 2 * Number(all
'6 cats and 8 dogs'
);](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-796-320.jpg)
![The replacement function gets the following parameters. Note how
similar they are to match objects. These parameters are all
positional, but I’ve included how one might name them:
all: complete match
g1: capture of positional group 1
Etc.
index: where did the match occur?
input: the string in which we are replacing
groups [ES2018]: captures of named groups (an object)
Exercise: Change quotes via .replace() and a named
group
exercises/regexps/change_quotes_test.mjs
43.5.7 Other methods for working with
regular expressions
String.prototype.split() is described in the chapter on strings. Its
first parameter of String.prototype.split() is either a string or a
regular expression. If it is the latter, then captures of groups appear
in the result:
> 'a:b : c'.split(':')
[ 'a', 'b ', ' c' ]
> 'a:b : c'.split(/ *: */)
[ 'a', 'b', 'c' ]
> 'a:b : c'.split(/( *):( *)/)
[ 'a', '', '', 'b', ' ', ' ', 'c' ]](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-797-320.jpg)
![Third, we can use .match() to count occurrences, which doesn’t
change or depend on .lastIndex.
function countMatches(regExp, str) {
const cloneFlags = regExp.flags + (regExp.global ? '' : 'g');
const clone = new RegExp(regExp, cloneFlags);
let count = 0;
while (clone.test(str)) {
count++;
}
return count;
}
function countMatches(regExp, str) {
if (!regExp.global) {
throw new Error('Flag /g of regExp must be set');
}
return (str.match(regExp) || []).length;
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-802-320.jpg)
![43.7 Techniques for working with
regular expressions
43.7.1 Escaping arbitrary text for regular
expressions
The following function escapes an arbitrary text so that it is matched
verbatim if you put it inside a regular expression:
In line A, we escape all syntax characters. We have to be selective
because the regular expression flag /u forbids many escapes – for
example: a : -
The regular expression method .replace() only lets you replace plain
text once. With escapeForRegExp(), we can work around that
limitation and replace plain text multiple times:
43.7.2 Matching everything or nothing
function escapeForRegExp(str) {
return str.replace(/[^$.*+?()[]{}|]/g, '$&'); // (A)
}
assert.equal(escapeForRegExp('[yes?]'), String.raw`[yes?]`);
assert.equal(escapeForRegExp('_g_'), String.raw`_g_`);
const plainText = ':-)';
const regExp = new RegExp(escapeForRegExp(plainText), 'ug');
assert.equal(
':-) :-) :-)'.replace(regExp, '🙂'), '🙂 🙂 🙂');](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-803-320.jpg)
![JavaScript has the global namespace object JSON that provides
methods for creating and parsing JSON.
"last": "Porter",
"married": true,
"born": 1890,
"friends": [ "Tarzan", "Cheeta" ]
}](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-822-320.jpg)
![45.3 Using the JSON API
The global namespace object JSON contains methods for working with
JSON data.
45.3.1 JSON.stringify(data, replacer?,
space?)
.stringify() converts JavaScript data to a JSON string. In this
section, we are ignoring the parameter replacer; it is explained in
§45.4 “Customizing stringification and parsing”.
45.3.1.1 Result: a single line of text
If you only provide the first argument, .stringify() returns a single
line of text:
45.3.1.2 Result: a tree of indented lines
If you provide a non-negative integer for space, then .stringify()
returns one or more lines and indents by space spaces per level of
nesting:
assert.equal(
JSON.stringify({foo: ['a', 'b']}),
'{"foo":["a","b"]}' );
assert.equal(
JSON.stringify({foo: ['a', 'b']}, null, 2),
`{
"foo": [](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-826-320.jpg)
![45.3.1.3 Details on how JavaScript data is stringified
Primitive values:
Supported primitive values are stringified as expected:
Unsupported numbers: 'null'
Other unsupported primitive values are not stringified; they
produce the result undefined:
Objects:
If an object has a method .toJSON(), then the result of that
method is stringified:
"a",
"b"
]
}`);
> JSON.stringify('abc')
'"abc"'
> JSON.stringify(123)
'123'
> JSON.stringify(null)
'null'
> JSON.stringify(NaN)
'null'
> JSON.stringify(Infinity)
'null'
> JSON.stringify(undefined)
undefined
> JSON.stringify(Symbol())
undefined](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-827-320.jpg)
![Dates have a method .toJSON() that returns a string:
Wrapped primitive values are unwrapped and stringified:
Arrays are stringified as Array literals. Unsupported Array
elements are stringified as if they were null:
All other objects – except for functions – are stringified as object
literals. Properties with unsupported values are omitted:
Functions are not stringified:
45.3.2 JSON.parse(text, reviver?)
.parse() converts a JSON text to a JavaScript value. In this section,
we are ignoring the parameter reviver; it is explained §45.4
> JSON.stringify({toJSON() {return true}})
'true'
> JSON.stringify(new Date(2999, 11, 31))
'"2999-12-30T23:00:00.000Z"'
> JSON.stringify(new Boolean(true))
'true'
> JSON.stringify(new Number(123))
'123'
> JSON.stringify([undefined, 123, Symbol()])
'[null,123,null]'
> JSON.stringify({a: Symbol(), b: true})
'{"b":true}'
> JSON.stringify(() => {})
undefined](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-828-320.jpg)
![“Customizing stringification and parsing”.
This is an example of using .parse():
45.3.3 Example: converting to and from
JSON
The following class implements conversions from (line A) and to
(line B) JSON.
Converting JSON to a point: We use the static method
Point.fromJson() to parse JSON and create an instance of Point.
> JSON.parse('{"foo":["a","b"]}')
{ foo: [ 'a', 'b' ] }
class Point {
static fromJson(jsonObj) { // (A)
return new Point(jsonObj.x, jsonObj.y);
}
constructor(x, y) {
this.x = x;
this.y = y;
}
toJSON() { // (B)
return {x: this.x, y: this.y};
}
}
assert.deepEqual(
Point.fromJson(JSON.parse('{"x":3,"y":5}')),
new Point(3, 5) );](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-829-320.jpg)
![45.4.2 .stringify() and .parse(): value
visitors
What I call a value visitor is a function that transforms JavaScript
data:
JSON.stringify() lets the value visitor in its parameter replacer
transform JavaScript data before it is stringified.
JSON.parse() lets the value visitor in its parameter reviver
transform parsed JavaScript data before it is returned.
In this section, JavaScript data is considered to be a tree of values. If
the data is atomic, it is a tree that only has a root. All values in the
tree are fed to the value visitor, one at a time. Depending on what the
visitor returns, the current value is omitted, changed, or preserved.
A value visitor has the following type signature:
The parameters are:
value: The current value.
this: Parent of current value. The parent of the root value r is
{'': r}.
c: 2,
d: 3,
}
};
assert.equal(
JSON.stringify(obj, ['b', 'c']),
'{"b":{"c":2}}');
type ValueVisitor = (key: string, value: any) => any;](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-832-320.jpg)
![Note: this is an implicit parameter and only available if the
value visitor is an ordinary function.
key: Key or index of the current value inside its parent. The key
of the root value is ''.
The value visitor can return:
value: means there won’t be any change.
A different value x: leads to value being replaced with x in the
output tree.
undefined: leads to value being omitted in the output tree.
45.4.3 Example: visiting values
The following code shows in which order a value visitor sees values:
const log = [];
function valueVisitor(key, value) {
log.push({this: this, key, value});
return value; // no change
}
const root = {
a: 1,
b: {
c: 2,
d: 3,
}
};
JSON.stringify(root, valueVisitor);
assert.deepEqual(log, [
{ this: { '': root }, key: '', value: root },
{ this: root , key: 'a', value: 1 },
{ this: root , key: 'b', value: root.b },
{ this: root.b , key: 'c', value: 2 },](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-833-320.jpg)
![As we can see, the replacer of JSON.stringify() visits values top-
down (root first, leaves last). The rationale for going in that direction
is that we are converting JavaScript values to JSON values. And a
single JavaScript object may be expanded into a tree of JSON-
compatible values.
In contrast, the reviver of JSON.parse() visits values bottom-up
(leaves first, root last). The rationale for going in that direction is
that we are assembling JSON values into JavaScript values.
Therefore, we need to convert the parts before we can convert the
whole.
45.4.4 Example: stringifying
unsupported values
JSON.stringify() has no special support for regular expression
objects – it stringifies them as if they were plain objects:
We can fix that via a replacer:
{ this: root.b , key: 'd', value: 3 },
]);
const obj = {
name: 'abc',
regex: /abc/ui,
};
assert.equal(
JSON.stringify(obj),
'{"name":"abc","regex":{}}');
function replacer(key, value) {
if (value instanceof RegExp) {
return {](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-834-320.jpg)
![45.5 FAQ
45.5.1 Why doesn’t JSON support
comments?
Douglas Crockford explains why in a Google+ post from 1 May 2012:
I removed comments from JSON because I saw people were
using them to hold parsing directives, a practice which would
have destroyed interoperability. I know that the lack of
comments makes some people sad, but it shouldn’t.
Suppose you are using JSON to keep configuration files, which
you would like to annotate. Go ahead and insert all the
comments you like. Then pipe it through JSMin [a minifier for
JavaScript] before handing it to your JSON parser.](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-837-320.jpg)
![constant
constructor function (role of an ordinary function)
continue
Converting to [type]
Coordinated Universal Time (UTC)
copy object deeply
copy object shallowly
currying
D
dash case
DataView
date
date time format
decimal floating point literal
decimal integer literal
decrementation operator (prefix)
decrementation operator (suffix)
deep copy of an object
default export
default value (destructuring)
default value (parameter)
delete
deleting a property
dense Array
descriptor of a property
destructive operation
destructuring](https://image.slidesharecdn.com/javascriptforimpatientprogrammers-220502021324/85/JavaScript-for-impatient-programmers-pdf-858-320.jpg)
JavaScript for impatient programmers.pdf
- 2. JavaScript for impatient programmers Dr. Axel Rauschmayer 2019
- 3. JavaScript for impatient programmers JavaScript for impatient programmers 1 About this book (ES2019 edition) 1.1 About the content 1.2 Previewing and buying this book 1.3 About the author 1.4 Acknowledgements 2 FAQ: Book and supplementary material 2.1 How to read this book 2.2 I own a digital edition 2.3 I own the print edition 2.4 Notations and conventions 3 Why JavaScript? (bonus) 3.1 The cons of JavaScript 3.2 The pros of JavaScript 3.3 Pro and con of JavaScript: innovation 4 The nature of JavaScript (bonus) 4.1 JavaScript’s influences 4.2 The nature of JavaScript 4.3 Tips for getting started with JavaScript 5 History and evolution of JavaScript 5.1 How JavaScript was created 5.2 Standardizing JavaScript
- 4. 5.3 Timeline of ECMAScript versions 5.4 Ecma Technical Committee 39 (TC39) 5.5 The TC39 process 5.6 FAQ: TC39 process 5.7 Evolving JavaScript: Don’t break the web 6 FAQ: JavaScript 6.1 What are good references for JavaScript? 6.2 How do I find out what JavaScript features are supported where? 6.3 Where can I look up what features are planned for JavaScript? 6.4 Why does JavaScript fail silently so often? 6.5 Why can’t we clean up JavaScript, by removing quirks and outdated features? 6.6 How can I quickly try out a piece of JavaScript code? 7 The big picture 7.1 What are you learning in this book? 7.2 The structure of browsers and Node.js 7.3 JavaScript references 7.4 Further reading 8 Syntax 8.1 An overview of JavaScript’s syntax 8.2 (Advanced) 8.3 Identifiers 8.4 Statement vs. expression 8.5 Ambiguous syntax 8.6 Semicolons
- 5. 8.7 Automatic semicolon insertion (ASI) 8.8 Semicolons: best practices 8.9 Strict mode vs. sloppy mode 9 Consoles: interactive JavaScript command lines 9.1 Trying out JavaScript code 9.2 The console.* API: printing data and more 10 Assertion API 10.1 Assertions in software development 10.2 How assertions are used in this book 10.3 Normal comparison vs. deep comparison 10.4 Quick reference: module assert 11 Getting started with quizzes and exercises 11.1 Quizzes 11.2 Exercises 11.3 Unit tests in JavaScript 12 Variables and assignment 12.1 let 12.2 const 12.3 Deciding between const and let 12.4 The scope of a variable 12.5 (Advanced) 12.6 Terminology: static vs. dynamic 12.7 Global variables and the global object 12.8 Declarations: scope and activation 12.9 Closures 12.10 Further reading 13 Values
- 6. 13.1 What’s a type? 13.2 JavaScript’s type hierarchy 13.3 The types of the language specification 13.4 Primitive values vs. objects 13.5 The operators typeof and instanceof: what’s the type of a value? 13.6 Classes and constructor functions 13.7 Converting between types 14 Operators 14.1 Making sense of operators 14.2 The plus operator (+) 14.3 Assignment operators 14.4 Equality: == vs. === 14.5 Ordering operators 14.6 Various other operators 15 The non-values undefined and null 15.1 undefined vs. null 15.2 Occurrences of undefined and null 15.3 Checking for undefined or null 15.4 undefined and null don’t have properties 15.5 The history of undefined and null 16 Booleans 16.1 Converting to boolean 16.2 Falsy and truthy values 16.3 Truthiness-based existence checks 16.4 Conditional operator (? :) 16.5 Binary logical operators: And (x && y), Or (x || y)
- 7. 16.6 Logical Not (!) 17 Numbers 17.1 JavaScript only has floating point numbers 17.2 Number literals 17.3 Arithmetic operators 17.4 Converting to number 17.5 Error values 17.6 Error value: NaN 17.7 Error value: Infinity 17.8 The precision of numbers: careful with decimal fractions 17.9 (Advanced) 17.10 Background: floating point precision 17.11 Integers in JavaScript 17.12 Bitwise operators 17.13 Quick reference: numbers 18 Math 18.1 Data properties 18.2 Exponents, roots, logarithms 18.3 Rounding 18.4 Trigonometric Functions 18.5 Various other functions 18.6 Sources 19 Unicode – a brief introduction (advanced) 19.1 Code points vs. code units 19.2 Encodings used in web development: UTF-16 and UTF-8 19.3 Grapheme clusters – the real characters 20 Strings
- 8. 20.1 Plain string literals 20.2 Accessing characters and code points 20.3 String concatenation via + 20.4 Converting to string 20.5 Comparing strings 20.6 Atoms of text: Unicode characters, JavaScript characters, grapheme clusters 20.7 Quick reference: Strings 21 Using template literals and tagged templates 21.1 Disambiguation: “template” 21.2 Template literals 21.3 Tagged templates 21.4 Raw string literals 21.5 (Advanced) 21.6 Multiline template literals and indentation 21.7 Simple templating via template literals 22 Symbols 22.1 Use cases for symbols 22.2 Publicly known symbols 22.3 Converting symbols 23 Control flow statements 23.1 Conditions of control flow statements 23.2 Controlling loops: break and continue 23.3 if statements 23.4 switch statements 23.5 while loops 23.6 do-while loops
- 9. 23.7 for loops 23.8 for-of loops 23.9 for-await-of loops 23.10 for-in loops (avoid) 24 Exception handling 24.1 Motivation: throwing and catching exceptions 24.2 throw 24.3 The try statement 24.4 Error classes 25 Callable values 25.1 Kinds of functions 25.2 Ordinary functions 25.3 Specialized functions 25.4 More kinds of functions and methods 25.5 Returning values from functions and methods 25.6 Parameter handling 25.7 Dynamically evaluating code: eval(), new Function() (advanced) 26 Environments: under the hood of variables (bonus) 26.1 Environment: data structure for managing variables 26.2 Recursion via environments 26.3 Nested scopes via environments 26.4 Closures and environments 27 Modules 27.1 Overview: syntax of ECMAScript modules 27.2 JavaScript source code formats 27.3 Before we had modules, we had scripts
- 10. 27.4 Module systems created prior to ES6 27.5 ECMAScript modules 27.6 Named exports and imports 27.7 Default exports and imports 27.8 More details on exporting and importing 27.9 npm packages 27.10 Naming modules 27.11 Module specifiers 27.12 Loading modules dynamically via import() 27.13 Preview: import.meta.url 27.14 Polyfills: emulating native web platform features (advanced) 28 Single objects 28.1 What is an object? 28.2 Objects as records 28.3 Spreading into object literals (...) 28.4 Methods 28.5 Objects as dictionaries (advanced) 28.6 Standard methods (advanced) 28.7 Advanced topics 29 Prototype chains and classes 29.1 Prototype chains 29.2 Classes 29.3 Private data for classes 29.4 Subclassing 29.5 FAQ: objects 30 Synchronous iteration
- 11. 30.1 What is synchronous iteration about? 30.2 Core iteration constructs: iterables and iterators 30.3 Iterating manually 30.4 Iteration in practice 30.5 Quick reference: synchronous iteration 31 Arrays (Array) 31.1 The two roles of Arrays in JavaScript 31.2 Basic Array operations 31.3 for-of and Arrays 31.4 Array-like objects 31.5 Converting iterable and Array-like values to Arrays 31.6 Creating and filling Arrays with arbitrary lengths 31.7 Multidimensional Arrays 31.8 More Array features (advanced) 31.9 Adding and removing elements (destructively and non- destructively) 31.10 Methods: iteration and transformation (.find(), .map(), .filter(), etc.) 31.11 .sort(): sorting Arrays 31.12 Quick reference: Array<T> 32 Typed Arrays: handling binary data (Advanced) 32.1 The basics of the API 32.2 Element types 32.3 More information on Typed Arrays 32.4 Quick references: indices vs. offsets 32.5 Quick reference: ArrayBuffers 32.6 Quick reference: Typed Arrays 32.7 Quick reference: DataViews
- 12. 33 Maps (Map) 33.1 Using Maps 33.2 Example: Counting characters 33.3 A few more details about the keys of Maps (advanced) 33.4 Missing Map operations 33.5 Quick reference: Map<K,V> 33.6 FAQ: Maps 34 WeakMaps (WeakMap) 34.1 WeakMaps are black boxes 34.2 The keys of a WeakMap are weakly held 34.3 Examples 34.4 WeakMap API 35 Sets (Set) 35.1 Using Sets 35.2 Examples of using Sets 35.3 What Set elements are considered equal? 35.4 Missing Set operations 35.5 Quick reference: Set<T> 35.6 FAQ: Sets 36 WeakSets (WeakSet) 36.1 Example: Marking objects as safe to use with a method 36.2 WeakSet API 37 Destructuring 37.1 A first taste of destructuring 37.2 Constructing vs. extracting 37.3 Where can we destructure? 37.4 Object-destructuring
- 13. 37.5 Array-destructuring 37.6 Examples of destructuring 37.7 What happens if a pattern part does not match anything? 37.8 What values can’t be destructured? 37.9 (Advanced) 37.10 Default values 37.11 Parameter definitions are similar to destructuring 37.12 Nested destructuring 38 Synchronous generators (advanced) 38.1 What are synchronous generators? 38.2 Calling generators from generators (advanced) 38.3 Background: external iteration vs. internal iteration 38.4 Use case for generators: reusing traversals 38.5 Advanced features of generators 39 Asynchronous programming in JavaScript 39.1 A roadmap for asynchronous programming in JavaScript 39.2 The call stack 39.3 The event loop 39.4 How to avoid blocking the JavaScript process 39.5 Patterns for delivering asynchronous results 39.6 Asynchronous code: the downsides 39.7 Resources 40 Promises for asynchronous programming 40.1 The basics of using Promises 40.2 Examples 40.3 Error handling: don’t mix rejections and exceptions
- 14. 40.4 Promise-based functions start synchronously, settle asynchronously 40.5 Promise.all(): concurrency and Arrays of Promises 40.6 Tips for chaining Promises 40.7 Advanced topics 41 Async functions 41.1 Async functions: the basics 41.2 Returning from async functions 41.3 await: working with Promises 41.4 (Advanced) 41.5 Immediately invoked async arrow functions 41.6 Concurrency and await 41.7 Tips for using async functions 42 Asynchronous iteration 42.1 Basic asynchronous iteration 42.2 Asynchronous generators 42.3 Async iteration over Node.js streams 43 Regular expressions (RegExp) 43.1 Creating regular expressions 43.2 Syntax 43.3 Flags 43.4 Properties of regular expression objects 43.5 Methods for working with regular expressions 43.6 Flag /g and its pitfalls 43.7 Techniques for working with regular expressions 44 Dates (Date) 44.1 Best practice: avoid the built-in Date
- 15. 44.2 Time standards 44.3 Background: date time formats (ISO) 44.4 Time values 44.5 Creating Dates 44.6 Getters and setters 44.7 Converting Dates to strings 45 Creating and parsing JSON (JSON) 45.1 The discovery and standardization of JSON 45.2 JSON syntax 45.3 Using the JSON API 45.4 Customizing stringification and parsing (advanced) 45.5 FAQ 46 Next steps: overview of web development (bonus) 46.1 Tips against feeling overwhelmed 46.2 Things worth learning for web development 46.3 Example: tool-based JavaScript workflow 46.4 An overview of JavaScript tools 46.5 Tools not related to JavaScript 47 Index
- 17. 1 About this book (ES2019 edition) 1.1 About the content 1.1.1 What’s in this book? 1.1.2 What is not covered by this book? 1.1.3 Isn’t this book too long for impatient people? 1.2 Previewing and buying this book 1.2.1 How can I preview the book, the exercises, and the quizzes? 1.2.2 How can I buy a digital edition of this book? 1.2.3 How can I buy the print edition of this book? 1.3 About the author 1.4 Acknowledgements
- 18. 1.1 About the content 1.1.1 What’s in this book? This book makes JavaScript less challenging to learn for newcomers by offering a modern view that is as consistent as possible. Highlights: Get started quickly by initially focusing on modern features. Test-driven exercises and quizzes available for most chapters. Covers all essential features of JavaScript, up to and including ES2019. Optional advanced sections let you dig deeper. No prior knowledge of JavaScript is required, but you should know how to program. 1.1.2 What is not covered by this book? Some advanced language features are not explained, but references to appropriate material are provided – for example, to my other JavaScript books at ExploringJS.com, which are free to read online. This book deliberately focuses on the language. Browser-only features, etc. are not described.
- 19. 1.1.3 Isn’t this book too long for impatient people? There are several ways in which you can read this book. One of them involves skipping much of the content in order to get started quickly. For details, see §2.1.1 “In which order should I read the content in this book?”.
- 20. 1.2 Previewing and buying this book 1.2.1 How can I preview the book, the exercises, and the quizzes? Go to the homepage of this book: All essential chapters of this book can be read online. The first half of the test-driven exercises can be downloaded. The first half of the quizzes can be tried online. 1.2.2 How can I buy a digital edition of this book? There are two digital editions of JavaScript for impatient programmers: Ebooks: PDF, EPUB, MOBI, HTML (all without DRM) Ebooks plus exercises and quizzes The home page of this book describes how you can buy them. 1.2.3 How can I buy the print edition of this book? The print edition of JavaScript for impatient programmers is available on Amazon.
- 21. 1.3 About the author Dr. Axel Rauschmayer specializes in JavaScript and web development. He has been developing web applications since 1995. In 1999, he was technical manager at a German internet startup that later expanded internationally. In 2006, he held his first talk on Ajax. In 2010, he received a PhD in Informatics from the University of Munich. Since 2011, he has been blogging about web development at 2ality.com and has written several books on JavaScript. He has held trainings and talks for companies such as eBay, Bank of America, and O’Reilly Media. He lives in Munich, Germany.
- 22. 1.4 Acknowledgements Cover by Fran Caye Parts of this book were edited by Adaobi Obi Tulton. Thanks for answering questions, discussing language topics, etc.: Allen Wirfs-Brock (@awbjs) Benedikt Meurer (@bmeurer) Brian Terlson (@bterlson) Daniel Ehrenberg (@littledan) Jordan Harband (@ljharb) Mathias Bynens (@mathias) Myles Borins (@MylesBorins) Rob Palmer (@robpalmer2) Šime Vidas (@simevidas) And many others Thanks for reviewing: Johannes Weber (@jowe) [Generated: 2019-08-31 17:39]
- 23. 2 FAQ: Book and supplementary material 2.1 How to read this book 2.1.1 In which order should I read the content in this book? 2.1.2 Why are some chapters and sections marked with “(advanced)”? 2.1.3 Why are some chapters marked with “(bonus)”? 2.2 I own a digital edition 2.2.1 How do I submit feedback and corrections? 2.2.2 How do I get updates for the downloads I bought at Payhip? 2.2.3 Can I upgrade from package “Ebooks” to package “Ebooks + exercises + quizzes”? 2.3 I own the print edition 2.3.1 Can I get a discount for a digital edition? 2.3.2 Can I submit an error or see submitted errors? 2.3.3 Is there an online list with the URLs in this book? 2.4 Notations and conventions 2.4.1 What is a type signature? Why am I seeing static types in this book? 2.4.2 What do the notes with icons mean? This chapter answers questions you may have and gives tips for reading this book.
- 24. 2.1 How to read this book 2.1.1 In which order should I read the content in this book? This book is three books in one: You can use it to get started with JavaScript as quickly as possible. This “mode” is for impatient people: Start reading with §7 “The big picture”. Skip all chapters and sections marked as “advanced”, and all quick references. It gives you a comprehensive look at current JavaScript. In this “mode”, you read everything and don’t skip advanced content and quick references. It serves as a reference. If there is a topic that you are interested in, you can find information on it via the table of contents or via the index. Due to basic and advanced content being mixed, everything you need is usually in a single location. The quizzes and exercises play an important part in helping you practice and retain what you have learned. 2.1.2 Why are some chapters and sections marked with “(advanced)”? Several chapters and sections are marked with “(advanced)”. The idea is that you can initially skip them. That is, you can get a quick
- 25. working knowledge of JavaScript by only reading the basic (non- advanced) content. As your knowledge evolves, you can later come back to some or all of the advanced content. 2.1.3 Why are some chapters marked with “(bonus)”? The bonus chapters are only available in the paid versions of this book (print and ebook). They are listed in the full table of contents.
- 26. 2.2 I own a digital edition 2.2.1 How do I submit feedback and corrections? The HTML version of this book (online, or ad-free archive in the paid version) has a link at the end of each chapter that enables you to give feedback. 2.2.2 How do I get updates for the downloads I bought at Payhip? The receipt email for the purchase includes a link. You’ll always be able to download the latest version of the files at that location. If you opted into emails while buying, you’ll get an email whenever there is new content. To opt in later, you must contact Payhip (see bottom of payhip.com). 2.2.3 Can I upgrade from package “Ebooks” to package “Ebooks + exercises + quizzes”? Yes. The instructions for doing so are on the homepage of this book.
- 27. 2.3 I own the print edition 2.3.1 Can I get a discount for a digital edition? If you bought the print edition, you can get a discount for a digital edition. The homepage of the print edition explains how. Alas, the reverse is not possible: you cannot get a discount for the print edition if you bought a digital edition. 2.3.2 Can I submit an error or see submitted errors? On the homepage of the print edition, you can submit errors and see submitted errors. 2.3.3 Is there an online list with the URLs in this book? The homepage of the print edition has a list with all the URLs that you see in the footnotes of the print edition.
- 28. 2.4 Notations and conventions 2.4.1 What is a type signature? Why am I seeing static types in this book? For example, you may see: That is called the type signature of Number.isFinite(). This notation, especially the static types number of num and boolean of the result, are not real JavaScript. The notation is borrowed from the compile-to- JavaScript language TypeScript (which is mostly just JavaScript plus static typing). Why is this notation being used? It helps give you a quick idea of how a function works. The notation is explained in detail in a 2ality blog post, but is usually relatively intuitive. 2.4.2 What do the notes with icons mean? Reading instructions Explains how to best read the content. External content Points to additional, external, content. Number.isFinite(num: number): boolean
- 29. Tip Gives a tip related to the current content. Question Asks and answers a question pertinent to the current content (think FAQ). Warning Warns about pitfalls, etc. Details Provides additional details, complementing the current content. It is similar to a footnote. Exercise Mentions the path of a test-driven exercise that you can do at that point. Quiz Indicates that there is a quiz for the current (part of a) chapter.
- 30. 3 Why JavaScript? (bonus) 3.1 The cons of JavaScript 3.2 The pros of JavaScript 3.2.1 Community 3.2.2 Practically useful 3.2.3 Language 3.3 Pro and con of JavaScript: innovation In this chapter, we examine the pros and cons of JavaScript. “ECMAScript 6” and “ES6” refer to versions of JavaScript ECMAScript is the name of the language standard; the number refers to the version of that standard. For more information, consult §5.2 “Standardizing JavaScript”.
- 31. 3.1 The cons of JavaScript Among programmers, JavaScript isn’t always well liked. One reason is that it has a fair amount of quirks. Some of them are just unusual ways of doing something. Others are considered bugs. Either way, learning why JavaScript does something the way it does, helps with dealing with the quirks and with accepting JavaScript (maybe even liking it). Hopefully, this book can be of assistance here. Additionally, many traditional quirks have been eliminated now. For example: Traditionally, JavaScript variables weren’t block-scoped. ES6 introduced let and const, which let you declare block-scoped variables. Prior to ES6, implementing object factories and inheritance via function and .prototype was clumsy. ES6 introduced classes, which provide more convenient syntax for these mechanisms. Traditionally, JavaScript did not have built-in modules. ES6 added them to the language. Lastly, JavaScript’s standard library is limited, but: There are plans for adding more functionality. Many libraries are easily available via the npm software registry.
- 32. 3.2 The pros of JavaScript On the plus side, JavaScript offers many benefits. 3.2.1 Community JavaScript’s popularity means that it’s well supported and well documented. Whenever you create something in JavaScript, you can rely on many people being (potentially) interested. And there is a large pool of JavaScript programmers from which you can hire, if you need to. No single party controls JavaScript – it is evolved by TC39, a committee comprising many organizations. The language is evolved via an open process that encourages feedback from the public. 3.2.2 Practically useful With JavaScript, you can write apps for many client platforms. These are a few example technologies: Progressive Web Apps can be installed natively on Android and many desktop operating systems. Electron lets you build cross-platform desktop apps. React Native lets you write apps for iOS and Android that have native user interfaces. Node.js provides extensive support for writing shell scripts (in addition to being a platform for web servers).
- 33. JavaScript is supported by many server platforms and services – for example: Node.js (many of the following services are based on Node.js or support its APIs) ZEIT Now Microsoft Azure Functions AWS Lambda Google Cloud Functions There are many data technologies available for JavaScript: many databases support it and intermediate layers (such as GraphQL) exist. Additionally, the standard data format JSON (JavaScript Object Notation) is based on JavaScript and supported by its standard library. Lastly, many, if not most, tools for JavaScript are written in JavaScript. That includes IDEs, build tools, and more. As a consequence, you install them the same way you install your libraries and you can customize them in JavaScript. 3.2.3 Language Many libraries are available, via the de-facto standard in the JavaScript universe, the npm software registry. If you are unhappy with “plain” JavaScript, it is relatively easy to add more features: You can compile future and modern language features to current and past versions of JavaScript, via Babel.
- 34. You can add static typing, via TypeScript and Flow. You can work with ReasonML, which is, roughly, OCaml with JavaScript syntax. It can be compiled to JavaScript or native code. The language is flexible: it is dynamic and supports both object- oriented programming and functional programming. JavaScript has become suprisingly fast for such a dynamic language. Whenever it isn’t fast enough, you can switch to WebAssembly, a universal virtual machine built into most JavaScript engines. It can run static code at nearly native speeds.
- 35. 3.3 Pro and con of JavaScript: innovation There is much innovation in the JavaScript ecosystem: new approaches to implementing user interfaces, new ways of optimizing the delivery of software, and more. The upside is that you will constantly learn new things. The downside is that the constant change can be exhausting at times. Thankfully, things have somewhat slowed down, recently: all of ES6 (which was a considerable modernization of the language) is becoming established, as are certain tools and workflows. Quiz See quiz app.
- 36. 4 The nature of JavaScript (bonus) 4.1 JavaScript’s influences 4.2 The nature of JavaScript 4.2.1 JavaScript often fails silently 4.3 Tips for getting started with JavaScript
- 37. 4.1 JavaScript’s influences When JavaScript was created in 1995, it was influenced by several programming languages: JavaScript’s syntax is largely based on Java. Self inspired JavaScript’s prototypal inheritance. Closures and environments were borrowed from Scheme. AWK influenced JavaScript’s functions (including the keyword function). JavaScript’s strings, Arrays, and regular expressions take cues from Perl. HyperTalk inspired event handling via onclick in web browsers. With ECMAScript 6, new influences came to JavaScript: Generators were borrowed from Python. The syntax of arrow functions came from CoffeeScript. C++ contributed the keyword const. Destructuring was inspired by Lisp’s destructuring bind. Template literals came from the E language (where they are called quasi literals).
- 38. 4.2 The nature of JavaScript These are a few traits of the language: Its syntax is part of the C family of languages (curly braces, etc.). It is a dynamic language: most objects can be changed in various ways at runtime, objects can be created directly, etc. It is a dynamically typed language: variables don’t have fixed static types and you can assign any value to a given (mutable) variable. It has functional programming features: first-class functions, closures, partial application via bind(), etc. It has object-oriented features: mutable state, objects, inheritance, classes, etc. It often fails silently: see the next subsection for details. It is deployed as source code. But that source code is often minified (rewritten to require less storage). And there are plans for a binary source code format. JavaScript is part of the web platform – it is the language built into web browsers. But it is also used elsewhere – for example, in Node.js, for server things, and shell scripting.
- 39. JavaScript engines often optimize less-efficient language mechanisms under the hood. For example, in principle, JavaScript Arrays are dictionaries. But under the hood, engines store Arrays contiguously if they have contiguous indices. 4.2.1 JavaScript often fails silently JavaScript often fails silently. Let’s look at two examples. First example: If the operands of an operator don’t have the appropriate types, they are converted as necessary. Second example: If an arithmetic computation fails, you get an error value, not an exception. The reason for the silent failures is historical: JavaScript did not have exceptions until ECMAScript 3. Since then, its designers have tried to avoid silent failures. > '3' * '5' 15 > 1 / 0 Infinity
- 40. 4.3 Tips for getting started with JavaScript These are a few tips to help you get started with JavaScript: Take your time to really get to know this language. The conventional C-style syntax hides that this is a very unconventional language. Learn especially the quirks and the rationales behind them. Then you will understand and appreciate the language better. In addition to details, this book also teaches simple rules of thumb to be safe – for example, “Always use === to determine if two values are equal, never ==.” Language tools make it easier to work with JavaScript. For example: You can statically type JavaScript via TypeScript or Flow. You can check for problems and anti-patterns via linters such as ESLint. You can format your code automatically via code formatters such as Prettier. Get in contact with the community: Twitter is popular among JavaScript programmers. As a mode of communication that sits between the spoken and the written word, it is well suited for exchanging knowledge. Many cities have regular free meetups where people come together to learn topics related to JavaScript.
- 41. JavaScript conferences are another convenient way of meeting other JavaScript programmers. Read books and blogs. Much material is free online!
- 42. 5 History and evolution of JavaScript 5.1 How JavaScript was created 5.2 Standardizing JavaScript 5.3 Timeline of ECMAScript versions 5.4 Ecma Technical Committee 39 (TC39) 5.5 The TC39 process 5.5.1 Tip: Think in individual features and stages, not ECMAScript versions 5.6 FAQ: TC39 process 5.6.1 How is [my favorite proposed feature] doing? 5.6.2 Is there an official list of ECMAScript features? 5.7 Evolving JavaScript: Don’t break the web
- 43. 5.1 How JavaScript was created JavaScript was created in May 1995 in 10 days, by Brendan Eich. Eich worked at Netscape and implemented JavaScript for their web browser, Netscape Navigator. The idea was that major interactive parts of the client-side web were to be implemented in Java. JavaScript was supposed to be a glue language for those parts and to also make HTML slightly more interactive. Given its role of assisting Java, JavaScript had to look like Java. That ruled out existing solutions such as Perl, Python, TCL, and others. Initially, JavaScript’s name changed several times: Its code name was Mocha. In the Netscape Navigator 2.0 betas (September 1995), it was called LiveScript. In Netscape Navigator 2.0 beta 3 (December 1995), it got its final name, JavaScript.
- 44. 5.2 Standardizing JavaScript There are two standards for JavaScript: ECMA-262 is hosted by Ecma International. It is the primary standard. ISO/IEC 16262 is hosted by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). This is a secondary standard. The language described by these standards is called ECMAScript, not JavaScript. A different name was chosen because Sun (now Oracle) had a trademark for the latter name. The “ECMA” in “ECMAScript” comes from the organization that hosts the primary standard. The original name of that organization was ECMA, an acronym for European Computer Manufacturers Association. It was later changed to Ecma International (with “Ecma” being a proper name, not an acronym) because the organization’s activities had expanded beyond Europe. The initial all-caps acronym explains the spelling of ECMAScript. In principle, JavaScript and ECMAScript mean the same thing. Sometimes the following distinction is made: The term JavaScript refers to the language and its implementations. The term ECMAScript refers to the language standard and language versions.
- 45. Therefore, ECMAScript 6 is a version of the language (its 6th edition).
- 46. 5.3 Timeline of ECMAScript versions This is a brief timeline of ECMAScript versions: ECMAScript 1 (June 1997): First version of the standard. ECMAScript 2 (June 1998): Small update to keep ECMA-262 in sync with the ISO standard. ECMAScript 3 (December 1999): Adds many core features – “[…] regular expressions, better string handling, new control statements [do-while, switch], try/catch exception handling, […]” ECMAScript 4 (abandoned in July 2008): Would have been a massive upgrade (with static typing, modules, namespaces, and more), but ended up being too ambitious and dividing the language’s stewards. ECMAScript 5 (December 2009): Brought minor improvements – a few standard library features and strict mode. ECMAScript 5.1 (June 2011): Another small update to keep Ecma and ISO standards in sync. ECMAScript 6 (June 2015): A large update that fulfilled many of the promises of ECMAScript 4. This version is the first one whose official name – ECMAScript 2015 – is based on the year of publication. ECMAScript 2016 (June 2016): First yearly release. The shorter release life cycle resulted in fewer new features compared to the large ES6.
- 47. ECMAScript 2017 (June 2017). Second yearly release. Subsequent ECMAScript versions (ES2018, etc.) are always ratified in June.
- 48. 5.4 Ecma Technical Committee 39 (TC39) TC39 is the committee that evolves JavaScript. Its member are, strictly speaking, companies: Adobe, Apple, Facebook, Google, Microsoft, Mozilla, Opera, Twitter, and others. That is, companies that are usually fierce competitors are working together for the good of the language. Every two months, TC39 has meetings that member-appointed delegates and invited experts attend. The minutes of those meetings are public in a GitHub repository.
- 49. 5.5 The TC39 process With ECMAScript 6, two issues with the release process used at that time became obvious: If too much time passes between releases then features that are ready early, have to wait a long time until they can be released. And features that are ready late, risk being rushed to make the deadline. Features were often designed long before they were implemented and used. Design deficiencies related to implementation and use were therefore discovered too late. In response to these issues, TC39 instituted the new TC39 process: ECMAScript features are designed independently and go through stages, starting at 0 (“strawman”), ending at 4 (“finished”). Especially the later stages require prototype implementations and real-world testing, leading to feedback loops between designs and implementations. ECMAScript versions are released once per year and include all features that have reached stage 4 prior to a release deadline. The result: smaller, incremental releases, whose features have already been field-tested. Fig. 1 illustrates the TC39 process.
- 50. Stage 0: strawman Stage 1: proposal Stage 2: draft Stage 3: candidate Stage 4: finished Pick champions First spec text, 2 implementations Spec complete Test 262 acceptance tests Review at TC39 meeting TC39 helps Likely to be standardized Done, needs feedback from implementations Ready for standardization Sketch Figure 1: Each ECMAScript feature proposal goes through stages that are numbered from 0 to 4. Champions are TC39 members that support the authors of a feature. Test 262 is a suite of tests that checks JavaScript engines for compliance with the language specification. ES2016 was the first ECMAScript version that was designed according to the TC39 process. 5.5.1 Tip: Think in individual features and stages, not ECMAScript versions Up to and including ES6, it was most common to think about JavaScript in terms of ECMAScript versions – for example, “Does this browser support ES6 yet?” Starting with ES2016, it’s better to think in individual features: once a feature reaches stage 4, you can safely use it (if it’s supported by the
- 51. JavaScript engines you are targeting). You don’t have to wait until the next ECMAScript release.
- 52. 5.6 FAQ: TC39 process 5.6.1 How is [my favorite proposed feature] doing? If you are wondering what stages various proposed features are in, consult the GitHub repository proposals. 5.6.2 Is there an official list of ECMAScript features? Yes, the TC39 repo lists finished proposals and mentions in which ECMAScript versions they were introduced.
- 53. 5.7 Evolving JavaScript: Don’t break the web One idea that occasionally comes up is to clean up JavaScript by removing old features and quirks. While the appeal of that idea is obvious, it has significant downsides. Let’s assume we create a new version of JavaScript that is not backward compatible and fix all of its flaws. As a result, we’d encounter the following problems: JavaScript engines become bloated: they need to support both the old and the new version. The same is true for tools such as IDEs and build tools. Programmers need to know, and be continually conscious of, the differences between the versions. You can either migrate all of an existing code base to the new version (which can be a lot of work). Or you can mix versions and refactoring becomes harder because you can’t move code between versions without changing it. You somehow have to specify per piece of code – be it a file or code embedded in a web page – what version it is written in. Every conceivable solution has pros and cons. For example, strict mode is a slightly cleaner version of ES5. One of the reasons why it wasn’t as popular as it should have been: it was a hassle to opt in via a directive at the beginning of a file or a function.
- 54. So what is the solution? Can we have our cake and eat it? The approach that was chosen for ES6 is called “One JavaScript”: New versions are always completely backward compatible (but there may occasionally be minor, hardly noticeable clean-ups). Old features aren’t removed or fixed. Instead, better versions of them are introduced. One example is declaring variables via let – which is an improved version of var. If aspects of the language are changed, it is done inside new syntactic constructs. That is, you opt in implicitly. For example, yield is only a keyword inside generators (which were introduced in ES6). And all code inside modules and classes (both introduced in ES6) is implicitly in strict mode. Quiz See quiz app.
- 55. 6 FAQ: JavaScript 6.1 What are good references for JavaScript? 6.2 How do I find out what JavaScript features are supported where? 6.3 Where can I look up what features are planned for JavaScript? 6.4 Why does JavaScript fail silently so often? 6.5 Why can’t we clean up JavaScript, by removing quirks and outdated features? 6.6 How can I quickly try out a piece of JavaScript code?
- 56. 6.1 What are good references for JavaScript? Please consult §7.3 “JavaScript references”.
- 57. 6.2 How do I find out what JavaScript features are supported where? This book usually mentions if a feature is part of ECMAScript 5 (as required by older browsers) or a newer version. For more detailed information (including pre-ES5 versions), there are several good compatibility tables available online: ECMAScript compatibility tables for various engines (by kangax, webbedspace, zloirock) Node.js compatibility tables (by William Kapke) Mozilla’s MDN web docs have tables for each feature that describe relevant ECMAScript versions and browser support. “Can I use…” documents what features (including JavaScript language features) are supported by web browsers.
- 58. 6.3 Where can I look up what features are planned for JavaScript? Please consult the following sources: §5.5 “The TC39 process” describes how upcoming features are planned. §5.6 “FAQ: TC39 process” answers various questions regarding upcoming features.
- 59. 6.4 Why does JavaScript fail silently so often? JavaScript often fails silently. Let’s look at two examples. First example: If the operands of an operator don’t have the appropriate types, they are converted as necessary. Second example: If an arithmetic computation fails, you get an error value, not an exception. The reason for the silent failures is historical: JavaScript did not have exceptions until ECMAScript 3. Since then, its designers have tried to avoid silent failures. > '3' * '5' 15 > 1 / 0 Infinity
- 60. 6.5 Why can’t we clean up JavaScript, by removing quirks and outdated features? This question is answered in §5.7 “Evolving JavaScript: Don’t break the web”.
- 61. 6.6 How can I quickly try out a piece of JavaScript code? §9.1 “Trying out JavaScript code” explains how to do that.
- 62. 7 The big picture 7.1 What are you learning in this book? 7.2 The structure of browsers and Node.js 7.3 JavaScript references 7.4 Further reading In this chapter, I’d like to paint the big picture: what are you learning in this book, and how does it fit into the overall landscape of web development?
- 63. 7.1 What are you learning in this book? This book teaches the JavaScript language. It focuses on just the language, but offers occasional glimpses at two platforms where JavaScript can be used: Web browser Node.js Node.js is important for web development in three ways: You can use it to write server-side software in JavaScript. You can also use it to write software for the command line (think Unix shell, Windows PowerShell, etc.). Many JavaScript-related tools are based on (and executed via) Node.js. Node’s software registry, npm, has become the dominant way of installing tools (such as compilers and build tools) and libraries – even for client-side development.
- 64. 7.2 The structure of browsers and Node.js JavaScript engine Platform core JS standard library Platform API Figure 2: The structure of the two JavaScript platforms web browser and Node.js. The APIs “standard library” and “platform API” are hosted on top of a foundational layer with a JavaScript engine and a platform-specific “core”. The structures of the two JavaScript platforms web browser and Node.js are similar (fig. 2): The foundational layer consists of the JavaScript engine and platform-specific “core” functionality. Two APIs are hosted on top of this foundation: The JavaScript standard library is part of JavaScript proper and runs on top of the engine. The platform API are also available from JavaScript – it provides access to platform-specific functionality. For example: In browsers, you need to use the platform-specific API if you want to do anything related to the user interface: react to mouse clicks, play sounds, etc. In Node.js, the platform-specific API lets you read and write files, download data via HTTP, etc.
- 65. 7.3 JavaScript references When you have a question about a JavaScript, a web search usually helps. I can recommend the following online sources: MDN web docs: cover various web technologies such as CSS, HTML, JavaScript, and more. An excellent reference. Node.js Docs: document the Node.js API. ExploringJS.com: My other books on JavaScript go into greater detail than this book and are free to read online. You can look up features by ECMAScript version: ES1–ES5: Speaking JavaScript ES6: Exploring ES6 ES2016–ES2017: Exploring ES2016 and ES2017 Etc.
- 66. 7.4 Further reading §46 “Next steps: overview of web development” provides a more comprehensive look at web development.
- 67. 8 Syntax 8.1 An overview of JavaScript’s syntax 8.1.1 Basic syntax 8.1.2 Modules 8.1.3 Legal variable and property names 8.1.4 Casing styles 8.1.5 Capitalization of names 8.1.6 More naming conventions 8.1.7 Where to put semicolons? 8.2 (Advanced) 8.3 Identifiers 8.3.1 Valid identifiers (variable names, etc.) 8.3.2 Reserved words 8.4 Statement vs. expression 8.4.1 Statements 8.4.2 Expressions 8.4.3 What is allowed where? 8.5 Ambiguous syntax 8.5.1 Same syntax: function declaration and function expression 8.5.2 Same syntax: object literal and block 8.5.3 Disambiguation 8.6 Semicolons 8.6.1 Rule of thumb for semicolons 8.6.2 Semicolons: control statements
- 68. 8.7 Automatic semicolon insertion (ASI) 8.7.1 ASI triggered unexpectedly 8.7.2 ASI unexpectedly not triggered 8.8 Semicolons: best practices 8.9 Strict mode vs. sloppy mode 8.9.1 Switching on strict mode 8.9.2 Improvements in strict mode
- 69. 8.1 An overview of JavaScript’s syntax 8.1.1 Basic syntax Comments: Primitive (atomic) values: An assertion describes what the result of a computation is expected to look like and throws an exception if those expectations aren’t correct. For example, the following assertion states that the result of the computation 7 plus 1 must be 8: // single-line comment /* Comment with multiple lines */ // Booleans true false // Numbers (JavaScript only has a single type for numbers) -123 1.141 // Strings (JavaScript has no type for characters) 'abc' "abc" assert.equal(7 + 1, 8);
- 70. assert.equal() is a method call (the object is assert, the method is .equal()) with two arguments: the actual result and the expected result. It is part of a Node.js assertion API that is explained later in this book. Logging to the console of a browser or Node.js: Operators: Declaring variables: // Printing a value to standard out (another method call) console.log('Hello!'); // Printing error information to standard error console.error('Something went wrong!'); // Operators for booleans assert.equal(true && false, false); // And assert.equal(true || false, true); // Or // Operators for numbers assert.equal(3 + 4, 7); assert.equal(5 - 1, 4); assert.equal(3 * 4, 12); assert.equal(9 / 3, 3); // Operators for strings assert.equal('a' + 'b', 'ab'); assert.equal('I see ' + 3 + ' monkeys', 'I see 3 monkeys'); // Comparison operators assert.equal(3 < 4, true); assert.equal(3 <= 4, true); assert.equal('abc' === 'abc', true); assert.equal('abc' !== 'def', true);
- 71. Control flow statements: Ordinary function declarations: Arrow function expressions (used especially as arguments of function calls and method calls): The previous code contains the following two arrow functions (the terms expression and statement are explained later in this chapter): let x; // declaring x (mutable) x = 3 * 5; // assign a value to x let y = 3 * 5; // declaring and assigning const z = 8; // declaring z (immutable) // Conditional statement if (x < 0) { // is x less than zero? x = -x; } // add1() has the parameters a and b function add1(a, b) { return a + b; } // Calling function add1() assert.equal(add1(5, 2), 7); const add2 = (a, b) => { return a + b }; // Calling function add2() assert.equal(add2(5, 2), 7); // Equivalent to add2: const add3 = (a, b) => a + b; // An arrow function whose body is a code block (a, b) => { return a + b }
- 72. Objects: Arrays (Arrays are also objects): 8.1.2 Modules Each module is a single file. Consider, for example, the following two files with modules in them: // An arrow function whose body is an expression (a, b) => a + b // Creating a plain object via an object literal const obj = { first: 'Jane', // property last: 'Doe', // property getFullName() { // property (method) return this.first + ' ' + this.last; }, }; // Getting a property value assert.equal(obj.first, 'Jane'); // Setting a property value obj.first = 'Janey'; // Calling the method assert.equal(obj.getFullName(), 'Janey Doe'); // Creating an Array via an Array literal const arr = ['a', 'b', 'c']; // Getting an Array element assert.equal(arr[1], 'b'); // Setting an Array element arr[1] = 'β';
- 73. file-tools.mjs main.mjs The module in file-tools.mjs exports its function isTextFilePath(): The module in main.mjs imports the whole module path and the function isTextFilePath(): 8.1.3 Legal variable and property names The grammatical category of variable names and property names is called identifier. Identifiers are allowed to have the following characters: Unicode letters: A–Z, a–z (etc.) $, _ Unicode digits: 0–9 (etc.) Variable names can’t start with a digit Some words have special meaning in JavaScript and are called reserved. Examples include: if, true, const. Reserved words can’t be used as variable names: export function isTextFilePath(filePath) { return filePath.endsWith('.txt'); } // Import whole module as namespace object `path` import * as path from 'path'; // Import a single export of module file-tools.mjs import {isTextFilePath} from './file-tools.mjs';
- 74. But they are allowed as names of properties: 8.1.4 Casing styles Common casing styles for concatenating words are: Camel case: threeConcatenatedWords Underscore case (also called snake case): three_concatenated_words Dash case (also called kebab case): three-concatenated-words 8.1.5 Capitalization of names In general, JavaScript uses camel case, except for constants. Lowercase: Functions, variables: myFunction Methods: obj.myMethod CSS: CSS entity: special-class Corresponding JavaScript variable: specialClass Uppercase: const if = 123; // SyntaxError: Unexpected token if > const obj = { if: 123 }; > obj.if 123
- 75. Classes: MyClass Constants: MY_CONSTANT Constants are also often written in camel case: myConstant 8.1.6 More naming conventions The following naming conventions are popular in JavaScript. If the name of a parameter starts with an underscore (or is an underscore) it means that this parameter is not used – for example: If the name of a property of an object starts with an underscore then that property is considered private: 8.1.7 Where to put semicolons? At the end of a statement: But not if that statement ends with a curly brace: arr.map((_x, i) => i) class ValueWrapper { constructor(value) { this._value = value; } } const x = 123; func(); while (false) { // ··· } // no semicolon
- 76. However, adding a semicolon after such a statement is not a syntax error – it is interpreted as an empty statement: Quiz: basic See quiz app. function func() { // ··· } // no semicolon // Function declaration followed by empty statement: function func() { // ··· };
- 77. 8.2 (Advanced) All remaining sections of this chapter are advanced.
- 78. 8.3 Identifiers 8.3.1 Valid identifiers (variable names, etc.) First character: Unicode letter (including accented characters such as é and ü and characters from non-latin alphabets, such as α) $ _ Subsequent characters: Legal first characters Unicode digits (including Eastern Arabic numerals) Some other Unicode marks and punctuations Examples: 8.3.2 Reserved words Reserved words can’t be variable names, but they can be property names. const ε = 0.0001; const строка = ''; let _tmp = 0; const $foo2 = true;
- 79. All JavaScript keywords are reserved words: await break case catch class const continue debugger default delete do else export extends finally for function if import in instanceof let new return static super switch this throw try typeof var void while with yield The following tokens are also keywords, but currently not used in the language: enum implements package protected interface private public The following literals are reserved words: true false null Technically, these words are not reserved, but you should avoid them, too, because they effectively are keywords: Infinity NaN undefined async You shouldn’t use the names of global variables (String, Math, etc.) for your own variables and parameters, either.
- 80. 8.4 Statement vs. expression In this section, we explore how JavaScript distinguishes two kinds of syntactic constructs: statements and expressions. Afterward, we’ll see that that can cause problems because the same syntax can mean different things, depending on where it is used. We pretend there are only statements and expressions For the sake of simplicity, we pretend that there are only statements and expressions in JavaScript. 8.4.1 Statements A statement is a piece of code that can be executed and performs some kind of action. For example, if is a statement: One more example of a statement: a function declaration. 8.4.2 Expressions let myStr; if (myBool) { myStr = 'Yes'; } else { myStr = 'No'; } function twice(x) { return x + x; }
- 81. An expression is a piece of code that can be evaluated to produce a value. For example, the code between the parentheses is an expression: The operator _?_:_ used between the parentheses is called the ternary operator. It is the expression version of the if statement. Let’s look at more examples of expressions. We enter expressions and the REPL evaluates them for us: 8.4.3 What is allowed where? The current location within JavaScript source code determines which kind of syntactic constructs you are allowed to use: The body of a function must be a sequence of statements: let myStr = (myBool ? 'Yes' : 'No'); > 'ab' + 'cd' 'abcd' > Number('123') 123 > true || false true function max(x, y) { if (x > y) { return x; } else { return y; } }
- 82. The arguments of a function call or a method call must be expressions: However, expressions can be used as statements. Then they are called expression statements. The opposite is not true: when the context requires an expression, you can’t use a statement. The following code demonstrates that any expression bar() can be either expression or statement – it depends on the context: console.log('ab' + 'cd', Number('123')); function f() { console.log(bar()); // bar() is expression bar(); // bar(); is (expression) statement }
- 83. 8.5 Ambiguous syntax JavaScript has several programming constructs that are syntactically ambiguous: the same syntax is interpreted differently, depending on whether it is used in statement context or in expression context. This section explores the phenomenon and the pitfalls it causes. 8.5.1 Same syntax: function declaration and function expression A function declaration is a statement: A function expression is an expression (right-hand side of =): 8.5.2 Same syntax: object literal and block In the following code, {} is an object literal: an expression that creates an empty object. This is an empty code block (a statement): function id(x) { return x; } const id = function me(x) { return x; }; const obj = {};
- 84. 8.5.3 Disambiguation The ambiguities are only a problem in statement context: If the JavaScript parser encounters ambiguous syntax, it doesn’t know if it’s a plain statement or an expression statement. For example: If a statement starts with function: Is it a function declaration or a function expression? If a statement starts with {: Is it an object literal or a code block? To resolve the ambiguity, statements starting with function or { are never interpreted as expressions. If you want an expression statement to start with either one of these tokens, you must wrap it in parentheses: In this code: 1. We first create a function via a function expression: 2. Then we invoke that function: ('abc') The code fragment shown in (1) is only interpreted as an expression because we wrap it in parentheses. If we didn’t, we would get a { } (function (x) { console.log(x) })('abc'); // Output: // 'abc' function (x) { console.log(x) }
- 85. syntax error because then JavaScript expects a function declaration and complains about the missing function name. Additionally, you can’t put a function call immediately after a function declaration. Later in this book, we’ll see more examples of pitfalls caused by syntactic ambiguity: Assigning via object destructuring Returning an object literal from an arrow function
- 86. 8.6 Semicolons 8.6.1 Rule of thumb for semicolons Each statement is terminated by a semicolon: except statements ending with blocks: The following case is slightly tricky: The whole const declaration (a statement) ends with a semicolon, but inside it, there is an arrow function expression. That is, it’s not the statement per se that ends with a curly brace; it’s the embedded arrow function expression. That’s why there is a semicolon at the end. 8.6.2 Semicolons: control statements const x = 3; someFunction('abc'); i++; function foo() { // ··· } if (y > 0) { // ··· } const func = () => {}; // semicolon!
- 87. The body of a control statement is itself a statement. For example, this is the syntax of the while loop: The body can be a single statement: But blocks are also statements and therefore legal bodies of control statements: If you want a loop to have an empty body, your first option is an empty statement (which is just a semicolon): Your second option is an empty block: while (condition) statement while (a > 0) a--; while (a > 0) { a--; } while (processNextItem() > 0); while (processNextItem() > 0) {}
- 88. 8.7 Automatic semicolon insertion (ASI) While I recommend to always write semicolons, most of them are optional in JavaScript. The mechanism that makes this possible is called automatic semicolon insertion (ASI). In a way, it corrects syntax errors. ASI works as follows. Parsing of a statement continues until there is either: A semicolon A line terminator followed by an illegal token In other words, ASI can be seen as inserting semicolons at line breaks. The next subsections cover the pitfalls of ASI. 8.7.1 ASI triggered unexpectedly The good news about ASI is that – if you don’t rely on it and always write semicolons – there is only one pitfall that you need to be aware of. It is that JavaScript forbids line breaks after some tokens. If you do insert a line break, a semicolon will be inserted, too. The token where this is most practically relevant is return. Consider, for example, the following code: return {
- 89. This code is parsed as: That is: Return statement without operand: return; Start of code block: { Expression statement 'jane'; with label first: End of code block: } Empty statement: ; Why does JavaScript do this? It protects against accidentally returning a value in a line after a return. 8.7.2 ASI unexpectedly not triggered In some cases, ASI is not triggered when you think it should be. That makes life more complicated for people who don’t like semicolons because they need to be aware of those cases. The following are three examples. There are more. Example 1: Unintended function call. first: 'jane' }; return; { first: 'jane'; } ; a = b + c (d + e).print()
- 90. Parsed as: Example 2: Unintended division. Parsed as: Example 3: Unintended property access. Executed as: a = b + c(d + e).print(); a = b /hi/g.exec(c).map(d) a = b / hi / g.exec(c).map(d); someFunction() ['ul', 'ol'].map(x => x + x) const propKey = ('ul','ol'); // comma operator assert.equal(propKey, 'ol'); someFunction()[propKey].map(x => x + x);
- 91. 8.8 Semicolons: best practices I recommend that you always write semicolons: I like the visual structure it gives code – you clearly see when a statement ends. There are less rules to keep in mind. The majority of JavaScript programmers use semicolons. However, there are also many people who don’t like the added visual clutter of semicolons. If you are one of them: Code without them is legal. I recommend that you use tools to help you avoid mistakes. The following are two examples: The automatic code formatter Prettier can be configured to not use semicolons. It then automatically fixes problems. For example, if it encounters a line that starts with a square bracket, it prefixes that line with a semicolon. The static checker ESLint has a rule that you tell your preferred style (always semicolons or as few semicolons as possible) and that warns you about critical issues.
- 92. 8.9 Strict mode vs. sloppy mode Starting with ECMAScript 5, JavaScript has two modes in which JavaScript can be executed: Normal “sloppy” mode is the default in scripts (code fragments that are a precursor to modules and supported by browsers). Strict mode is the default in modules and classes, and can be switched on in scripts (how, is explained later). In this mode, several pitfalls of normal mode are removed and more exceptions are thrown. You’ll rarely encounter sloppy mode in modern JavaScript code, which is almost always located in modules. In this book, I assume that strict mode is always switched on. 8.9.1 Switching on strict mode In script files and CommonJS modules, you switch on strict mode for a complete file, by putting the following code in the first line: The neat thing about this “directive” is that ECMAScript versions before 5 simply ignore it: it’s an expression statement that does nothing. You can also switch on strict mode for just a single function: 'use strict';
- 93. 8.9.2 Improvements in strict mode Let’s look at three things that strict mode does better than sloppy mode. Just in this one section, all code fragments are executed in sloppy mode. 8.9.2.1 Sloppy mode pitfall: changing an undeclared variable creates a global variable In non-strict mode, changing an undeclared variable creates a global variable. Strict mode does it better and throws a ReferenceError. That makes it easier to detect typos. function functionInStrictMode() { 'use strict'; } function sloppyFunc() { undeclaredVar1 = 123; } sloppyFunc(); // Created global variable `undeclaredVar1`: assert.equal(undeclaredVar1, 123); function strictFunc() { 'use strict'; undeclaredVar2 = 123; } assert.throws( () => strictFunc(), { name: 'ReferenceError', message: 'undeclaredVar2 is not defined', });
- 94. The assert.throws() states that its first argument, a function, throws a ReferenceError when it is called. 8.9.2.2 Function declarations are block-scoped in strict mode, function-scoped in sloppy mode In strict mode, a variable created via a function declaration only exists within the innermost enclosing block: In sloppy mode, function declarations are function-scoped: 8.9.2.3 Sloppy mode doesn’t throw exceptions when changing immutable data function strictFunc() { 'use strict'; { function foo() { return 123 } } return foo(); // ReferenceError } assert.throws( () => strictFunc(), { name: 'ReferenceError', message: 'foo is not defined', }); function sloppyFunc() { { function foo() { return 123 } } return foo(); // works } assert.equal(sloppyFunc(), 123);
- 95. In strict mode, you get an exception if you try to change immutable data: In sloppy mode, the assignment fails silently: Further reading: sloppy mode For more information on how sloppy mode differs from strict mode, see MDN. Quiz: advanced See quiz app. function strictFunc() { 'use strict'; true.prop = 1; // TypeError } assert.throws( () => strictFunc(), { name: 'TypeError', message: "Cannot create property 'prop' on boolean 'true'", }); function sloppyFunc() { true.prop = 1; // fails silently return true.prop; } assert.equal(sloppyFunc(), undefined);
- 96. 9 Consoles: interactive JavaScript command lines 9.1 Trying out JavaScript code 9.1.1 Browser consoles 9.1.2 The Node.js REPL 9.1.3 Other options 9.2 The console.* API: printing data and more 9.2.1 Printing values: console.log() (stdout) 9.2.2 Printing error information: console.error() (stderr) 9.2.3 Printing nested objects via JSON.stringify()
- 97. 9.1 Trying out JavaScript code You have many options for quickly running pieces of JavaScript code. The following subsections describe a few of them. 9.1.1 Browser consoles Web browsers have so-called consoles: interactive command lines to which you can print text via console.log() and where you can run pieces of code. How to open the console differs from browser to browser. Fig. 3 shows the console of Google Chrome. To find out how to open the console in your web browser, you can do a web search for “console «name-of-your-browser»”. These are pages for a few commonly used web browsers: Apple Safari Google Chrome Microsoft Edge Mozilla Firefox
- 98. Figure 3: The console of the web browser “Google Chrome” is open (in the bottom half of window) while visiting a web page. 9.1.2 The Node.js REPL REPL stands for read-eval-print loop and basically means command line. To use it, you must first start Node.js from an operating system command line, via the command node. Then an interaction with it looks as depicted in fig. 4: The text after > is input from the user; everything else is output from Node.js.
- 99. Figure 4: Starting and using the Node.js REPL (interactive command line). Reading: REPL interactions I occasionally demonstrate JavaScript via REPL interactions. Then I also use greater-than symbols (>) to mark input – for example: 9.1.3 Other options Other options include: There are many web apps that let you experiment with JavaScript in web browsers – for example, Babel’s REPL. There are also native apps and IDE plugins for running JavaScript. Consoles often run in non-strict mode In modern JavaScript, most code (e.g., modules) is executed in strict mode. However, consoles often run in non-strict mode. Therefore, you may occasionally get slightly different results when using a console to execute code from this book. > 3 + 5 8
- 100. 9.2 The console.* API: printing data and more In browsers, the console is something you can bring up that is normally hidden. For Node.js, the console is the terminal that Node.js is currently running in. The full console.* API is documented on MDN web docs and on the Node.js website. It is not part of the JavaScript language standard, but much functionality is supported by both browsers and Node.js. In this chapter, we only look at the following two methods for printing data (“printing” means displaying in the console): console.log() console.error() 9.2.1 Printing values: console.log() (stdout) There are two variants of this operation: 9.2.1.1 Printing multiple values The first variant prints (text representations of) values on the console: console.log(...values: any[]): void console.log(pattern: string, ...values: any[]): void
- 101. At the end, console.log() always prints a newline. Therefore, if you call it with zero arguments, it just prints a newline. 9.2.1.2 Printing a string with substitutions The second variant performs string substitution: These are some of the directives you can use for substitutions: %s converts the corresponding value to a string and inserts it. %o inserts a string representation of an object. %j converts a value to a JSON string and inserts it. %% inserts a single %. console.log('abc', 123, true); // Output: // abc 123 true console.log('Test: %s %j', 123, 'abc'); // Output: // Test: 123 "abc" console.log('%s %s', 'abc', 123); // Output: // abc 123 console.log('%o', {foo: 123, bar: 'abc'}); // Output: // { foo: 123, bar: 'abc' } console.log('%j', {foo: 123, bar: 'abc'}); // Output: // {"foo":123,"bar":"abc"}
- 102. 9.2.2 Printing error information: console.error() (stderr) console.error() works the same as console.log(), but what it logs is considered error information. For Node.js, that means that the output goes to stderr instead of stdout on Unix. 9.2.3 Printing nested objects via JSON.stringify() JSON.stringify() is occasionally useful for printing nested objects: Output: { "first": "Jane", "last": "Doe" } console.log('%s%%', 99); // Output: // 99% console.log(JSON.stringify({first: 'Jane', last: 'Doe'}, null, 2
- 103. 10 Assertion API 10.1 Assertions in software development 10.2 How assertions are used in this book 10.2.1 Documenting results in code examples via assertions 10.2.2 Implementing test-driven exercises via assertions 10.3 Normal comparison vs. deep comparison 10.4 Quick reference: module assert 10.4.1 Normal equality 10.4.2 Deep equality 10.4.3 Expecting exceptions 10.4.4 Another tool function
- 104. 10.1 Assertions in software development In software development, assertions state facts about values or pieces of code that must be true. If they aren’t, an exception is thrown. Node.js supports assertions via its built-in module assert – for example: This assertion states that the expected result of 3 plus 5 is 8. The import statement uses the recommended strict version of assert. import {strict as assert} from 'assert'; assert.equal(3 + 5, 8);
- 105. 10.2 How assertions are used in this book In this book, assertions are used in two ways: to document results in code examples and to implement test-driven exercises. 10.2.1 Documenting results in code examples via assertions In code examples, assertions express expected results. Take, for example, the following function: id() returns its parameter. We can show it in action via an assertion: In the examples, I usually omit the statement for importing assert. The motivation behind using assertions is: You can specify precisely what is expected. Code examples can be tested automatically, which ensures that they really work. 10.2.2 Implementing test-driven exercises via assertions function id(x) { return x; } assert.equal(id('abc'), 'abc');
- 106. The exercises for this book are test-driven, via the test framework AVA. Checks inside the tests are made via methods of assert. The following is an example of such a test: For more information, consult §11 “Getting started with quizzes and exercises”. // For the exercise, you must implement the function hello(). // The test checks if you have done it properly. test('First exercise', t => { assert.equal(hello('world'), 'Hello world!'); assert.equal(hello('Jane'), 'Hello Jane!'); assert.equal(hello('John'), 'Hello John!'); assert.equal(hello(''), 'Hello !'); });
- 107. 10.3 Normal comparison vs. deep comparison The strict equal() uses === to compare values. Therefore, an object is only equal to itself – even if another object has the same content (because === does not compare the contents of objects, only their identities): deepEqual() is a better choice for comparing objects: This method works for Arrays, too: assert.notEqual({foo: 1}, {foo: 1}); assert.deepEqual({foo: 1}, {foo: 1}); assert.notEqual(['a', 'b', 'c'], ['a', 'b', 'c']); assert.deepEqual(['a', 'b', 'c'], ['a', 'b', 'c']);
- 108. 10.4 Quick reference: module assert For the full documentation, see the Node.js docs. 10.4.1 Normal equality function equal(actual: any, expected: any, message?: string): void actual === expected must be true. If not, an AssertionError is thrown. function notEqual(actual: any, expected: any, message?: string): void actual !== expected must be true. If not, an AssertionError is thrown. The optional last parameter message can be used to explain what is asserted. If the assertion fails, the message is used to set up the AssertionError that is thrown. assert.equal(3+3, 6); assert.notEqual(3+3, 22); let e; try { const x = 3; assert.equal(x, 8, 'x must be equal to 8') } catch (err) { assert.equal(
- 109. 10.4.2 Deep equality function deepEqual(actual: any, expected: any, message?: string): void actual must be deeply equal to expected. If not, an AssertionError is thrown. function notDeepEqual(actual: any, expected: any, message?: string): void actual must not be deeply equal to expected. If it is, an AssertionError is thrown. 10.4.3 Expecting exceptions If you want to (or expect to) receive an exception, you need throws(): This function calls its first parameter, the function block, and only succeeds if it throws an exception. Additional parameters can be used to specify what that exception must look like. function throws(block: Function, message?: string): void String(err), 'AssertionError [ERR_ASSERTION]: x must be equal to 8'); } assert.deepEqual([1,2,3], [1,2,3]); assert.deepEqual([], []); // To .equal(), an object is only equal to itself: assert.notEqual([], []); assert.notDeepEqual([1,2,3], [1,2]);
- 110. function throws(block: Function, error: Function, message?: string): void function throws(block: Function, error: RegExp, message?: string): void function throws(block: Function, error: Object, message?: string): void 10.4.4 Another tool function assert.throws( () => { null.prop; } ); assert.throws( () => { null.prop; }, TypeError ); assert.throws( () => { null.prop; }, /^TypeError: Cannot read property 'prop' of null$/ ); assert.throws( () => { null.prop; }, { name: 'TypeError', message: `Cannot read property 'prop' of null`, } );
- 111. function fail(message: string | Error): never Always throws an AssertionError when it is called. That is occasionally useful for unit testing. Quiz See quiz app. try { functionThatShouldThrow(); assert.fail(); } catch (_) { // Success }
- 112. 11 Getting started with quizzes and exercises 11.1 Quizzes 11.2 Exercises 11.2.1 Installing the exercises 11.2.2 Running exercises 11.3 Unit tests in JavaScript 11.3.1 A typical test 11.3.2 Asynchronous tests in AVA Throughout most chapters, there are quizzes and exercises. These are a paid feature, but a comprehensive preview is available. This chapter explains how to get started with them.
- 113. 11.1 Quizzes Installation: Download and unzip impatient-js-quiz.zip Running the quiz app: Open impatient-js-quiz/index.html in a web browser You’ll see a TOC of all the quizzes.
- 114. 11.2 Exercises 11.2.1 Installing the exercises To install the exercises: Download and unzip impatient-js-code.zip Follow the instructions in README.txt 11.2.2 Running exercises Exercises are referred to by path in this book. For example: exercises/quizzes- exercises/first_module_test.mjs Within each file: The first line contains the command for running the exercise. The following lines describe what you have to do.
- 115. 11.3 Unit tests in JavaScript All exercises in this book are tests that are run via the test framework AVA. This section gives a brief introduction. 11.3.1 A typical test Typical test code is split into two parts: Part 1: the code to be tested. Part 2: the tests for the code. Take, for example, the following two files: id.mjs (code to be tested) id_test.mjs (tests) 11.3.1.1 Part 1: the code The code itself resides in id.mjs: The key thing here is: everything you want to test must be exported. Otherwise, the test code can’t access it. 11.3.1.2 Part 2: the tests export function id(x) { return x; }
- 116. Don’t worry about the exact details of tests You don’t need to worry about the exact details of tests: They are always implemented for you. Therefore, you only need to read them, but not write them. The tests for the code reside in id_test.mjs: The core of this test file is line E – an assertion: assert.equal() specifies that the expected result of id('abc') is 'abc'. As for the other lines: The comment at the very beginning shows the shell command for running the test. Line A: We import the test framework. Line B: We import the assertion library. AVA has built-in assertions, but module assert lets us remain compatible with plain Node.js. Line C: We import the function to test. Line D: We define a test. This is done by calling the function test(): First parameter: the name of the test. // npm t demos/quizzes-exercises/id_test.mjs import test from 'ava'; // (A) import {strict as assert} from 'assert'; // (B) import {id} from './id.mjs'; // (C) test('My test', t => { // (D) assert.equal(id('abc'), 'abc'); // (E) });
- 117. Second parameter: the test code, which is provided via an arrow function. The parameter t gives us access to AVA’s testing API (assertions, etc.). To run the test, we execute the following in a command line: npm t demos/quizzes-exercises/id_test.mjs The t is an abbreviation for test. That is, the long version of this command is: npm test demos/quizzes-exercises/id_test.mjs Exercise: Your first exercise The following exercise gives you a first taste of what exercises are like: exercises/quizzes-exercises/first_module_test.mjs 11.3.2 Asynchronous tests in AVA Reading You can postpone reading this section until you get to the chapters on asynchronous programming. Writing tests for asynchronous code requires extra work: The test receives its results later and has to signal to AVA that it isn’t finished yet when it returns. The following subsections examine three ways of doing so.
- 118. 11.3.2.1 Asynchronicity via callbacks If we call test.cb() instead of test(), AVA switches to callback- based asynchronicity. When we are done with our asynchronous work, we have to call t.end(): 11.3.2.2 Asynchronicity via Promises If a test returns a Promise, AVA switches to Promise-based asynchronicity. A test is considered successful if the Promise is fulfilled and failed if the Promise is rejected. 11.3.2.3 Async functions as test “bodies” Async functions always return Promises. Therefore, an async function is a convenient way of implementing an asynchronous test. The following code is equivalent to the previous example. test.cb('divideCallback', t => { divideCallback(8, 4, (error, result) => { if (error) { t.end(error); } else { assert.strictEqual(result, 2); t.end(); } }); }); test('dividePromise 1', t => { return dividePromise(8, 4) .then(result => { assert.strictEqual(result, 2); }); });
- 119. You don’t need to explicitly return anything: The implicitly returned undefined is used to fulfill the Promise returned by this async function. And if the test code throws an exception, then the async function takes care of rejecting the returned Promise. test('dividePromise 2', async t => { const result = await dividePromise(8, 4); assert.strictEqual(result, 2); // No explicit return necessary! });
- 120. 12 Variables and assignment 12.1 let 12.2 const 12.2.1 const and immutability 12.2.2 const and loops 12.3 Deciding between const and let 12.4 The scope of a variable 12.4.1 Shadowing variables 12.5 (Advanced) 12.6 Terminology: static vs. dynamic 12.6.1 Static phenomenon: scopes of variables 12.6.2 Dynamic phenomenon: function calls 12.7 Global variables and the global object 12.7.1 globalThis 12.8 Declarations: scope and activation 12.8.1 const and let: temporal dead zone 12.8.2 Function declarations and early activation 12.8.3 Class declarations are not activated early 12.8.4 var: hoisting (partial early activation) 12.9 Closures 12.9.1 Bound variables vs. free variables 12.9.2 What is a closure? 12.9.3 Example: A factory for incrementors 12.9.4 Use cases for closures 12.10 Further reading
- 121. These are JavaScript’s main ways of declaring variables: let declares mutable variables. const declares constants (immutable variables). Before ES6, there was also var. But it has several quirks, so it’s best to avoid it in modern JavaScript. You can read more about it in Speaking JavaScript.
- 122. 12.1 let Variables declared via let are mutable: You can also declare and assign at the same time: let i; i = 0; i = i + 1; assert.equal(i, 1); let i = 0;
- 123. 12.2 const Variables declared via const are immutable. You must always initialize immediately: 12.2.1 const and immutability In JavaScript, const only means that the binding (the association between variable name and variable value) is immutable. The value itself may be mutable, like obj in the following example. const i = 0; // must initialize assert.throws( () => { i = i + 1 }, { name: 'TypeError', message: 'Assignment to constant variable.', } ); const obj = { prop: 0 }; // Allowed: changing properties of `obj` obj.prop = obj.prop + 1; assert.equal(obj.prop, 1); // Not allowed: assigning to `obj` assert.throws( () => { obj = {} }, { name: 'TypeError', message: 'Assignment to constant variable.', } );
- 124. 12.2.2 const and loops You can use const with for-of loops, where a fresh binding is created for each iteration: In plain for loops, you must use let, however: const arr = ['hello', 'world']; for (const elem of arr) { console.log(elem); } // Output: // 'hello' // 'world' const arr = ['hello', 'world']; for (let i=0; i<arr.length; i++) { const elem = arr[i]; console.log(elem); }
- 125. 12.3 Deciding between const and let I recommend the following rules to decide between const and let: const indicates an immutable binding and that a variable never changes its value. Prefer it. let indicates that the value of a variable changes. Use it only when you can’t use const. Exercise: const exercises/variables-assignment/const_exrc.mjs
- 126. 12.4 The scope of a variable The scope of a variable is the region of a program where it can be accessed. Consider the following code. Scope A is the (direct) scope of x. Scopes B and C are inner scopes of scope A. Scope A is an outer scope of scope B and scope C. Each variable is accessible in its direct scope and all scopes nested within that scope. { // // Scope A. Accessible: x const x = 0; assert.equal(x, 0); { // Scope B. Accessible: x, y const y = 1; assert.equal(x, 0); assert.equal(y, 1); { // Scope C. Accessible: x, y, z const z = 2; assert.equal(x, 0); assert.equal(y, 1); assert.equal(z, 2); } } } // Outside. Not accessible: x, y, z assert.throws( () => console.log(x), { name: 'ReferenceError', message: 'x is not defined', } );
- 127. The variables declared via const and let are called block-scoped because their scopes are always the innermost surrounding blocks. 12.4.1 Shadowing variables You can’t declare the same variable twice at the same level: Why eval()? eval() delays parsing (and therefore the SyntaxError), until the callback of assert.throws() is executed. If we didn’t use it, we’d already get an error when this code is parsed and assert.throws() wouldn’t even be executed. You can, however, nest a block and use the same variable name x that you used outside the block: assert.throws( () => { eval('let x = 1; let x = 2;'); }, { name: 'SyntaxError', message: "Identifier 'x' has already been declared", }); const x = 1; assert.equal(x, 1); { const x = 2; assert.equal(x, 2); } assert.equal(x, 1);
- 128. Inside the block, the inner x is the only accessible variable with that name. The inner x is said to shadow the outer x. Once you leave the block, you can access the old value again. Quiz: basic See quiz app.
- 129. 12.5 (Advanced) All remaining sections are advanced.
- 130. 12.6 Terminology: static vs. dynamic These two adjectives describe phenomena in programming languages: Static means that something is related to source code and can be determined without executing code. Dynamic means at runtime. Let’s look at examples for these two terms. 12.6.1 Static phenomenon: scopes of variables Variable scopes are a static phenomenon. Consider the following code: x is statically (or lexically) scoped. That is, its scope is fixed and doesn’t change at runtime. Variable scopes form a static tree (via static nesting). 12.6.2 Dynamic phenomenon: function calls function f() { const x = 3; // ··· }
- 131. Function calls are a dynamic phenomenon. Consider the following code: Whether or not the function call in line A happens, can only be decided at runtime. Function calls form a dynamic tree (via dynamic calls). function g(x) {} function h(y) { if (Math.random()) g(y); // (A) }
- 132. 12.7 Global variables and the global object JavaScript’s variable scopes are nested. They form a tree: The outermost scope is the root of the tree. The scopes directly contained in that scope are the children of the root. And so on. The root is also called the global scope. In web browsers, the only location where one is directly in that scope is at the top level of a script. The variables of the global scope are called global variables and accessible everywhere. There are two kinds of global variables: Global declarative variables are normal variables. They can only be created while at the top level of a script, via const, `let, and class declarations. Global object variables are stored in properties of the so-called global object. They are created in the top level of a script, via var and function declarations. The global object can be accessed via the global variable globalThis. It can be used to create, read, and delete global object variables. Other than that, global object variables work like normal variables.
- 133. The following HTML fragment demonstrates globalThis and the two kinds of global variables. Each ECMAScript module has its own scope. Therefore, variables that exist at the top level of a module are not global. Fig. 5 illustrates how the various scopes are related. Object variables Global scope Module scope 1 ··· Declarative variables Top level of scripts: var, function declarations const, let, class declarations Module scope 2 Figure 5: The global scope is JavaScript’s outermost scope. It has two kinds of variables: object variables (managed via the global object) and normal declarative variables. Each ECMAScript module has its own scope which is contained in the global scope. <script> const declarativeVariable = 'd'; var objectVariable = 'o'; </script> <script> // All scripts share the same top-level scope: console.log(declarativeVariable); // 'd' console.log(objectVariable); // 'o' // Not all declarations create properties of the global object console.log(globalThis.declarativeVariable); // undefined console.log(globalThis.objectVariable); // 'o' </script>
- 134. 12.7.1 globalThis globalThis is new globalThis is a new feature. Be sure that the JavaScript engines you are targeting support it. If they don’t, switch to one of the alternatives mentioned below. The global variable globalThis is the new standard way of accessing the global object. It got its name from the fact that it has the same value as this in global scope. globalThis does not always directly point to the global object For example, in browsers, there is an indirection. That indirection is normally not noticable, but it is there and can be observed. 12.7.1.1 Alternatives to globalThis Older ways of accessing the global object depend on the platform: Global variable window: is the classic way of referring to the global object. But it doesn’t work in Node.js and in Web Workers. Global variable self: is available in Web Workers and browsers in general. But it isn’t supported by Node.js. Global variable global: is only available in Node.js. 12.7.1.2 Use cases for globalThis
- 135. The global object is now considered a mistake that JavaScript can’t get rid of, due to backward compatibility. It affects performance negatively and is generally confusing. ECMAScript 6 introduced several features that make it easier to avoid the global object – for example: const, let, and class declarations don’t create global object properties when used in global scope. Each ECMAScript module has its own local scope. It is usually better to access global object variables via variables and not via properties of globalThis. The former has always worked the same on all JavaScript platforms. Tutorials on the web occasionally access global variables globVar via window.globVar. But the prefix “window.” is not necessary and I recommend to omit it: Therefore, there are relatively few use cases for globalThis – for example: Polyfills that add new features to old JavaScript engines. Feature detection, to find out what features a JavaScript engine supports. window.encodeURIComponent(str); // no encodeURIComponent(str); // yes
- 136. 12.8 Declarations: scope and activation These are two key aspects of declarations: Scope: Where can a declared entity be seen? This is a static trait. Activation: When can I access an entity? This is a dynamic trait. Some entities can be accessed as soon as we enter their scopes. For others, we have to wait until execution reaches their declarations. Tbl. 1 summarizes how various declarations handle these aspects. Table 1: Aspects of declarations. “Duplicates” describes if a declaration can be used twice with the same name (per scope). “Global prop.” describes if a declaration adds a property to the global object, when it is executed in the global scope of a script. TDZ means temporal dead zone (which is explained later). (*) Function declarations are normally block-scoped, but function-scoped in sloppy mode. Scope Activation Duplicates Global prop. const Block decl. (TDZ) ✘ ✘ let Block decl. (TDZ) ✘ ✘ function Block (*) start ✔ ✔ class Block decl. (TDZ) ✘ ✘ import Module same as export ✘ ✘
- 137. Scope Activation Duplicates Global prop. var Function start, partially ✔ ✔ import is described in §27.5 “ECMAScript modules”. The following sections describe the other constructs in more detail. 12.8.1 const and let: temporal dead zone For JavaScript, TC39 needed to decide what happens if you access a constant in its direct scope, before its declaration: Some possible approaches are: 1. The name is resolved in the scope surrounding the current scope. 2. You get undefined. 3. There is an error. Approach 1 was rejected because there is no precedent in the language for this approach. It would therefore not be intuitive to JavaScript programmers. Approach 2 was rejected because then x wouldn’t be a constant – it would have different values before and after its declaration. { console.log(x); // What happens here? const x; }
- 138. let uses the same approach 3 as const, so that both work similarly and it’s easy to switch between them. The time between entering the scope of a variable and executing its declaration is called the temporal dead zone (TDZ) of that variable: During this time, the variable is considered to be uninitialized (as if that were a special value it has). If you access an uninitialized variable, you get a ReferenceError. Once you reach a variable declaration, the variable is set to either the value of the initializer (specified via the assignment symbol) or undefined – if there is no initializer. The following code illustrates the temporal dead zone: The next example shows that the temporal dead zone is truly temporal (related to time): if (true) { // entering scope of `tmp`, TDZ starts // `tmp` is uninitialized: assert.throws(() => (tmp = 'abc'), ReferenceError); assert.throws(() => console.log(tmp), ReferenceError); let tmp; // TDZ ends assert.equal(tmp, undefined); } if (true) { // entering scope of `myVar`, TDZ starts const func = () => { console.log(myVar); // executed later }; // We are within the TDZ: // Accessing `myVar` causes `ReferenceError`
- 139. Even though func() is located before the declaration of myVar and uses that variable, we can call func(). But we have to wait until the temporal dead zone of myVar is over. 12.8.2 Function declarations and early activation More information on functions In this section, we are using functions – before we had a chance to learn them properly. Hopefully, everything still makes sense. Whenever it doesn’t, please see §25 “Callable values”. A function declaration is always executed when entering its scope, regardless of where it is located within that scope. That enables you to call a function foo() before it is declared: The early activation of foo() means that the previous code is equivalent to: If you declare a function via const or let, then it is not activated early. In the following example, you can only use bar() after its declaration. let myVar = 3; // TDZ ends func(); // OK, called outside TDZ } assert.equal(foo(), 123); // OK function foo() { return 123; } function foo() { return 123; } assert.equal(foo(), 123);
- 140. 12.8.2.1 Calling ahead without early activation Even if a function g() is not activated early, it can be called by a preceding function f() (in the same scope) if we adhere to the following rule: f() must be invoked after the declaration of g(). The functions of a module are usually invoked after its complete body is executed. Therefore, in modules, you rarely need to worry about the order of functions. Lastly, note how early activation automatically keeps the aforementioned rule: when entering a scope, all function declarations are executed first, before any calls are made. 12.8.2.2 A pitfall of early activation If you rely on early activation to call a function before its declaration, then you need to be careful that it doesn’t access data that isn’t activated early. assert.throws( () => bar(), // before declaration ReferenceError); const bar = () => { return 123; }; assert.equal(bar(), 123); // after declaration const f = () => g(); const g = () => 123; // We call f() after g() was declared: assert.equal(f(), 123);
- 141. The problem goes away if you make the call to funcDecl() after the declaration of MY_STR. 12.8.2.3 The pros and cons of early activation We have seen that early activation has a pitfall and that you can get most of its benefits without using it. Therefore, it is better to avoid early activation. But I don’t feel strongly about this and, as mentioned before, often use function declarations because I like their syntax. 12.8.3 Class declarations are not activated early Even though they are similar to function declarations in some ways, class declarations are not activated early: funcDecl(); const MY_STR = 'abc'; function funcDecl() { assert.throws( () => MY_STR, ReferenceError); } assert.throws( () => new MyClass(), ReferenceError); class MyClass {} assert.equal(new MyClass() instanceof MyClass, true);
- 142. Why is that? Consider the following class declaration: The operand of extends is an expression. Therefore, you can do things like this: Evaluating such an expression must be done at the location where it is mentioned. Anything else would be confusing. That explains why class declarations are not activated early. 12.8.4 var: hoisting (partial early activation) var is an older way of declaring variables that predates const and let (which are preferred now). Consider the following var declaration. This declaration has two parts: Declaration var x: The scope of a var-declared variable is the innermost surrounding function and not the innermost surrounding block, as for most other declarations. Such a variable is already active at the beginning of its scope and initialized with undefined. Assignment x = 123: The assignment is always executed in place. class MyClass extends Object {} const identity = x => x; class MyClass extends identity(Object) {} var x = 123;
- 143. The following code demonstrates the effects of var: function f() { // Partial early activation: assert.equal(x, undefined); if (true) { var x = 123; // The assignment is executed in place: assert.equal(x, 123); } // Scope is function, not block: assert.equal(x, 123); }
- 144. 12.9 Closures Before we can explore closures, we need to learn about bound variables and free variables. 12.9.1 Bound variables vs. free variables Per scope, there is a set of variables that are mentioned. Among these variables we distinguish: Bound variables are declared within the scope. They are parameters and local variables. Free variables are declared externally. They are also called non- local variables. Consider the following code: In the body of func(), x and y are bound variables. z is a free variable. 12.9.2 What is a closure? What is a closure then? A closure is a function plus a connection to the variables that exist at its “birth place”. function func(x) { const y = 123; console.log(z); }
- 145. What is the point of keeping this connection? It provides the values for the free variables of the function – for example: funcFactory returns a closure that is assigned to func. Because func has the connection to the variables at its birth place, it can still access the free variable value when it is called in line A (even though it “escaped” its scope). All functions in JavaScript are closures Static scoping is supported via closures in JavaScript. Therefore, every function is a closure. 12.9.3 Example: A factory for incrementors The following function returns incrementors (a name that I just made up). An incrementor is a function that internally stores a number. When it is called, it updates that number by adding the argument to it and returns the new value. function funcFactory(value) { return () => { return value; }; } const func = funcFactory('abc'); assert.equal(func(), 'abc'); // (A) function createInc(startValue) { return (step) => { // (A) startValue += step; return startValue;
- 146. We can see that the function created in line A keeps its internal number in the free variable startValue. This time, we don’t just read from the birth scope, we use it to store data that we change and that persists across function calls. We can create more storage slots in the birth scope, via local variables: 12.9.4 Use cases for closures What are closures good for? For starters, they are simply an implementation of static scoping. As such, they provide context data for callbacks. They can also be used by functions to store state that persists across function calls. createInc() is an example of that. }; } const inc = createInc(5); assert.equal(inc(2), 7); function createInc(startValue) { let index = -1; return (step) => { startValue += step; index++; return [index, startValue]; }; } const inc = createInc(5); assert.deepEqual(inc(2), [0, 7]); assert.deepEqual(inc(2), [1, 9]); assert.deepEqual(inc(2), [2, 11]);
- 147. And they can provide private data for objects (produced via literals or classes). The details of how that works are explained in Exploring ES6. Quiz: advanced See quiz app.
- 148. 12.10 Further reading For more information on how variables are handled under the hood (as described in the ECMAScript specification), consult §26.4 “Closures and environments”.
- 149. 13 Values 13.1 What’s a type? 13.2 JavaScript’s type hierarchy 13.3 The types of the language specification 13.4 Primitive values vs. objects 13.4.1 Primitive values (short: primitives) 13.4.2 Objects 13.5 The operators typeof and instanceof: what’s the type of a value? 13.5.1 typeof 13.5.2 instanceof 13.6 Classes and constructor functions 13.6.1 Constructor functions associated with primitive types 13.7 Converting between types 13.7.1 Explicit conversion between types 13.7.2 Coercion (automatic conversion between types) In this chapter, we’ll examine what kinds of values JavaScript has. Supporting tool: === In this chapter, we’ll occasionally use the strict equality operator. a === b evaluates to true if a and b are equal. What exactly that means is explained in §14.4.2 “Strict equality (=== and !==)”.
- 150. 13.1 What’s a type? For this chapter, I consider types to be sets of values – for example, the type boolean is the set { false, true }.
- 151. 13.2 JavaScript’s type hierarchy (any) (object) (primitive value) boolean number string symbol undefined null Object Array Map Set Function RegExp Date Figure 6: A partial hierarchy of JavaScript’s types. Missing are the classes for errors, the classes associated with primitive types, and more. The diagram hints at the fact that not all objects are instances of Object. Fig. 6 shows JavaScript’s type hierarchy. What do we learn from that diagram? JavaScript distinguishes two kinds of values: primitive values and objects. We’ll see soon what the difference is. The diagram differentiates objects and instances of class Object. Each instance of Object is also an object, but not vice versa. However, virtually all objects that you’ll encounter in practice are instances of Object – for example, objects created via object literals. More details on this topic are explained in §29.4.3.4 “Objects that aren’t instances of Object”.
- 152. 13.3 The types of the language specification The ECMAScript specification only knows a total of seven types. The names of those types are (I’m using TypeScript’s names, not the spec’s names): undefined with the only element undefined null with the only element null boolean with the elements false and true number the type of all numbers (e.g., -123, 3.141) string the type of all strings (e.g., 'abc') symbol the type of all symbols (e.g., Symbol('My Symbol')) object the type of all objects (different from Object, the type of all instances of class Object and its subclasses)
- 153. 13.4 Primitive values vs. objects The specification makes an important distinction between values: Primitive values are the elements of the types undefined, null, boolean, number, string, symbol. All other values are objects. In contrast to Java (that inspired JavaScript here), primitive values are not second-class citizens. The difference between them and objects is more subtle. In a nutshell: Primitive values: are atomic building blocks of data in JavaScript. They are passed by value: when primitive values are assigned to variables or passed to functions, their contents are copied. They are compared by value: when comparing two primitive values, their contents are compared. Objects: are compound pieces of data. They are passed by identity (my term): when objects are assigned to variables or passed to functions, their identities (think pointers) are copied. They are compared by identity (my term): when comparing two objects, their identities are compared. Other than that, primitive values and objects are quite similar: they both have properties (key-value entries) and can be used in the same
- 154. locations. Next, we’ll look at primitive values and objects in more depth. 13.4.1 Primitive values (short: primitives) 13.4.1.1 Primitives are immutable You can’t change, add, or remove properties of primitives: 13.4.1.2 Primitives are passed by value Primitives are passed by value: variables (including parameters) store the contents of the primitives. When assigning a primitive value to a variable or passing it as an argument to a function, its content is copied. 13.4.1.3 Primitives are compared by value Primitives are compared by value: when comparing two primitive values, we compare their contents. let str = 'abc'; assert.equal(str.length, 3); assert.throws( () => { str.length = 1 }, /^TypeError: Cannot assign to read only property 'length'/ ); let x = 123; let y = x; assert.equal(y, 123);
- 155. To see what’s so special about this way of comparing, read on and find out how objects are compared. 13.4.2 Objects Objects are covered in detail in §28 “Single objects” and the following chapter. Here, we mainly focus on how they differ from primitive values. Let’s first explore two common ways of creating objects: Object literal: The object literal starts and ends with curly braces {}. It creates an object with two properties. The first property has the key 'first' (a string) and the value 'Jane'. The second property has the key 'last' and the value 'Doe'. For more information on object literals, consult §28.2.1 “Object literals: properties”. Array literal: The Array literal starts and ends with square brackets []. It creates an Array with two elements: 'foo' and 'bar'. For more assert.equal(123 === 123, true); assert.equal('abc' === 'abc', true); const obj = { first: 'Jane', last: 'Doe', }; const arr = ['foo', 'bar'];
- 156. information on Array literals, consult §31.2.1 “Creating, reading, writing Arrays”. 13.4.2.1 Objects are mutable by default By default, you can freely change, add, and remove the properties of objects: 13.4.2.2 Objects are passed by identity Objects are passed by identity (my term): variables (including parameters) store the identities of objects. The identity of an object is like a pointer (or a transparent reference) to the object’s actual data on the heap (think shared main memory of a JavaScript engine). When assigning an object to a variable or passing it as an argument to a function, its identity is copied. Each object literal creates a fresh object on the heap and returns its identity. const obj = {}; obj.foo = 'abc'; // add a property assert.equal(obj.foo, 'abc'); obj.foo = 'def'; // change a property assert.equal(obj.foo, 'def'); const a = {}; // fresh empty object // Pass the identity in `a` to `b`: const b = a; // Now `a` and `b` point to the same object
- 157. JavaScript uses garbage collection to automatically manage memory: Now the old value { prop: 'value' } of obj is garbage (not used anymore). JavaScript will automatically garbage-collect it (remove it from memory), at some point in time (possibly never if there is enough free memory). Details: passing by identity “Passing by identity” means that the identity of an object (a transparent reference) is passed by value. This approach is also called “passing by sharing”. 13.4.2.3 Objects are compared by identity Objects are compared by identity (my term): two variables are only equal if they contain the same object identity. They are not equal if they refer to different objects with the same content. // (they “share” that object): assert.equal(a === b, true); // Changing `a` also changes `b`: a.foo = 123; assert.equal(b.foo, 123); let obj = { prop: 'value' }; obj = {}; const obj = {}; // fresh empty object assert.equal(obj === obj, true); // same identity assert.equal({} === {}, false); // different identities, same co
- 158. 13.5 The operators typeof and instanceof: what’s the type of a value? The two operators typeof and instanceof let you determine what type a given value x has: How do they differ? typeof distinguishes the 7 types of the specification (minus one omission, plus one addition). instanceof tests which class created a given value. Rule of thumb: typeof is for primitive values; instanceof is for objects 13.5.1 typeof Table 2: The results of the typeof operator. x typeof x undefined 'undefined' null 'object' Boolean 'boolean' Number 'number' if (typeof x === 'string') ··· if (x instanceof Array) ···
- 159. x typeof x String 'string' Symbol 'symbol' Function 'function' All other objects 'object' Tbl. 2 lists all results of typeof. They roughly correspond to the 7 types of the language specification. Alas, there are two differences, and they are language quirks: typeof null returns 'object' and not 'null'. That’s a bug. Unfortunately, it can’t be fixed. TC39 tried to do that, but it broke too much code on the web. typeof of a function should be 'object' (functions are objects). Introducing a separate category for functions is confusing. Exercises: Two exercises on typeof exercises/values/typeof_exrc.mjs Bonus: exercises/values/is_object_test.mjs 13.5.2 instanceof This operator answers the question: has a value x been created by a class C? For example: x instanceof C
- 160. Primitive values are not instances of anything: Exercise: instanceof exercises/values/instanceof_exrc.mjs > (function() {}) instanceof Function true > ({}) instanceof Object true > [] instanceof Array true > 123 instanceof Number false > '' instanceof String false > '' instanceof Object false
- 161. 13.6 Classes and constructor functions JavaScript’s original factories for objects are constructor functions: ordinary functions that return “instances” of themselves if you invoke them via the new operator. ES6 introduced classes, which are mainly better syntax for constructor functions. In this book, I’m using the terms constructor function and class interchangeably. Classes can be seen as partitioning the single type object of the specification into subtypes – they give us more types than the limited 7 ones of the specification. Each class is the type of the objects that were created by it. 13.6.1 Constructor functions associated with primitive types Each primitive type (except for the spec-internal types for undefined and null) has an associated constructor function (think class): The constructor function Boolean is associated with booleans. The constructor function Number is associated with numbers. The constructor function String is associated with strings. The constructor function Symbol is associated with symbols.
- 162. Each of these functions plays several roles – for example, Number: You can use it as a function and convert values to numbers: Number.prototype provides the properties for numbers – for example, method .toString(): Number is a namespace/container object for tool functions for numbers – for example: Lastly, you can also use Number as a class and create number objects. These objects are different from real numbers and should be avoided. 13.6.1.1 Wrapping primitive values The constructor functions related to primitive types are also called wrapper types because they provide the canonical way of converting primitive values to objects. In the process, primitive values are “wrapped” in objects. assert.equal(Number('123'), 123); assert.equal((123).toString, Number.prototype.toString); assert.equal(Number.isInteger(123), true); assert.notEqual(new Number(123), 123); assert.equal(new Number(123).valueOf(), 123); const prim = true; assert.equal(typeof prim, 'boolean'); assert.equal(prim instanceof Boolean, false); const wrapped = Object(prim);
- 163. Wrapping rarely matters in practice, but it is used internally in the language specification, to give primitives properties. assert.equal(typeof wrapped, 'object'); assert.equal(wrapped instanceof Boolean, true); assert.equal(wrapped.valueOf(), prim); // unwrap
- 164. 13.7 Converting between types There are two ways in which values are converted to other types in JavaScript: Explicit conversion: via functions such as String(). Coercion (automatic conversion): happens when an operation receives operands/parameters that it can’t work with. 13.7.1 Explicit conversion between types The function associated with a primitive type explicitly converts values to that type: You can also use Object() to convert values to objects: 13.7.2 Coercion (automatic conversion between types) For many operations, JavaScript automatically converts the operands/parameters if their types don’t fit. This kind of automatic > Boolean(0) false > Number('123') 123 > String(123) '123' > typeof Object(123) 'object'
- 165. conversion is called coercion. For example, the multiplication operator coerces its operands to numbers: Many built-in functions coerce, too. For example, parseInt() coerces its parameter to string (parsing stops at the first character that is not a digit): Exercise: Converting values to primitives exercises/values/conversion_exrc.mjs Quiz See quiz app. > '7' * '3' 21 > parseInt(123.45) 123
- 166. 14 Operators 14.1 Making sense of operators 14.1.1 Operators coerce their operands to appropriate types 14.1.2 Most operators only work with primitive values 14.2 The plus operator (+) 14.3 Assignment operators 14.3.1 The plain assignment operator 14.3.2 Compound assignment operators 14.3.3 A list of all compound assignment operators 14.4 Equality: == vs. === 14.4.1 Loose equality (== and !=) 14.4.2 Strict equality (=== and !==) 14.4.3 Recommendation: always use strict equality 14.4.4 Even stricter than ===: Object.is() 14.5 Ordering operators 14.6 Various other operators 14.6.1 Comma operator 14.6.2 void operator
- 167. 14.1 Making sense of operators JavaScript’s operators may seem quirky. With the following two rules, they are easier to understand: Operators coerce their operands to appropriate types Most operators only work with primitive values 14.1.1 Operators coerce their operands to appropriate types If an operator gets operands that don’t have the proper types, it rarely throws an exception. Instead, it coerces (automatically converts) the operands so that it can work with them. Let’s look at two examples. First, the multiplication operator can only work with numbers. Therefore, it converts strings to numbers before computing its result. Second, the square brackets operator ([ ]) for accessing the properties of an object can only handle strings and symbols. All other values are coerced to string: > '7' * '3' 21 const obj = {}; obj['true'] = 123; // Coerce true to the string 'true' assert.equal(obj[true], 123);
- 168. 14.1.2 Most operators only work with primitive values As mentioned before, most operators only work with primitive values. If an operand is an object, it is usually coerced to a primitive value – for example: Why? The plus operator first coerces its operands to primitive values: Next, it concatenates the two strings: > [1,2,3] + [4,5,6] '1,2,34,5,6' > String([1,2,3]) '1,2,3' > String([4,5,6]) '4,5,6' > '1,2,3' + '4,5,6' '1,2,34,5,6'
- 169. 14.2 The plus operator (+) The plus operator works as follows in JavaScript: First, it converts both operands to primitive values. Then it switches to one of two modes: String mode: If one of the two primitive values is a string, then it converts the other one to a string, concatenates both strings, and returns the result. Number mode: Otherwise, It converts both operands to numbers, adds them, and returns the result. String mode lets us use + to assemble strings: Number mode means that if neither operand is a string (or an object that becomes a string) then everything is coerced to numbers: Number(true) is 1. > 'There are ' + 3 + ' items' 'There are 3 items' > 4 + true 5
- 170. 14.3 Assignment operators 14.3.1 The plain assignment operator The plain assignment operator is used to change storage locations: Initializers in variable declarations can also be viewed as a form of assignment: 14.3.2 Compound assignment operators Given an operator op, the following two ways of assigning are equivalent: myvar op= value myvar = myvar op value If, for example, op is +, then we get the operator += that works as follows. x = value; // assign to a previously declared variable obj.propKey = value; // assign to a property arr[index] = value; // assign to an Array element const x = value; let y = value; let str = ''; str += '<b>'; str += 'Hello!'; str += '</b>'; assert.equal(str, '<b>Hello!</b>');
- 171. 14.3.3 A list of all compound assignment operators Arithmetic operators: += -= *= /= %= **= += also works for string concatenation Bitwise operators: <<= >>= >>>= &= ^= |=
- 172. 14.4 Equality: == vs. === JavaScript has two kinds of equality operators: loose equality (==) and strict equality (===). The recommendation is to always use the latter. Other names for == and === == is also called double equals. Its official name in the language specification is abstract equality comparison. === is also called triple equals. 14.4.1 Loose equality (== and !=) Loose equality is one of JavaScript’s quirks. It often coerces operands. Some of those coercions make sense: Others less so: Objects are coerced to primitives if (and only if!) the other operand is primitive: > '123' == 123 true > false == 0 true > '' == 0 true > [1, 2, 3] == '1,2,3' true
- 173. If both operands are objects, they are only equal if they are the same object: Lastly, == considers undefined and null to be equal: 14.4.2 Strict equality (=== and !==) Strict equality never coerces. Two values are only equal if they have the same type. Let’s revisit our previous interaction with the == operator and see what the === operator does: An object is only equal to another value if that value is the same object: > ['1', '2', '3'] == '1,2,3' true > [1, 2, 3] == ['1', '2', '3'] false > [1, 2, 3] == [1, 2, 3] false > const arr = [1, 2, 3]; > arr == arr true > undefined == null true > false === 0 false > '123' === 123 false > [1, 2, 3] === '1,2,3' false > ['1', '2', '3'] === '1,2,3'
- 174. The === operator does not consider undefined and null to be equal: 14.4.3 Recommendation: always use strict equality I recommend to always use ===. It makes your code easier to understand and spares you from having to think about the quirks of ==. Let’s look at two use cases for == and what I recommend to do instead. 14.4.3.1 Use case for ==: comparing with a number or a string == lets you check if a value x is a number or that number as a string – with a single comparison: false > [1, 2, 3] === ['1', '2', '3'] false > [1, 2, 3] === [1, 2, 3] false > const arr = [1, 2, 3]; > arr === arr true > undefined === null false if (x == 123) { // x is either 123 or '123'
- 175. I prefer either of the following two alternatives: You can also convert x to a number when you first encounter it. 14.4.3.2 Use case for ==: comparing with undefined or null Another use case for == is to check if a value x is either undefined or null: The problem with this code is that you can’t be sure if someone meant to write it that way or if they made a typo and meant === null. I prefer either of the following two alternatives: A downside of the second alternative is that it accepts values other than undefined and null, but it is a well-established pattern in JavaScript (to be explained in detail in §16.3 “Truthiness-based existence checks”). The following three conditions are also roughly equivalent: } if (x === 123 || x === '123') ··· if (Number(x) === 123) ··· if (x == null) { // x is either null or undefined } if (x === undefined || x === null) ··· if (!x) ··· if (x != null) ··· if (x !== undefined && x !== null) ··· if (x) ···
- 176. 14.4.4 Even stricter than ===: Object.is() Method Object.is() compares two values: It is even stricter than ===. For example, it considers NaN, the error value for computations involving numbers, to be equal to itself: That is occasionally useful. For example, you can use it to implement an improved version of the Array method .indexOf(): myIndexOf() finds NaN in an Array, while .indexOf() doesn’t: The result -1 means that .indexOf() couldn’t find its argument in the Array. > Object.is(123, 123) true > Object.is(123, '123') false > Object.is(NaN, NaN) true > NaN === NaN false const myIndexOf = (arr, elem) => { return arr.findIndex(x => Object.is(x, elem)); }; > myIndexOf([0,NaN,2], NaN) 1 > [0,NaN,2].indexOf(NaN) -1
- 177. 14.5 Ordering operators Table 3: JavaScript’s ordering operators. Operator name < less than <= Less than or equal > Greater than >= Greater than or equal JavaScript’s ordering operators (tbl. 3) work for both numbers and strings: <= and >= are based on strict equality. The ordering operators don’t work well for human languages The ordering operators don’t work well for comparing text in a human language, e.g., when capitalization or accents are involved. The details are explained in §20.5 “Comparing strings”. > 5 >= 2 true > 'bar' < 'foo' true
- 178. 14.6 Various other operators Operators for booleans, strings, numbers, objects: are covered elsewhere in this book. The next two subsections discuss two operators that are rarely used. 14.6.1 Comma operator The comma operator has two operands, evaluates both of them and returns the second one: For more information on this operator, see Speaking JavaScript. 14.6.2 void operator The void operator evaluates its operand and returns undefined: For more information on this operator, see Speaking JavaScript. Quiz See quiz app. > 'a', 'b' 'b' > void (3 + 2) undefined
- 179. 15 The non-values undefined and null 15.1 undefined vs. null 15.2 Occurrences of undefined and null 15.2.1 Occurrences of undefined 15.2.2 Occurrences of null 15.3 Checking for undefined or null 15.4 undefined and null don’t have properties 15.5 The history of undefined and null Many programming languages have one “non-value” called null. It indicates that a variable does not currently point to an object – for example, when it hasn’t been initialized yet. In contrast, JavaScript has two of them: undefined and null.
- 180. 15.1 undefined vs. null Both values are very similar and often used interchangeably. How they differ is therefore subtle. The language itself makes the following distinction: undefined means “not initialized” (e.g., a variable) or “not existing” (e.g., a property of an object). null means “the intentional absence of any object value” (a quote from the language specification). Programmers may make the following distinction: undefined is the non-value used by the language (when something is uninitialized, etc.). null means “explicitly switched off”. That is, it helps implement a type that comprises both meaningful values and a meta-value that stands for “no meaningful value”. Such a type is called option type or maybe type in functional programming.
- 181. 15.2 Occurrences of undefined and null The following subsections describe where undefined and null appear in the language. We’ll encounter several mechanisms that are explained in more detail later in this book. 15.2.1 Occurrences of undefined Uninitialized variable myVar: Parameter x is not provided: Property .unknownProp is missing: If you don’t explicitly specify the result of a function via a return statement, JavaScript returns undefined for you: 15.2.2 Occurrences of null let myVar; assert.equal(myVar, undefined); function func(x) { return x; } assert.equal(func(), undefined); const obj = {}; assert.equal(obj.unknownProp, undefined); function func() {} assert.equal(func(), undefined);
- 182. The prototype of an object is either an object or, at the end of a chain of prototypes, null. Object.prototype does not have a prototype: If you match a regular expression (such as /a/) against a string (such as 'x'), you either get an object with matching data (if matching was successful) or null (if matching failed): The JSON data format does not support undefined, only null: > Object.getPrototypeOf(Object.prototype) null > /a/.exec('x') null > JSON.stringify({a: undefined, b: null}) '{"b":null}'
- 183. 15.3 Checking for undefined or null Checking for either: Does x have a value? Is x either undefined or null? Truthy means “is true if coerced to boolean”. Falsy means “is false if coerced to boolean”. Both concepts are explained properly in §16.2 “Falsy and truthy values”. if (x === null) ··· if (x === undefined) ··· if (x !== undefined && x !== null) { // ··· } if (x) { // truthy? // x is neither: undefined, null, false, 0, NaN, '' } if (x === undefined || x === null) { // ··· } if (!x) { // falsy? // x is: undefined, null, false, 0, NaN, '' }
- 184. 15.4 undefined and null don’t have properties undefined and null are the two only JavaScript values where you get an exception if you try to read a property. To explore this phenomenon, let’s use the following function, which reads (“gets”) property .foo and returns the result. If we apply getFoo() to various values, we can see that it only fails for undefined and null: function getFoo(x) { return x.foo; } > getFoo(undefined) TypeError: Cannot read property 'foo' of undefined > getFoo(null) TypeError: Cannot read property 'foo' of null > getFoo(true) undefined > getFoo({}) undefined
- 185. 15.5 The history of undefined and null In Java (which inspired many aspects of JavaScript), initialization values depend on the static type of a variable: Variables with object types are initialized with null. Each primitive type has its own initialization value. For example, int variables are initialized with 0. In JavaScript, each variable can hold both object values and primitive values. Therefore, if null means “not an object”, JavaScript also needs an initialization value that means “neither an object nor a primitive value”. That initialization value is undefined. Quiz See quiz app.
- 186. 16 Booleans 16.1 Converting to boolean 16.2 Falsy and truthy values 16.2.1 Checking for truthiness or falsiness 16.3 Truthiness-based existence checks 16.3.1 Pitfall: truthiness-based existence checks are imprecise 16.3.2 Use case: was a parameter provided? 16.3.3 Use case: does a property exist? 16.4 Conditional operator (? :) 16.5 Binary logical operators: And (x && y), Or (x || y) 16.5.1 Logical And (x && y) 16.5.2 Logical Or (||) 16.5.3 Default values via logical Or (||) 16.6 Logical Not (!) The primitive type boolean comprises two values – false and true: > typeof false 'boolean' > typeof true 'boolean'
- 187. 16.1 Converting to boolean The meaning of “converting to [type]” “Converting to [type]” is short for “Converting arbitrary values to values of type [type]”. These are three ways in which you can convert an arbitrary value x to a boolean. Boolean(x) Most descriptive; recommended. x ? true : false Uses the conditional operator (explained later in this chapter). !!x Uses the logical Not operator (!). This operator coerces its operand to boolean. It is applied a second time to get a non- negated result. Tbl. 4 describes how various values are converted to boolean. Table 4: Converting values to booleans. x Boolean(x) undefined false null false boolean value x (no change) number value 0 → false, NaN → false
- 188. x Boolean(x) other numbers → true string value '' → false other strings → true object value always true
- 189. 16.2 Falsy and truthy values When checking the condition of an if statement, a while loop, or a do-while loop, JavaScript works differently than you may expect. Take, for example, the following condition: In many programming languages, this condition is equivalent to: However, in JavaScript, it is equivalent to: That is, JavaScript checks if value is true when converted to boolean. This kind of check is so common that the following names were introduced: A value is called truthy if it is true when converted to boolean. A value is called falsy if it is false when converted to boolean. Each value is either truthy or falsy. Consulting tbl. 4, we can make an exhaustive list of falsy values: undefined, null false 0, NaN '' if (value) {} if (value === true) {} if (Boolean(value) === true) {}
- 190. All other values (including all objects) are truthy: 16.2.1 Checking for truthiness or falsiness The conditional operator that is used in the last line, is explained later in this chapter. Exercise: Truthiness exercises/booleans/truthiness_exrc.mjs > Boolean('abc') true > Boolean([]) true > Boolean({}) true if (x) { // x is truthy } if (!x) { // x is falsy } if (x) { // x is truthy } else { // x is falsy } const result = x ? 'truthy' : 'falsy';
- 191. 16.3 Truthiness-based existence checks In JavaScript, if you read something that doesn’t exist (e.g., a missing parameter or a missing property), you usually get undefined as a result. In these cases, an existence check amounts to comparing a value with undefined. For example, the following code checks if object obj has the property .prop: Due to undefined being falsy, we can shorten this check to: 16.3.1 Pitfall: truthiness-based existence checks are imprecise Truthiness-based existence checks have one pitfall: they are not very precise. Consider this previous example: The body of the if statement is skipped if: if (obj.prop !== undefined) { // obj has property .prop } if (obj.prop) { // obj has property .prop } if (obj.prop) { // obj has property .prop }
- 192. obj.prop is missing (in which case, JavaScript returns undefined). However, it is also skipped if: obj.prop is undefined. obj.prop is any other falsy value (null, 0, '', etc.). In practice, this rarely causes problems, but you have to be aware of this pitfall. 16.3.2 Use case: was a parameter provided? A truthiness check is often used to determine if the caller of a function provided a parameter: On the plus side, this pattern is established and short. It correctly throws errors for undefined and null. On the minus side, there is the previously mentioned pitfall: the code also throws errors for all other falsy values. An alternative is to check for undefined: function func(x) { if (!x) { throw new Error('Missing parameter x'); } // ··· } if (x === undefined) { throw new Error('Missing parameter x');
- 193. 16.3.3 Use case: does a property exist? Truthiness checks are also often used to determine if a property exists: This pattern is also established and has the usual caveat: it not only throws if the property is missing, but also if it exists and has any of the falsy values. If you truly want to check if the property exists, you have to use the in operator: } function readFile(fileDesc) { if (!fileDesc.path) { throw new Error('Missing property: .path'); } // ··· } readFile({ path: 'foo.txt' }); // no error if (! ('path' in fileDesc)) { throw new Error('Missing property: .path'); }
- 194. 16.4 Conditional operator (? :) The conditional operator is the expression version of the if statement. Its syntax is: «condition» ? «thenExpression» : «elseExpression» It is evaluated as follows: If condition is truthy, evaluate and return thenExpression. Otherwise, evaluate and return elseExpression. The conditional operator is also called ternary operator because it has three operands. Examples: The following code demonstrates that whichever of the two branches “then” and “else” is chosen via the condition, only that branch is evaluated. The other branch isn’t. > true ? 'yes' : 'no' 'yes' > false ? 'yes' : 'no' 'no' > '' ? 'yes' : 'no' 'no' const x = (true ? console.log('then') : console.log('else')); // Output: // 'then'
- 195. 16.5 Binary logical operators: And (x && y), Or (x || y) The operators && and || are value-preserving and short-circuiting. What does that mean? Value-preservation means that operands are interpreted as booleans but returned unchanged: Short-circuiting means if the first operand already determines the result, then the second operand is not evaluated. The only other operator that delays evaluating its operands is the conditional operator. Usually, all operands are evaluated before performing an operation. For example, logical And (&&) does not evaluate its second operand if the first one is falsy: If the first operand is truthy, console.log() is executed: > 12 || 'hello' 12 > 0 || 'hello' 'hello' const x = false && console.log('hello'); // No output const x = true && console.log('hello'); // Output: // 'hello'
- 196. 16.5.1 Logical And (x && y) The expression a && b (“a And b”) is evaluated as follows: 1. Evaluate a. 2. Is the result falsy? Return it. 3. Otherwise, evaluate b and return the result. In other words, the following two expressions are roughly equivalent: Examples: 16.5.2 Logical Or (||) The expression a || b (“a Or b”) is evaluated as follows: 1. Evaluate a. 2. Is the result truthy? Return it. 3. Otherwise, evaluate b and return the result. a && b !a ? a : b > false && true false > false && 'abc' false > true && false false > true && 'abc' 'abc' > '' && 'abc' ''
- 197. In other words, the following two expressions are roughly equivalent: Examples: 16.5.3 Default values via logical Or (||) Sometimes you receive a value and only want to use it if it isn’t either null or undefined. Otherwise, you’d like to use a default value, as a fallback. You can do that via the || operator: The following code shows a real-world example: If there are one or more matches for regex inside str then .match() returns an Array. If there are no matches, it unfortunately returns a || b a ? a : b > true || false true > true || 'abc' true > false || true true > false || 'abc' 'abc' > 'abc' || 'def' 'abc' const valueToUse = valueReceived || defaultValue; function countMatches(regex, str) { const matchResult = str.match(regex); // null or Array return (matchResult || []).length; }
- 198. null (and not the empty Array). We fix that via the || operator. Exercise: Default values via the Or operator (||) exercises/booleans/default_via_or_exrc.mjs
- 199. 16.6 Logical Not (!) The expression !x (“Not x”) is evaluated as follows: 1. Evaluate x. 2. Is it truthy? Return false. 3. Otherwise, return true. Examples: Quiz See quiz app. > !false true > !true false > !0 true > !123 false > !'' true > !'abc' false
- 200. 17 Numbers 17.1 JavaScript only has floating point numbers 17.2 Number literals 17.2.1 Integer literals 17.2.2 Floating point literals 17.2.3 Syntactic pitfall: properties of integer literals 17.3 Arithmetic operators 17.3.1 Binary arithmetic operators 17.3.2 Unary plus (+) and negation (-) 17.3.3 Incrementing (++) and decrementing (--) 17.4 Converting to number 17.5 Error values 17.6 Error value: NaN 17.6.1 Checking for NaN 17.6.2 Finding NaN in Arrays 17.7 Error value: Infinity 17.7.1 Infinity as a default value 17.7.2 Checking for Infinity 17.8 The precision of numbers: careful with decimal fractions 17.9 (Advanced) 17.10 Background: floating point precision 17.10.1 A simplified representation of floating point numbers 17.11 Integers in JavaScript 17.11.1 Converting to integer
- 201. 17.11.2 Ranges of integers in JavaScript 17.11.3 Safe integers 17.12 Bitwise operators 17.12.1 Internally, bitwise operators work with 32-bit integers 17.12.2 Binary bitwise operators 17.12.3 Bitwise Not 17.12.4 Bitwise shift operators 17.12.5 b32(): displaying unsigned 32-bit integers in binary notation 17.13 Quick reference: numbers 17.13.1 Global functions for numbers 17.13.2 Static properties of Number 17.13.3 Static methods of Number 17.13.4 Methods of Number.prototype 17.13.5 Sources This chapter covers JavaScript’s single type for numbers, number.
- 202. 17.1 JavaScript only has floating point numbers You can express both integers and floating point numbers in JavaScript: However, there is only a single type for all numbers: they are all doubles, 64-bit floating point numbers implemented according to the IEEE Standard for Floating-Point Arithmetic (IEEE 754). Integers are simply floating point numbers without a decimal fraction: Note that, under the hood, most JavaScript engines are often able to use real integers, with all associated performance and storage size benefits. 98 123.45 > 98 === 98.0 true
- 203. 17.2 Number literals Let’s examine literals for numbers. 17.2.1 Integer literals Several integer literals let you express integers with various bases: 17.2.2 Floating point literals Floating point numbers can only be expressed in base 10. Fractions: Exponent: eN means ×10N // Binary (base 2) assert.equal(0b11, 3); // Octal (base 8) assert.equal(0o10, 8); // Decimal (base 10): assert.equal(35, 35); // Hexadecimal (base 16) assert.equal(0xE7, 231); > 35.0 35 > 3e2 300 > 3e-2
- 204. 17.2.3 Syntactic pitfall: properties of integer literals Accessing a property of an integer literal entails a pitfall: If the integer literal is immediately followed by a dot, then that dot is interpreted as a decimal dot: 7.toString(); // syntax error There are four ways to work around this pitfall: 0.03 > 0.3e2 30 7.0.toString() (7).toString() 7..toString() 7 .toString() // space before dot
- 205. 17.3 Arithmetic operators 17.3.1 Binary arithmetic operators Tbl. 5 lists JavaScript’s binary arithmetic operators. Table 5: Binary arithmetic operators. Operator Name Example n + m Addition ES1 3 + 4 → 7 n - m Subtraction ES1 9 - 1 → 8 n * m Multiplication ES1 3 * 2.25 → 6.75 n / m Division ES1 5.625 / 5 → 1.125 n % m Remainder ES1 8 % 5 → 3 -8 % 5 → -3 n ** m Exponentiation ES2016 4 ** 2 → 16 17.3.1.1 % is a remainder operator % is a remainder operator, not a modulo operator. Its result has the sign of the first operand: For more information on the difference between remainder and modulo, see the blog post “Remainder operator vs. modulo operator > 5 % 3 2 > -5 % 3 -2
- 206. (with JavaScript code)” on 2ality. 17.3.2 Unary plus (+) and negation (-) Tbl. 6 summarizes the two operators unary plus (+) and negation (-). Table 6: The operators unary plus (+) and negation (-). Operator Name Example +n Unary plus ES1 +(-7) → -7 -n Unary negation ES1 -(-7) → 7 Both operators coerce their operands to numbers: Thus, unary plus lets us convert arbitrary values to numbers. 17.3.3 Incrementing (++) and decrementing (--) The incrementation operator ++ exists in a prefix version and a suffix version. In both versions, it destructively adds one to its operand. Therefore, its operand must be a storage location that can be changed. > +'5' 5 > +'-12' -12 > -'9' -9
- 207. The decrementation operator -- works the same, but subtracts one from its operand. The next two examples explain the difference between the prefix and the suffix version. Tbl. 7 summarizes the incrementation and decrementation operators. Table 7: Incrementation operators and decrementation operators. Operator Name Example v++ Increment ES1 let v=0; [v++, v] → [0, 1] ++v Increment ES1 let v=0; [++v, v] → [1, 1] v-- Decrement ES1 let v=1; [v--, v] → [1, 0] --v Decrement ES1 let v=1; [--v, v] → [0, 0] Next, we’ll look at examples of these operators in use. Prefix ++ and prefix -- change their operands and then return them. Suffix ++ and suffix -- return their operands and then change them. let foo = 3; assert.equal(++foo, 4); assert.equal(foo, 4); let bar = 3; assert.equal(--bar, 2); assert.equal(bar, 2); let foo = 3; assert.equal(foo++, 3); assert.equal(foo, 4); let bar = 3;
- 208. 17.3.3.1 Operands: not just variables You can also apply these operators to property values: And to Array elements: Exercise: Number operators exercises/numbers-math/is_odd_test.mjs assert.equal(bar--, 3); assert.equal(bar, 2); const obj = { a: 1 }; ++obj.a; assert.equal(obj.a, 2); const arr = [ 4 ]; arr[0]++; assert.deepEqual(arr, [5]);
- 209. 17.4 Converting to number These are three ways of converting values to numbers: Number(value) +value parseFloat(value) (avoid; different than the other two!) Recommendation: use the descriptive Number(). Tbl. 8 summarizes how it works. Table 8: Converting values to numbers. x Number(x) undefined NaN null 0 boolean false → 0, true → 1 number x (no change) string '' → 0 other → parsed number, ignoring leading/trailing whitespace object configurable (e.g. via .valueOf()) Examples: assert.equal(Number(123.45), 123.45); assert.equal(Number(''), 0); assert.equal(Number('n 123.45 t'), 123.45); assert.equal(Number('xyz'), NaN);
- 210. How objects are converted to numbers can be configured – for example, by overriding .valueOf(): Exercise: Converting to number exercises/numbers-math/parse_number_test.mjs > Number({ valueOf() { return 123 } }) 123
- 211. 17.5 Error values Two number values are returned when errors happen: NaN Infinity
- 212. 17.6 Error value: NaN NaN is an abbreviation of “not a number”. Ironically, JavaScript considers it to be a number: When is NaN returned? NaN is returned if a number can’t be parsed: NaN is returned if an operation can’t be performed: NaN is returned if an operand or argument is NaN (to propagate errors): 17.6.1 Checking for NaN > typeof NaN 'number' > Number('$$$') NaN > Number(undefined) NaN > Math.log(-1) NaN > Math.sqrt(-1) NaN > NaN - 3 NaN > 7 ** NaN NaN
- 213. NaN is the only JavaScript value that is not strictly equal to itself: These are several ways of checking if a value x is NaN: In the last line, we use the comparison quirk to detect NaN. 17.6.2 Finding NaN in Arrays Some Array methods can’t find NaN: Others can: Alas, there is no simple rule of thumb. You have to check for each method how it handles NaN. const n = NaN; assert.equal(n === n, false); const x = NaN; assert.equal(Number.isNaN(x), true); // preferred assert.equal(Object.is(x, NaN), true); assert.equal(x !== x, true); > [NaN].indexOf(NaN) -1 > [NaN].includes(NaN) true > [NaN].findIndex(x => Number.isNaN(x)) 0 > [NaN].find(x => Number.isNaN(x)) NaN
- 214. 17.7 Error value: Infinity When is the error value Infinity returned? Infinity is returned if a number is too large: Infinity is returned if there is a division by zero: 17.7.1 Infinity as a default value Infinity is larger than all other numbers (except NaN), making it a good default value: 17.7.2 Checking for Infinity > Math.pow(2, 1023) 8.98846567431158e+307 > Math.pow(2, 1024) Infinity > 5 / 0 Infinity > -5 / 0 -Infinity function findMinimum(numbers) { let min = Infinity; for (const n of numbers) { if (n < min) min = n; } return min; } assert.equal(findMinimum([5, -1, 2]), -1); assert.equal(findMinimum([]), Infinity);
- 215. These are two common ways of checking if a value x is Infinity: Exercise: Comparing numbers exercises/numbers-math/find_max_test.mjs const x = Infinity; assert.equal(x === Infinity, true); assert.equal(Number.isFinite(x), false);
- 216. 17.8 The precision of numbers: careful with decimal fractions Internally, JavaScript floating point numbers are represented with base 2 (according to the IEEE 754 standard). That means that decimal fractions (base 10) can’t always be represented precisely: You therefore need to take rounding errors into consideration when performing arithmetic in JavaScript. Read on for an explanation of this phenomenon. Quiz: basic See quiz app. > 0.1 + 0.2 0.30000000000000004 > 1.3 * 3 3.9000000000000004 > 1.4 * 100000000000000 139999999999999.98
- 217. 17.9 (Advanced) All remaining sections of this chapter are advanced.
- 218. 17.10 Background: floating point precision In JavaScript, computations with numbers don’t always produce correct results – for example: To understand why, we need to explore how JavaScript represents floating point numbers internally. It uses three integers to do so, which take up a total of 64 bits of storage (double precision): Component Size Integer range Sign 1 bit [0, 1] Fraction 52 bits [0, 252−1] Exponent 11 bits [−1023, 1024] The floating point number represented by these integers is computed as follows: (–1)sign × 0b1.fraction × 2exponent This representation can’t encode a zero because its second component (involving the fraction) always has a leading 1. Therefore, a zero is encoded via the special exponent −1023 and a fraction 0. 17.10.1 A simplified representation of floating point numbers > 0.1 + 0.2 0.30000000000000004
- 219. To make further discussions easier, we simplify the previous representation: Instead of base 2 (binary), we use base 10 (decimal) because that’s what most people are more familiar with. The fraction is a natural number that is interpreted as a fraction (digits after a point). We switch to a mantissa, an integer that is interpreted as itself. As a consequence, the exponent is used differently, but its fundamental role doesn’t change. As the mantissa is an integer (with its own sign), we don’t need a separate sign, anymore. The new representation works like this: mantissa × 10exponent Let’s try out this representation for a few floating point numbers. For the integer −123, we mainly need the mantissa: For the number 1.5, we imagine there being a point after the mantissa. We use a negative exponent to move that point one digit to the left: For the number 0.25, we move the point two digits to the left: > -123 * (10 ** 0) -123 > 15 * (10 ** -1) 1.5 > 25 * (10 ** -2) 0.25
- 220. Representations with negative exponents can also be written as fractions with positive exponents in the denominators: These fractions help with understanding why there are numbers that our encoding cannot represent: 1/10 can be represented. It already has the required format: a power of 10 in the denominator. 1/2 can be represented as 5/10. We turned the 2 in the denominator into a power of 10 by multiplying the numerator and denominator by 5. 1/4 can be represented as 25/100. We turned the 4 in the denominator into a power of 10 by multiplying the numerator and denominator by 25. 1/3 cannot be represented. There is no way to turn the denominator into a power of 10. (The prime factors of 10 are 2 and 5. Therefore, any denominator that only has these prime factors can be converted to a power of 10, by multiplying both the numerator and denominator by enough twos and fives. If a denominator has a different prime factor, then there’s nothing we can do.) To conclude our excursion, we switch back to base 2: > 15 * (10 ** -1) === 15 / (10 ** 1) true > 25 * (10 ** -2) === 25 / (10 ** 2) true
- 221. 0.5 = 1/2 can be represented with base 2 because the denominator is already a power of 2. 0.25 = 1/4 can be represented with base 2 because the denominator is already a power of 2. 0.1 = 1/10 cannot be represented because the denominator cannot be converted to a power of 2. 0.2 = 2/10 cannot be represented because the denominator cannot be converted to a power of 2. Now we can see why 0.1 + 0.2 doesn’t produce a correct result: internally, neither of the two operands can be represented precisely. The only way to compute precisely with decimal fractions is by internally switching to base 10. For many programming languages, base 2 is the default and base 10 an option. For example, Java has the class BigDecimal and Python has the module decimal. There are tentative plans to add something similar to JavaScript: the ECMAScript proposal “Decimal” is currently at stage 0.
- 222. 17.11 Integers in JavaScript JavaScript doesn’t have a special type for integers. Instead, they are simply normal (floating point) numbers without a decimal fraction: In this section, we’ll look at a few tools for working with these pseudo-integers. 17.11.1 Converting to integer The recommended way of converting numbers to integers is to use one of the rounding methods of the Math object: Math.floor(n): returns the largest integer i ≤ n Math.ceil(n): returns the smallest integer i ≥ n Math.round(n): returns the integer that is “closest” to n with __.5 being rounded up – for example: > 1 === 1.0 true > Number.isInteger(1.0) true > Math.floor(2.1) 2 > Math.floor(2.9) 2 > Math.ceil(2.1) 3 > Math.ceil(2.9) 3
- 223. Math.trunc(n): removes any decimal fraction (after the point) that n has, therefore turning it into an integer. For more information on rounding, consult §18.3 “Rounding”. 17.11.2 Ranges of integers in JavaScript These are important ranges of integers in JavaScript: Safe integers: can be represented “safely” by JavaScript (more on what that means in the next subsection) Precision: 53 bits plus sign Range: (−253, 253) Array indices Precision: 32 bits, unsigned Range: [0, 232−1) (excluding the maximum length) Typed Arrays have a larger range of 53 bits (safe and unsigned) Bitwise operators (bitwise Or, etc.) Precision: 32 bits Range of unsigned right shift (>>>): unsigned, [0, 232) Range of all other bitwise operators: signed, [−231, 231) > Math.round(2.4) 2 > Math.round(2.5) 3 > Math.trunc(2.1) 2 > Math.trunc(2.9) 2
- 224. 17.11.3 Safe integers This is the range of integers that are safe in JavaScript (53 bits plus a sign): [–253–1, 253–1] An integer is safe if it is represented by exactly one JavaScript number. Given that JavaScript numbers are encoded as a fraction multiplied by 2 to the power of an exponent, higher integers can also be represented, but then there are gaps between them. For example (18014398509481984 is 254): The following properties of Number help determine if an integer is safe: > 18014398509481984 18014398509481984 > 18014398509481985 18014398509481984 > 18014398509481986 18014398509481984 > 18014398509481987 18014398509481988 assert.equal(Number.MAX_SAFE_INTEGER, (2 ** 53) - 1); assert.equal(Number.MIN_SAFE_INTEGER, -Number.MAX_SAFE_INTEGER); assert.equal(Number.isSafeInteger(5), true); assert.equal(Number.isSafeInteger('5'), false); assert.equal(Number.isSafeInteger(5.1), false); assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER), true assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER+1), fa
- 225. Exercise: Detecting safe integers exercises/numbers-math/is_safe_integer_test.mjs 17.11.3.1 Safe computations Let’s look at computations involving unsafe integers. The following result is incorrect and unsafe, even though both of its operands are safe: The following result is safe, but incorrect. The first operand is unsafe; the second operand is safe: Therefore, the result of an expression a op b is correct if and only if: That is, both operands and the result must be safe. > 9007199254740990 + 3 9007199254740992 > 9007199254740995 - 10 9007199254740986 isSafeInteger(a) && isSafeInteger(b) && isSafeInteger(a op b)
- 226. 17.12 Bitwise operators 17.12.1 Internally, bitwise operators work with 32-bit integers Internally, JavaScript’s bitwise operators work with 32-bit integers. They produce their results in the following steps: Input (JavaScript numbers): The 1–2 operands are first converted to JavaScript numbers (64-bit floating point numbers) and then to 32-bit integers. Computation (32-bit integers): The actual operation processes 32-bit integers and produces a 32-bit integer. Output (JavaScript number): Before returning the result, it is converted back to a JavaScript number. 17.12.1.1 The types of operands and results For each bitwise operator, this book mentions the types of its operands and its result. Each type is always one of the following two: Type Description Size Range Int32 signed 32-bit integer 32 bits incl. sign [−231, 231) Uint32 unsigned 32-bit integer 32 bits [0, 232) Considering the previously mentioned steps, I recommend to pretend that bitwise operators internally work with unsigned 32-bit
- 227. integers (step “computation”) and that Int32 and Uint32 only affect how JavaScript numbers are converted to and from integers (steps “input” and “output”). 17.12.1.2 Displaying JavaScript numbers as unsigned 32-bit integers While exploring the bitwise operators, it occasionally helps to display JavaScript numbers as unsigned 32-bit integers in binary notation. That’s what b32() does (whose implementation is shown later): 17.12.2 Binary bitwise operators Table 9: Binary bitwise operators. Operation Name Type signature num1 & num2 Bitwise And Int32 × Int32 → Int32 ES1 num1 ¦ num2 Bitwise Or Int32 × Int32 → Int32 ES1 num1 ^ num2 Bitwise Xor Int32 × Int32 → Int32 ES1 The binary bitwise operators (tbl. 9) combine the bits of their operands to produce their results: assert.equal( b32(-1), '11111111111111111111111111111111'); assert.equal( b32(1), '00000000000000000000000000000001'); assert.equal( b32(2 ** 31), '10000000000000000000000000000000');
- 228. 17.12.3 Bitwise Not Table 10: The bitwise Not operator. Operation Name Type signature ~num Bitwise Not, ones’ complement Int32 → Int32 ES1 The bitwise Not operator (tbl. 10) inverts each binary digit of its operand: 17.12.4 Bitwise shift operators Table 11: Bitwise shift operators. Operation Name Type signature num << count Left shift Int32 × Uint32 → Int32 ES1 num >> count Signed right shift Int32 × Uint32 → Int32 ES1 num >>> count Unsigned right shift Uint32 × Uint32 → Uint32 ES1 > (0b1010 & 0b0011).toString(2).padStart(4, '0') '0010' > (0b1010 | 0b0011).toString(2).padStart(4, '0') '1011' > (0b1010 ^ 0b0011).toString(2).padStart(4, '0') '1001' > b32(~0b100) '11111111111111111111111111111011'
- 229. The shift operators (tbl. 11) move binary digits to the left or to the right: >> preserves highest bit, >>> doesn’t: 17.12.5 b32(): displaying unsigned 32-bit integers in binary notation We have now used b32() a few times. The following code is an implementation of it: n >>> 0 means that we are shifting n zero bits to the right. Therefore, in principle, the >>> operator does nothing, but it still coerces n to an unsigned 32-bit integer: > (0b10 << 1).toString(2) '100' > b32(0b10000000000000000000000000000010 >> 1) '11000000000000000000000000000001' > b32(0b10000000000000000000000000000010 >>> 1) '01000000000000000000000000000001' /** * Return a string representing n as a 32-bit unsigned integer, * in binary notation. */ function b32(n) { // >>> ensures highest bit isn’t interpreted as a sign return (n >>> 0).toString(2).padStart(32, '0'); } assert.equal( b32(6), '00000000000000000000000000000110'); > 12 >>> 0 12
- 230. > -12 >>> 0 4294967284 > (2**32 + 1) >>> 0 1
- 231. 17.13 Quick reference: numbers 17.13.1 Global functions for numbers JavaScript has the following four global functions for numbers: isFinite() isNaN() parseFloat() parseInt() However, it is better to use the corresponding methods of Number (Number.isFinite(), etc.), which have fewer pitfalls. They were introduced with ES6 and are discussed below. 17.13.2 Static properties of Number .EPSILON: number [ES6] The difference between 1 and the next representable floating point number. In general, a machine epsilon provides an upper bound for rounding errors in floating point arithmetic. Approximately: 2.2204460492503130808472633361816 × 10-16 .MAX_SAFE_INTEGER: number [ES6]
- 232. The largest integer that JavaScript can represent unambiguously (253−1). .MAX_VALUE: number [ES1] The largest positive finite JavaScript number. Approximately: 1.7976931348623157 × 10308 .MIN_SAFE_INTEGER: number [ES6] The smallest integer that JavaScript can represent unambiguously (−253+1). .MIN_VALUE: number [ES1] The smallest positive JavaScript number. Approximately 5 × 10−324. .NaN: number [ES1] The same as the global variable NaN. .NEGATIVE_INFINITY: number [ES1] The same as -Number.POSITIVE_INFINITY. .POSITIVE_INFINITY: number [ES1] The same as the global variable Infinity. 17.13.3 Static methods of Number
- 233. .isFinite(num: number): boolean [ES6] Returns true if num is an actual number (neither Infinity nor - Infinity nor NaN). .isInteger(num: number): boolean [ES6] Returns true if num is a number and does not have a decimal fraction. .isNaN(num: number): boolean [ES6] Returns true if num is the value NaN: > Number.isFinite(Infinity) false > Number.isFinite(-Infinity) false > Number.isFinite(NaN) false > Number.isFinite(123) true > Number.isInteger(-17) true > Number.isInteger(33) true > Number.isInteger(33.1) false > Number.isInteger('33') false > Number.isInteger(NaN) false > Number.isInteger(Infinity) false > Number.isNaN(NaN) true > Number.isNaN(123)
- 234. .isSafeInteger(num: number): boolean [ES6] Returns true if num is a number and unambiguously represents an integer. .parseFloat(str: string): number [ES6] Coerces its parameter to string and parses it as a floating point number. For converting strings to numbers, Number() (which ignores leading and trailing whitespace) is usually a better choice than Number.parseFloat() (which ignores leading whitespace and illegal trailing characters and can hide problems). .parseInt(str: string, radix=10): number [ES6] Coerces its parameter to string and parses it as an integer, ignoring leading whitespace and illegal trailing characters: The parameter radix specifies the base of the number to be parsed: false > Number.isNaN('abc') false > Number.parseFloat(' 123.4#') 123.4 > Number(' 123.4#') NaN > Number.parseInt(' 123#') 123
- 235. Do not use this method to convert numbers to integers: coercing to string is inefficient. And stopping before the first non-digit is not a good algorithm for removing the fraction of a number. Here is an example where it goes wrong: It is better to use one of the rounding functions of Math to convert a number to an integer: 17.13.4 Methods of Number.prototype (Number.prototype is where the methods of numbers are stored.) .toExponential(fractionDigits?: number): string [ES3] Returns a string that represents the number via exponential notation. With fractionDigits, you can specify, how many digits should be shown of the number that is multiplied with the exponent (the default is to show as many digits as necessary). Example: number too small to get a positive exponent via .toString(). > Number.parseInt('101', 2) 5 > Number.parseInt('FF', 16) 255 > Number.parseInt(1e21, 10) // wrong 1 > Math.trunc(1e21) // correct 1e+21
- 236. Example: fraction not small enough to get a negative exponent via .toString(). .toFixed(fractionDigits=0): string [ES3] Returns an exponent-free representation of the number, rounded to fractionDigits digits. If the number is 1021 or greater, even .toFixed() uses an exponent: .toPrecision(precision?: number): string [ES3] > 1234..toString() '1234' > 1234..toExponential() // 3 fraction digits '1.234e+3' > 1234..toExponential(5) '1.23400e+3' > 1234..toExponential(1) '1.2e+3' > 0.003.toString() '0.003' > 0.003.toExponential() '3e-3' > 0.00000012.toString() // with exponent '1.2e-7' > 0.00000012.toFixed(10) // no exponent '0.0000001200' > 0.00000012.toFixed() '0' > (10 ** 21).toFixed() '1e+21'
- 237. Works like .toString(), but precision specifies how many digits should be shown. If precision is missing, .toString() is used. .toString(radix=10): string [ES1] Returns a string representation of the number. By default, you get a base 10 numeral as a result: If you want the numeral to have a different base, you can specify it via radix: > 1234..toPrecision(3) // requires exponential notation '1.23e+3' > 1234..toPrecision(4) '1234' > 1234..toPrecision(5) '1234.0' > 1.234.toPrecision(3) '1.23' > 123.456.toString() '123.456' > 4..toString(2) // binary (base 2) '100' > 4.5.toString(2) '100.1' > 255..toString(16) // hexadecimal (base 16) 'ff' > 255.66796875.toString(16) 'ff.ab'
- 238. parseInt() provides the inverse operation: it converts a string that contains an integer (no fraction!) numeral with a given base, to a number. 17.13.5 Sources Wikipedia TypeScript’s built-in typings MDN web docs for JavaScript ECMAScript language specification Quiz: advanced See quiz app. > 1234567890..toString(36) 'kf12oi' > parseInt('kf12oi', 36) 1234567890
- 239. 18 Math 18.1 Data properties 18.2 Exponents, roots, logarithms 18.3 Rounding 18.4 Trigonometric Functions 18.5 Various other functions 18.6 Sources Math is an object with data properties and methods for processing numbers. You can see it as a poor man’s module: It was created long before JavaScript had modules.
- 240. 18.1 Data properties Math.E: number [ES1] Euler’s number, base of the natural logarithms, approximately 2.7182818284590452354. Math.LN10: number [ES1] The natural logarithm of 10, approximately 2.302585092994046. Math.LN2: number [ES1] The natural logarithm of 2, approximately 0.6931471805599453. Math.LOG10E: number [ES1] The logarithm of e to base 10, approximately 0.4342944819032518. Math.LOG2E: number [ES1] The logarithm of e to base 2, approximately 1.4426950408889634. Math.PI: number [ES1] The mathematical constant π, ratio of a circle’s circumference to its diameter, approximately 3.1415926535897932.
- 241. Math.SQRT1_2: number [ES1] The square root of 1/2, approximately 0.7071067811865476. Math.SQRT2: number [ES1] The square root of 2, approximately 1.4142135623730951.
- 242. 18.2 Exponents, roots, logarithms Math.cbrt(x: number): number [ES6] Returns the cube root of x. Math.exp(x: number): number [ES1] Returns ex (e being Euler’s number). The inverse of Math.log(). Math.expm1(x: number): number [ES6] Returns Math.exp(x)-1. The inverse of Math.log1p(). Very small numbers (fractions close to 0) are represented with a higher precision. Therefore, this function returns more precise values whenever .exp() returns values close to 1. Math.log(x: number): number [ES1] Returns the natural logarithm of x (to base e, Euler’s number). The inverse of Math.exp(). > Math.cbrt(8) 2 > Math.exp(0) 1 > Math.exp(1) === Math.E true > Math.log(1) 0 > Math.log(Math.E) 1
- 243. Math.log1p(x: number): number [ES6] Returns Math.log(1 + x). The inverse of Math.expm1(). Very small numbers (fractions close to 0) are represented with a higher precision. Therefore, you can provide this function with a more precise argument whenever the argument for .log() is close to 1. Math.log10(x: number): number [ES6] Returns the logarithm of x to base 10. The inverse of 10 ** x. Math.log2(x: number): number [ES6] Returns the logarithm of x to base 2. The inverse of 2 ** x. Math.pow(x: number, y: number): number [ES1] Returns xy , x to the power of y. The same as x ** y. > Math.log(Math.E ** 2) 2 > Math.log10(1) 0 > Math.log10(10) 1 > Math.log10(100) 2 > Math.log2(1) 0 > Math.log2(2) 1 > Math.log2(4) 2
- 244. Math.sqrt(x: number): number [ES1] Returns the square root of x. The inverse of x ** 2. > Math.pow(2, 3) 8 > Math.pow(25, 0.5) 5 > Math.sqrt(9) 3
- 245. 18.3 Rounding Rounding means converting an arbitrary number to an integer (a number without a decimal fraction). The following functions implement different approaches to rounding. Math.ceil(x: number): number [ES1] Returns the smallest (closest to −∞) integer i with x ≤ i. Math.floor(x: number): number [ES1] Returns the largest (closest to +∞) integer i with i ≤ x. Math.round(x: number): number [ES1] Returns the integer that is closest to x. If the decimal fraction of x is .5 then .round() rounds up (to the integer closer to positive infinity): > Math.ceil(2.1) 3 > Math.ceil(2.9) 3 > Math.floor(2.1) 2 > Math.floor(2.9) 2 > Math.round(2.4) 2 > Math.round(2.5) 3
- 246. Math.trunc(x: number): number [ES6] Removes the decimal fraction of x and returns the resulting integer. Tbl. 12 shows the results of the rounding functions for a few representative inputs. Table 12: Rounding functions of Math. Note how things change with negative numbers because “larger” always means “closer to positive infinity”. -2.9 -2.5 -2.1 2.1 2.5 2.9 Math.floor -3 -3 -3 2 2 2 Math.ceil -2 -2 -2 3 3 3 Math.round -3 -2 -2 2 3 3 Math.trunc -2 -2 -2 2 2 2 > Math.trunc(2.1) 2 > Math.trunc(2.9) 2
- 247. 18.4 Trigonometric Functions All angles are specified in radians. Use the following two functions to convert between degrees and radians. Math.acos(x: number): number [ES1] Returns the arc cosine (inverse cosine) of x. Math.acosh(x: number): number [ES6] Returns the inverse hyperbolic cosine of x. Math.asin(x: number): number [ES1] Returns the arc sine (inverse sine) of x. function degreesToRadians(degrees) { return degrees / 180 * Math.PI; } assert.equal(degreesToRadians(90), Math.PI/2); function radiansToDegrees(radians) { return radians / Math.PI * 180; } assert.equal(radiansToDegrees(Math.PI), 180); > Math.acos(0) 1.5707963267948966 > Math.acos(1) 0 > Math.asin(0) 0 > Math.asin(1) 1.5707963267948966
- 248. Math.asinh(x: number): number [ES6] Returns the inverse hyperbolic sine of x. Math.atan(x: number): number [ES1] Returns the arc tangent (inverse tangent) of x. Math.atanh(x: number): number [ES6] Returns the inverse hyperbolic tangent of x. Math.atan2(y: number, x: number): number [ES1] Returns the arc tangent of the quotient y/x. Math.cos(x: number): number [ES1] Returns the cosine of x. Math.cosh(x: number): number [ES6] Returns the hyperbolic cosine of x. Math.hypot(...values: number[]): number [ES6] Returns the square root of the sum of the squares of values (Pythagoras’ theorem): > Math.cos(0) 1 > Math.cos(Math.PI) -1
- 249. Math.sin(x: number): number [ES1] Returns the sine of x. Math.sinh(x: number): number [ES6] Returns the hyperbolic sine of x. Math.tan(x: number): number [ES1] Returns the tangent of x. Math.tanh(x: number): number; [ES6] Returns the hyperbolic tangent of x. > Math.hypot(3, 4) 5 > Math.sin(0) 0 > Math.sin(Math.PI / 2) 1 > Math.tan(0) 0 > Math.tan(1) 1.5574077246549023
- 250. 18.5 Various other functions Math.abs(x: number): number [ES1] Returns the absolute value of x. Math.clz32(x: number): number [ES6] Counts the leading zero bits in the 32-bit integer x. Used in DSP algorithms. Math.max(...values: number[]): number [ES1] Converts values to numbers and returns the largest one. Math.min(...values: number[]): number [ES1] > Math.abs(3) 3 > Math.abs(-3) 3 > Math.abs(0) 0 > Math.clz32(0b01000000000000000000000000000000) 1 > Math.clz32(0b00100000000000000000000000000000) 2 > Math.clz32(2) 30 > Math.clz32(1) 31 > Math.max(3, -5, 24) 24
- 251. Converts values to numbers and returns the smallest one. Math.random(): number [ES1] Returns a pseudo-random number n where 0 ≤ n < 1. Computing a random integer i where 0 ≤ i < max: Math.sign(x: number): number [ES6] Returns the sign of a number: > Math.min(3, -5, 24) -5 function getRandomInteger(max) { return Math.floor(Math.random() * max); } > Math.sign(-8) -1 > Math.sign(0) 0 > Math.sign(3) 1
- 252. 18.6 Sources Wikipedia TypeScript’s built-in typings MDN web docs for JavaScript ECMAScript language specification
- 253. 19 Unicode – a brief introduction (advanced) 19.1 Code points vs. code units 19.1.1 Code points 19.1.2 Encoding Unicode code points: UTF-32, UTF-16, UTF-8 19.2 Encodings used in web development: UTF-16 and UTF-8 19.2.1 Source code internally: UTF-16 19.2.2 Strings: UTF-16 19.2.3 Source code in files: UTF-8 19.3 Grapheme clusters – the real characters Unicode is a standard for representing and managing text in most of the world’s writing systems. Virtually all modern software that works with text, supports Unicode. The standard is maintained by the Unicode Consortium. A new version of the standard is published every year (with new emojis, etc.). Unicode version 1.0.0 was published in October 1991.
- 254. 19.1 Code points vs. code units Two concepts are crucial for understanding Unicode: Code points are numbers that represent Unicode characters. Code units are numbers that encode code points, to store or transmit Unicode text. One or more code units encode a single code point. Each code unit has the same size, which depends on the encoding format that is used. The most popular format, UTF-8, has 8-bit code units. 19.1.1 Code points The first version of Unicode had 16-bit code points. Since then, the number of characters has grown considerably and the size of code points was extended to 21 bits. These 21 bits are partitioned in 17 planes, with 16 bits each: Plane 0: Basic Multilingual Plane (BMP), 0x0000–0xFFFF Contains characters for almost all modern languages (Latin characters, Asian characters, etc.) and many symbols. Plane 1: Supplementary Multilingual Plane (SMP), 0x10000– 0x1FFFF Supports historic writing systems (e.g., Egyptian hieroglyphs and cuneiform) and additional modern writing systems. Supports emojis and many other symbols.
- 255. Plane 2: Supplementary Ideographic Plane (SIP), 0x20000– 0x2FFFF Contains additional CJK (Chinese, Japanese, Korean) ideographs. Plane 3–13: Unassigned Plane 14: Supplementary Special-Purpose Plane (SSP), 0xE0000–0xEFFFF Contains non-graphical characters such as tag characters and glyph variation selectors. Plane 15–16: Supplementary Private Use Area (S PUA A/B), 0x0F0000–0x10FFFF Available for character assignment by parties outside the ISO and the Unicode Consortium. Not standardized. Planes 1-16 are called supplementary planes or astral planes. Let’s check the code points of a few characters: The hexadecimal numbers of the code points tell us that the first three characters reside in plane 0 (within 16 bits), while the emoji resides in plane 1. > 'A'.codePointAt(0).toString(16) '41' > 'ü'.codePointAt(0).toString(16) 'fc' > 'π'.codePointAt(0).toString(16) '3c0' > '🙂'.codePointAt(0).toString(16) '1f642'
- 256. 19.1.2 Encoding Unicode code points: UTF-32, UTF-16, UTF-8 The main ways of encoding code points are three Unicode Transformation Formats (UTFs): UTF-32, UTF-16, UTF-8. The number at the end of each format indicates the size (in bits) of its code units. 19.1.2.1 UTF-32 (Unicode Transformation Format 32) UTF-32 uses 32 bits to store code units, resulting in one code unit per code point. This format is the only one with fixed-length encoding; all others use a varying number of code units to encode a single code point. 19.1.2.2 UTF-16 (Unicode Transformation Format 16) UTF-16 uses 16-bit code units. It encodes code points as follows: The BMP (first 16 bits of Unicode) is stored in single code units. Astral planes: The BMP comprises 0x10_000 code points. Given that Unicode has a total of 0x110_000 code points, we still need to encode the remaining 0x100_000 code points (20 bits). The BMP has two ranges of unassigned code points that provide the necessary storage: Most significant 10 bits (leading surrogate): 0xD800- 0xDBFF
- 257. Least significant 10 bits (trailing surrogate): 0xDC00- 0xDFFF In other words, the two hexadecimal digits at the end contribute 8 bits. But we can only use those 8 bits if a BMP starts with one of the following 2-digit pairs: D8, D9, DA, DB DC, DD, DE, DF Per surrogate, we have a choice between 4 pairs, which is where the remaining 2 bits come from. As a consequence, each UTF-16 code unit is always either a leading surrogate, a trailing surrogate, or encodes a BMP code point. These are two examples of UTF-16-encoded code points: Code point 0x03C0 (π) is in the BMP and can therefore be represented by a single UTF-16 code unit: 0x03C0. Code point 0x1F642 (🙂) is in an astral plane and represented by two code units: 0xD83D and 0xDE42. 19.1.2.3 UTF-8 (Unicode Transformation Format 8) UTF-8 has 8-bit code units. It uses 1–4 code units to encode a code point: Code points Code units 0000–007F 0bbbbbbb (7 bits)
- 258. Code points Code units 0080–07FF 110bbbbb, 10bbbbbb (5+6 bits) 0800–FFFF 1110bbbb, 10bbbbbb, 10bbbbbb (4+6+6 bits) 10000– 1FFFFF 11110bbb, 10bbbbbb, 10bbbbbb, 10bbbbbb (3+6+6+6 bits) Notes: The bit prefix of each code unit tells us: Is it first in a series of code units? If yes, how many code units will follow? Is it second or later in a series of code units? The character mappings in the 0000–007F range are the same as ASCII, which leads to a degree of backward compatibility with older software. Three examples: Character Code point Code units A 0x0041 01000001 π 0x03C0 11001111, 10000000 🙂 0x1F642 11110000, 10011111, 10011001, 10000010
- 259. 19.2 Encodings used in web development: UTF-16 and UTF-8 The Unicode encoding formats that are used in web development are: UTF-16 and UTF-8. 19.2.1 Source code internally: UTF-16 The ECMAScript specification internally represents source code as UTF-16. 19.2.2 Strings: UTF-16 The characters in JavaScript strings are based on UTF-16 code units: For more information on Unicode and strings, consult §20.6 “Atoms of text: Unicode characters, JavaScript characters, grapheme clusters”. 19.2.3 Source code in files: UTF-8 HTML and JavaScript are almost always encoded as UTF-8 these days. > const smiley = '🙂'; > smiley.length 2 > smiley === 'uD83DuDE42' // code units true
- 260. For example, this is how HTML files usually start now: For HTML modules loaded in web browsers, the standard encoding is also UTF-8. <!doctype html> <html> <head> <meta charset="UTF-8"> ···
- 261. 19.3 Grapheme clusters – the real characters The concept of a character becomes remarkably complex once you consider many of the world’s writing systems. On one hand, there are Unicode characters, as represented by code points. On the other hand, there are grapheme clusters. A grapheme cluster corresponds most closely to a symbol displayed on screen or paper. It is defined as “a horizontally segmentable unit of text”. Therefore, official Unicode documents also call it a user-perceived character. One or more code point characters are needed to encode a grapheme cluster. For example, the Devanagari kshi is encoded by 4 code points. We use spreading (...) to split a string into an Array with code point characters (for details, consult §20.6.1 “Working with code points”): Flag emojis are also grapheme clusters and composed of two code point characters – for example, the flag of Japan: More information on grapheme clusters
- 262. For more information, consult “Let’s Stop Ascribing Meaning to Code Points” by Manish Goregaokar. Quiz See quiz app.
- 263. 20 Strings 20.1 Plain string literals 20.1.1 Escaping 20.2 Accessing characters and code points 20.2.1 Accessing JavaScript characters 20.2.2 Accessing Unicode code point characters via for-of and spreading 20.3 String concatenation via + 20.4 Converting to string 20.4.1 Stringifying objects 20.4.2 Customizing the stringification of objects 20.4.3 An alternate way of stringifying values 20.5 Comparing strings 20.6 Atoms of text: Unicode characters, JavaScript characters, grapheme clusters 20.6.1 Working with code points 20.6.2 Working with code units (char codes) 20.6.3 Caveat: grapheme clusters 20.7 Quick reference: Strings 20.7.1 Converting to string 20.7.2 Numeric values of characters 20.7.3 String operators 20.7.4 String.prototype: finding and matching 20.7.5 String.prototype: extracting 20.7.6 String.prototype: combining
- 264. 20.7.7 String.prototype: transforming 20.7.8 Sources Strings are primitive values in JavaScript and immutable. That is, string-related operations always produce new strings and never change existing strings.
- 265. 20.1 Plain string literals Plain string literals are delimited by either single quotes or double quotes: Single quotes are used more often because it makes it easier to mention HTML, where double quotes are preferred. The next chapter covers template literals, which give you: String interpolation Multiple lines Raw string literals (backslash has no special meaning) 20.1.1 Escaping The backslash lets you create special characters: Unix line break: 'n' Windows line break: 'rn' Tab: 't' Backslash: '' The backslash also lets you use the delimiter of a string literal inside that literal: const str1 = 'abc'; const str2 = "abc"; assert.equal(str1, str2);
- 266. assert.equal( 'She said: "Let's go!"', "She said: "Let's go!"");
- 267. 20.2 Accessing characters and code points 20.2.1 Accessing JavaScript characters JavaScript has no extra data type for characters – characters are always represented as strings. 20.2.2 Accessing Unicode code point characters via for-of and spreading Iterating over strings via for-of or spreading (...) visits Unicode code point characters. Each code point character is encoded by 1–2 JavaScript characters. For more information, see §20.6 “Atoms of text: Unicode characters, JavaScript characters, grapheme clusters”. This is how you iterate over the code point characters of a string via for-of: const str = 'abc'; // Reading a character at a given index assert.equal(str[1], 'b'); // Counting the characters in a string: assert.equal(str.length, 3); for (const ch of 'x🙂y') { console.log(ch); } // Output:
- 268. And this is how you convert a string into an Array of code point characters via spreading: // 'x' // '🙂' // 'y' assert.deepEqual([...'x🙂y'], ['x', '🙂', 'y']);
- 269. 20.3 String concatenation via + If at least one operand is a string, the plus operator (+) converts any non-strings to strings and concatenates the result: The assignment operator += is useful if you want to assemble a string, piece by piece: Concatenating via + is efficient Using + to assemble strings is quite efficient because most JavaScript engines internally optimize it. Exercise: Concatenating strings exercises/strings/concat_string_array_test.mjs assert.equal(3 + ' times ' + 4, '3 times 4'); let str = ''; // must be `let`! str += 'Say it'; str += ' one more'; str += ' time'; assert.equal(str, 'Say it one more time');
- 270. 20.4 Converting to string These are three ways of converting a value x to a string: String(x) ''+x x.toString() (does not work for undefined and null) Recommendation: use the descriptive and safe String(). Examples: Pitfall for booleans: If you convert a boolean to a string via String(), you generally can’t convert it back via Boolean(): The only string for which Boolean() returns false, is the empty string. 20.4.1 Stringifying objects assert.equal(String(undefined), 'undefined'); assert.equal(String(null), 'null'); assert.equal(String(false), 'false'); assert.equal(String(true), 'true'); assert.equal(String(123.45), '123.45'); > String(false) 'false' > Boolean('false') true
- 271. Plain objects have a default string representation that is not very useful: Arrays have a better string representation, but it still hides much information: Stringifying functions, returns their source code: 20.4.2 Customizing the stringification of objects You can override the built-in way of stringifying objects by implementing the method toString(): > String({a: 1}) '[object Object]' > String(['a', 'b']) 'a,b' > String(['a', ['b']]) 'a,b' > String([1, 2]) '1,2' > String(['1', '2']) '1,2' > String([true]) 'true' > String(['true']) 'true' > String(true) 'true' > String(function f() {return 4}) 'function f() {return 4}'
- 272. 20.4.3 An alternate way of stringifying values The JSON data format is a text representation of JavaScript values. Therefore, JSON.stringify() can also be used to convert values to strings: The caveat is that JSON only supports null, booleans, numbers, strings, Arrays, and objects (which it always treats as if they were created by object literals). Tip: The third parameter lets you switch on multiline output and specify how much to indent – for example: This statement produces the following output: { "first": "Jane", "last": "Doe" } const obj = { toString() { return 'hello'; } }; assert.equal(String(obj), 'hello'); > JSON.stringify({a: 1}) '{"a":1}' > JSON.stringify(['a', ['b']]) '["a",["b"]]' console.log(JSON.stringify({first: 'Jane', last: 'Doe'}, null, 2
- 273. 20.5 Comparing strings Strings can be compared via the following operators: < <= > >= There is one important caveat to consider: These operators compare based on the numeric values of JavaScript characters. That means that the order that JavaScript uses for strings is different from the one used in dictionaries and phone books: Properly comparing text is beyond the scope of this book. It is supported via the ECMAScript Internationalization API (Intl). > 'A' < 'B' // ok true > 'a' < 'B' // not ok false > 'ä' < 'b' // not ok false
- 274. 20.6 Atoms of text: Unicode characters, JavaScript characters, grapheme clusters Quick recap of §19 “Unicode – a brief introduction”: Unicode characters are represented by code points – numbers which have a range of 21 bits. In JavaScript strings, Unicode is implemented via code units based on the encoding format UTF-16. Each code unit is a 16-bit number. One to two of code units are needed to encode a single code point. Therefore, each JavaScript character is represented by a code unit. In the JavaScript standard library, code units are also called char codes. Which is what they are: numbers for JavaScript characters. Grapheme clusters (user-perceived characters) are written symbols, as displayed on screen or paper. One or more Unicode characters are needed to encode a single grapheme cluster. The following code demonstrates that a single Unicode character comprises one or two JavaScript characters. We count the latter via .length: // 3 Unicode characters, 3 JavaScript characters: assert.equal('abc'.length, 3); // 1 Unicode character, 2 JavaScript characters: assert.equal('🙂'.length, 2);
- 275. The following table summarizes the concepts we have just explored: Entity Numeric representation Size Encoded via Grapheme cluster 1+ code points Unicode character Code point 21 bits 1–2 code units JavaScript character UTF-16 code unit 16 bits – 20.6.1 Working with code points Let’s explore JavaScript’s tools for working with code points. A code point escape lets you specify a code point hexadecimally. It produces one or two JavaScript characters. String.fromCodePoint() converts a single code point to 1–2 JavaScript characters: .codePointAt() converts 1–2 JavaScript characters to a single code point: > 'u{1F642}' '🙂' > String.fromCodePoint(0x1F642) '🙂' > '🙂'.codePointAt(0).toString(16) '1f642'
- 276. You can iterate over a string, which visits Unicode characters (not JavaScript characters). Iteration is described later in this book. One way of iterating is via a for-of loop: Spreading (...) into Array literals is also based on iteration and visits Unicode characters: That makes it a good tool for counting Unicode characters: 20.6.2 Working with code units (char codes) Indices and lengths of strings are based on JavaScript characters (as represented by UTF-16 code units). To specify a code unit hexadecimally, you can use a code unit escape: const str = '🙂a'; assert.equal(str.length, 3); for (const codePointChar of str) { console.log(codePointChar); } // Output: // '🙂' // 'a' > [...'🙂a'] [ '🙂', 'a' ] > [...'🙂a'].length 2 > '🙂a'.length 3
- 277. And you can use String.fromCharCode(). Char code is the standard library’s name for code unit: To get the char code of a character, use .charCodeAt(): 20.6.3 Caveat: grapheme clusters When working with text that may be written in any human language, it’s best to split at the boundaries of grapheme clusters, not at the boundaries of Unicode characters. TC39 is working on Intl.Segmenter, a proposal for the ECMAScript Internationalization API to support Unicode segmentation (along grapheme cluster boundaries, word boundaries, sentence boundaries, etc.). Until that proposal becomes a standard, you can use one of several libraries that are available (do a web search for “JavaScript grapheme”). > 'uD83DuDE42' '🙂' > String.fromCharCode(0xD83D) + String.fromCharCode(0xDE42) '🙂' > '🙂'.charCodeAt(0).toString(16) 'd83d'
- 278. 20.7 Quick reference: Strings Strings are immutable; none of the string methods ever modify their strings. 20.7.1 Converting to string Tbl. 13 describes how various values are converted to strings. Table 13: Converting values to strings. x String(x) undefined 'undefined' null 'null' Boolean value false → 'false', true → 'true' Number value Example: 123 → '123' String value x (input, unchanged) An object Configurable via, e.g., toString() 20.7.2 Numeric values of characters Char code: represents a JavaScript character numerically. JavaScript’s name for Unicode code unit. Size: 16 bits, unsigned Convert number to character: String.fromCharCode() [ES1] Convert character to number: string method .charCodeAt() [ES1] Code point: represents a Unicode character numerically.
- 279. Size: 21 bits, unsigned (17 planes, 16 bits each) Convert number to character: String.fromCodePoint() [ES6] Convert character to number: string method .codePointAt() [ES6] 20.7.3 String operators 20.7.4 String.prototype: finding and matching (String.prototype is where the methods of strings are stored.) .endsWith(searchString: string, endPos=this.length): boolean [ES6] Returns true if the string would end with searchString if its length were endPos. Returns false otherwise. .includes(searchString: string, startPos=0): boolean [ES6] // Access characters via [] const str = 'abc'; assert.equal(str[1], 'b'); // Concatenate strings via + assert.equal('a' + 'b' + 'c', 'abc'); assert.equal('take ' + 3 + ' oranges', 'take 3 oranges'); > 'foo.txt'.endsWith('.txt') true > 'abcde'.endsWith('cd', 4) true
- 280. Returns true if the string contains the searchString and false otherwise. The search starts at startPos. .indexOf(searchString: string, minIndex=0): number [ES1] Returns the lowest index at which searchString appears within the string or -1, otherwise. Any returned index will beminIndex` or higher. .lastIndexOf(searchString: string, maxIndex=Infinity): number [ES1] Returns the highest index at which searchString appears within the string or -1, otherwise. Any returned index will bemaxIndex` or lower. > 'abc'.includes('b') true > 'abc'.includes('b', 2) false > 'abab'.indexOf('a') 0 > 'abab'.indexOf('a', 1) 2 > 'abab'.indexOf('c') -1 > 'abab'.lastIndexOf('ab', 2) 2 > 'abab'.lastIndexOf('ab', 1) 0 > 'abab'.lastIndexOf('ab') 2
- 281. [1 of 2] .match(regExp: string | RegExp): RegExpMatchArray | null [ES3] If regExp is a regular expression with flag /g not set, then .match() returns the first match for regExp within the string. Or null if there is no match. If regExp is a string, it is used to create a regular expression (think parameter of new RegExp()) before performing the previously mentioned steps. The result has the following type: Numbered capture groups become Array indices (which is why this type extends Array). Named capture groups (ES2018) become properties of .groups. In this mode, .match() works like RegExp.prototype.exec(). Examples: [2 of 2] .match(regExp: RegExp): string[] | null [ES3] interface RegExpMatchArray extends Array<string> { index: number; input: string; groups: undefined | { [key: string]: string }; } > 'ababb'.match(/a(b+)/) { 0: 'ab', 1: 'b', index: 0, input: 'ababb', groups: undefin > 'ababb'.match(/a(?<foo>b+)/) { 0: 'ab', 1: 'b', index: 0, input: 'ababb', groups: { foo: > 'abab'.match(/x/) null
- 282. If flag /g of regExp is set, .match() returns either an Array with all matches or null if there was no match. .search(regExp: string | RegExp): number [ES3] Returns the index at which regExp occurs within the string. If regExp is a string, it is used to create a regular expression (think parameter of new RegExp()). .startsWith(searchString: string, startPos=0): boolean [ES6] Returns true if searchString occurs in the string at index startPos. Returns false otherwise. 20.7.5 String.prototype: extracting .slice(start=0, end=this.length): string [ES3] > 'ababb'.match(/a(b+)/g) [ 'ab', 'abb' ] > 'ababb'.match(/a(?<foo>b+)/g) [ 'ab', 'abb' ] > 'abab'.match(/x/g) null > 'a2b'.search(/[0-9]/) 1 > 'a2b'.search('[0-9]') 1 > '.gitignore'.startsWith('.') true > 'abcde'.startsWith('bc', 1) true
- 283. Returns the substring of the string that starts at (including) index start and ends at (excluding) index end. If an index is negative, it is added to .length before it is used (-1 becomes this.length-1, etc.). .split(separator: string | RegExp, limit?: number): string[] [ES3] Splits the string into an Array of substrings – the strings that occur between the separators. The separator can be a string: It can also be a regular expression: The last invocation demonstrates that captures made by groups in the regular expression become elements of the returned Array. Warning: .split('') splits a string into JavaScript characters. That doesn’t work well when dealing with astral > 'abc'.slice(1, 3) 'bc' > 'abc'.slice(1) 'bc' > 'abc'.slice(-2) 'bc' > 'a | b | c'.split('|') [ 'a ', ' b ', ' c' ] > 'a : b : c'.split(/ *: */) [ 'a', 'b', 'c' ] > 'a : b : c'.split(/( *):( *)/) [ 'a', ' ', ' ', 'b', ' ', ' ', 'c' ]
- 284. Unicode characters (which are encoded as two JavaScript characters). For example, emojis are astral: Instead, it is better to use spreading: .substring(start: number, end=this.length): string [ES1] Use .slice() instead of this method. .substring() wasn’t implemented consistently in older engines and doesn’t support negative indices. 20.7.6 String.prototype: combining .concat(...strings: string[]): string [ES3] Returns the concatenation of the string and strings. 'a'.concat('b') is equivalent to 'a'+'b'. The latter is much more popular. .padEnd(len: number, fillString=' '): string [ES2017] Appends (fragments of) fillString to the string until it has the desired length len. If it already has or exceeds len, then it is returned without any changes. > '🙂X🙂'.split('') [ 'uD83D', 'uDE42', 'X', 'uD83D', 'uDE42' ] > [...'🙂X🙂'] [ '🙂', 'X', '🙂' ] > 'ab'.concat('cd', 'ef', 'gh') 'abcdefgh'
- 285. .padStart(len: number, fillString=' '): string [ES2017] Prepends (fragments of) fillString to the string until it has the desired length len. If it already has or exceeds len, then it is returned without any changes. .repeat(count=0): string [ES6] Returns the string, concatenated count times. 20.7.7 String.prototype: transforming .normalize(form: 'NFC'|'NFD'|'NFKC'|'NFKD' = 'NFC'): string [ES6] Normalizes the string according to the Unicode Normalization Forms. > '#'.padEnd(2) '# ' > 'abc'.padEnd(2) 'abc' > '#'.padEnd(5, 'abc') '#abca' > '#'.padStart(2) ' #' > 'abc'.padStart(2) 'abc' > '#'.padStart(5, 'abc') 'abca#' > '*'.repeat() '' > '*'.repeat(3) '***'
- 286. [1 of 2] .replace(searchValue: string | RegExp, replaceValue: string): string [ES3] Replace matches of searchValue with replaceValue. If searchValue is a string, only the first verbatim occurrence is replaced. If searchValue is a regular expression without flag /g, only the first match is replaced. If searchValue is a regular expression with /g then all matches are replaced. Special characters in replaceValue are: $$: becomes $ $n: becomes the capture of numbered group n (alas, $0 stands for the string '$0', it does not refer to the complete match) $&: becomes the complete match $`: becomes everything before the match $': becomes everything after the match Examples: > 'x.x.'.replace('.', '#') 'x#x.' > 'x.x.'.replace(/./, '#') '#.x.' > 'x.x.'.replace(/./g, '#') '####' > 'a 2020-04 b'.replace(/([0-9]{4})-([0-9]{2})/, '|$2|') 'a |04| b' > 'a 2020-04 b'.replace(/([0-9]{4})-([0-9]{2})/, '|$&|') 'a |2020-04| b' > 'a 2020-04 b'.replace(/([0-9]{4})-([0-9]{2})/, '|$`|') 'a |a | b'
- 287. Named capture groups (ES2018) are supported, too: $<name> becomes the capture of named group name Example: [2 of 2] .replace(searchValue: string | RegExp, replacer: (...args: any[]) => string): string [ES3] If the second parameter is a function, occurrences are replaced with the strings it returns. Its parameters args are: matched: string. The complete match g1: string|undefined. The capture of numbered group 1 g2: string|undefined. The capture of numbered group 2 (Etc.) offset: number. Where was the match found in the input string? input: string. The whole input string Named capture groups (ES2018) are supported, too. If there are any, an argument is added at the end with an object whose properties contain the captures: assert.equal( 'a 2020-04 b'.replace( /(?<year>[0-9]{4})-(?<month>[0-9]{2})/, '|$<month>|'), 'a |04| b'); const regexp = /([0-9]{4})-([0-9]{2})/; const replacer = (all, year, month) => '|' + all + '|'; assert.equal( 'a 2020-04 b'.replace(regexp, replacer), 'a |2020-04| b');
- 288. .toUpperCase(): string [ES1] Returns a copy of the string in which all lowercase alphabetic characters are converted to uppercase. How well that works for various alphabets, depends on the JavaScript engine. .toLowerCase(): string [ES1] Returns a copy of the string in which all uppercase alphabetic characters are converted to lowercase. How well that works for various alphabets, depends on the JavaScript engine. .trim(): string [ES5] Returns a copy of the string in which all leading and trailing whitespace (spaces, tabs, line terminators, etc.) is gone. const regexp = /(?<year>[0-9]{4})-(?<month>[0-9]{2})/; const replacer = (...args) => { const groups=args.pop(); return '|' + groups.month + '|'; }; assert.equal( 'a 2020-04 b'.replace(regexp, replacer), 'a |04| b'); > '-a2b-'.toUpperCase() '-A2B-' > 'αβγ'.toUpperCase() 'ΑΒΓ' > '-A2B-'.toLowerCase() '-a2b-' > 'ΑΒΓ'.toLowerCase() 'αβγ'
- 289. .trimEnd(): string [ES2019] Similar to .trim() but only the end of the string is trimmed: .trimStart(): string [ES2019] Similar to .trim() but only the beginning of the string is trimmed: 20.7.8 Sources TypeScript’s built-in typings MDN web docs for JavaScript ECMAScript language specification Exercise: Using string methods exercises/strings/remove_extension_test.mjs Quiz See quiz app. > 'rn#t '.trim() '#' > ' abc '.trim() 'abc' > ' abc '.trimEnd() ' abc' > ' abc '.trimStart() 'abc '
- 290. 21 Using template literals and tagged templates 21.1 Disambiguation: “template” 21.2 Template literals 21.3 Tagged templates 21.3.1 Cooked vs. raw template strings (advanced) 21.3.2 Tag function library: lit-html 21.3.3 Tag function library: re-template-tag 21.3.4 Tag function library: graphql-tag 21.4 Raw string literals 21.5 (Advanced) 21.6 Multiline template literals and indentation 21.6.1 Fix: template tag for dedenting 21.6.2 Fix: .trim() 21.7 Simple templating via template literals 21.7.1 A more complex example 21.7.2 Simple HTML-escaping Before we dig into the two features template literal and tagged template, let’s first examine the multiple meanings of the term template.
- 291. 21.1 Disambiguation: “template” The following three things are significantly different despite all having template in their names and despite all of them looking similar: A text template is a function from data to text. It is frequently used in web development and often defined via text files. For example, the following text defines a template for the library Handlebars: This template has two blanks to be filled in: title and body. It is used like this: A template literal is similar to a string literal, but has additional features – for example, interpolation. It is delimited by backticks: <div class="entry"> <h1>{{title}}</h1> <div class="body"> {{body}} </div> </div> // First step: retrieve the template text, e.g. from a text const tmplFunc = Handlebars.compile(TMPL_TEXT); // compile s const data = {title: 'My page', body: 'Welcome to my page!'} const html = tmplFunc(data); const num = 5; assert.equal(`Count: ${num}!`, 'Count: 5!');
- 292. Syntactically, a tagged template is a template literal that follows a function (or rather, an expression that evaluates to a function). That leads to the function being called. Its arguments are derived from the contents of the template literal. Note that getArgs() receives both the text of the literal and the data interpolated via ${}. const getArgs = (...args) => args; assert.deepEqual( getArgs`Count: ${5}!`, [['Count: ', '!'], 5] );
- 293. 21.2 Template literals A template literal has two new features compared to a normal string literal. First, it supports string interpolation: if you put a dynamically computed value inside a ${}, it is converted to a string and inserted into the string returned by the literal. Second, template literals can span multiple lines: Template literals always produce strings. const MAX = 100; function doSomeWork(x) { if (x > MAX) { throw new Error(`At most ${MAX} allowed: ${x}!`); } // ··· } assert.throws( () => doSomeWork(101), {message: 'At most 100 allowed: 101!'}); const str = `this is a text with multiple lines`;
- 294. 21.3 Tagged templates The expression in line A is a tagged template. It is equivalent to invoking tagFunc() with the arguments listed in the Array in line B. The function tagFunc before the first backtick is called a tag function. Its arguments are: Template strings (first argument): an Array with the text fragments surrounding the interpolations ${}. In the example: ['Setting ', ' is ', '!'] Substitutions (remaining arguments): the interpolated values. In the example: 'dark mode' and true The static (fixed) parts of the literal (the template strings) are kept separate from the dynamic parts (the substitutions). A tag function can return arbitrary values. function tagFunc(...args) { return args; } const setting = 'dark mode'; const value = true; assert.deepEqual( tagFunc`Setting ${setting} is ${value}!`, // (A) [['Setting ', ' is ', '!'], 'dark mode', true] // (B) );
- 295. 21.3.1 Cooked vs. raw template strings (advanced) So far, we have only seen the cooked interpretation of template strings. But tag functions actually get two interpretations: A cooked interpretation where backslashes have special meaning. For example, t produces a tab character. This interpretation of the template strings is stored as an Array in the first argument. A raw interpretation where backslashes do not have special meaning. For example, t produces two characters – a backslash and a t. This interpretation of the template strings is stored in property .raw of the first argument (an Array). The following tag function cookedRaw uses both interpretations: function cookedRaw(templateStrings, ...substitutions) { return { cooked: [...templateStrings], // copy just the Array element raw: templateStrings.raw, substitutions, }; } assert.deepEqual( cookedRaw`tab${'subst'}newline`, { cooked: ['tab', 'newline'], raw: ['tab', 'newline'], substitutions: ['subst'], });
- 296. The raw interpretation enables raw string literals via String.raw (described later) and similar applications. Tagged templates are great for supporting small embedded languages (so-called domain-specific languages). We’ll continue with a few examples. 21.3.2 Tag function library: lit-html lit-html is a templating library that is based on tagged templates and used by the frontend framework Polymer: repeat() is a custom function for looping. Its 2nd parameter produces unique keys for the values returned by the 3rd parameter. Note the nested tagged template used by that parameter. 21.3.3 Tag function library: re-template- tag import {html, render} from 'lit-html'; const template = (items) => html` <ul> ${ repeat(items, (item) => item.id, (item, index) => html`<li>${index}. ${item.name}</li>` ) } </ul> `;
- 297. re-template-tag is a simple library for composing regular expressions. Templates tagged with re produce regular expressions. The main benefit is that you can interpolate regular expressions and plain text via ${} (line A): 21.3.4 Tag function library: graphql-tag The library graphql-tag lets you create GraphQL queries via tagged templates: Additionally, there are plugins for pre-compiling such queries in Babel, TypeScript, etc. const RE_YEAR = re`(?<year>[0-9]{4})`; const RE_MONTH = re`(?<month>[0-9]{2})`; const RE_DAY = re`(?<day>[0-9]{2})`; const RE_DATE = re`/${RE_YEAR}-${RE_MONTH}-${RE_DAY}/u`; // (A) const match = RE_DATE.exec('2017-01-27'); assert.equal(match.groups.year, '2017'); import gql from 'graphql-tag'; const query = gql` { user(id: 5) { firstName lastName } } `;
- 298. 21.4 Raw string literals Raw string literals are implemented via the tag function String.raw. They are string literals where backslashes don’t do anything special (such as escaping characters, etc.): This helps whenever data contains backslashes – for example, strings with regular expressions: All three regular expressions are equivalent. With a normal string literal, you have to write the backslash twice, to escape it for that literal. With a raw string literal, you don’t have to do that. Raw string literals are also useful for specifying Windows filename paths: assert.equal(String.raw`back`, 'back'); const regex1 = /^./; const regex2 = new RegExp('^.'); const regex3 = new RegExp(String.raw`^.`); const WIN_PATH = String.raw`C:foobar`; assert.equal(WIN_PATH, 'C:foobar');
- 299. 21.5 (Advanced) All remaining sections are advanced
- 300. 21.6 Multiline template literals and indentation If you put multiline text in template literals, two goals are in conflict: On one hand, the template literal should be indented to fit inside the source code. On the other hand, the lines of its content should start in the leftmost column. For example: Due to the indentation, the template literal fits well into the source code. Alas, the output is also indented. And we don’t want the return at the beginning and the return plus two spaces at the end. function div(text) { return ` <div> ${text} </div> `; } console.log('Output:'); console.log( div('Hello!') // Replace spaces with mid-dots: .replace(/ /g, '·') // Replace n with #n: .replace(/n/g, '#n') ); Output: # ····<div># ······Hello!#
- 301. There are two ways to fix this: via a tagged template or by trimming the result of the template literal. 21.6.1 Fix: template tag for dedenting The first fix is to use a custom template tag that removes the unwanted whitespace. It uses the first line after the initial line break to determine in which column the text starts and shortens the indentation everywhere. It also removes the line break at the very beginning and the indentation at the very end. One such template tag is dedent by Desmond Brand: This time, the output is not indented: 21.6.2 Fix: .trim() ····</div># ·· import dedent from 'dedent'; function divDedented(text) { return dedent` <div> ${text} </div> `.replace(/n/g, '#n'); } console.log('Output:'); console.log(divDedented('Hello!')); Output: <div># Hello!# </div>
- 302. The second fix is quicker, but also dirtier: The string method .trim() removes the superfluous whitespace at the beginning and at the end, but the content itself must start in the leftmost column. The advantage of this solution is that you don’t need a custom tag function. The downside is that it looks ugly. The output is the same as with dedent: function divDedented(text) { return ` <div> ${text} </div> `.trim().replace(/n/g, '#n'); } console.log('Output:'); console.log(divDedented('Hello!')); Output: <div># Hello!# </div>
- 303. 21.7 Simple templating via template literals While template literals look like text templates, it is not immediately obvious how to use them for (text) templating: A text template gets its data from an object, while a template literal gets its data from variables. The solution is to use a template literal in the body of a function whose parameter receives the templating data – for example: 21.7.1 A more complex example As a more complex example, we’d like to take an Array of addresses and produce an HTML table. This is the Array: The function tmpl() that produces the HTML table looks as follows: const tmpl = (data) => `Hello ${data.name}!`; assert.equal(tmpl({name: 'Jane'}), 'Hello Jane!'); const addresses = [ { first: '<Jane>', last: 'Bond' }, { first: 'Lars', last: '<Croft>' }, ]; onst tmpl = (addrs) => ` table> ${addrs.map( (addr) => ` <tr> <td>${escapeHtml(addr.first)}</td> <td>${escapeHtml(addr.last)}</td> </tr>
- 304. This code contains two templating functions: The first one (line 1) takes addrs, an Array with addresses, and returns a string with a table. The second one (line 4) takes addr, an object containing an address, and returns a string with a table row. Note the .trim() at the end, which removes unnecessary whitespace. The first templating function produces its result by wrapping a table element around an Array that it joins into a string (line 10). That Array is produced by mapping the second templating function to each element of addrs (line 3). It therefore contains strings with table rows. The helper function escapeHtml() is used to escape special HTML characters (line 6 and line 7). Its implementation is shown in the next subsection. Let us call tmpl() with the addresses and log the result: The output is: `.trim() ).join('')} /table> .trim(); console.log(tmpl(addresses)); <table> <tr> <td><Jane></td> <td>Bond</td> </tr><tr> <td>Lars</td>
- 305. 21.7.2 Simple HTML-escaping The following function escapes plain text so that it is displayed verbatim in HTML: Exercise: HTML templating Exercise with bonus challenge: exercises/template- literals/templating_test.mjs Quiz See quiz app. <td><Croft></td> </tr> </table> function escapeHtml(str) { return str .replace(/&/g, '&') // first! .replace(/>/g, '>') .replace(/</g, '<') .replace(/"/g, '"') .replace(/'/g, ''') .replace(/`/g, '`') ; } assert.equal( escapeHtml('Rock & Roll'), 'Rock & Roll'); assert.equal( escapeHtml('<blank>'), '<blank>');
- 306. 22 Symbols 22.1 Use cases for symbols 22.1.1 Symbols: values for constants 22.1.2 Symbols: unique property keys 22.2 Publicly known symbols 22.3 Converting symbols Symbols are primitive values that are created via the factory function Symbol(): The parameter is optional and provides a description, which is mainly useful for debugging. On one hand, symbols are like objects in that each value created by Symbol() is unique and not compared by value: On the other hand, they also behave like primitive values. They have to be categorized via typeof: And they can be property keys in objects: const mySymbol = Symbol('mySymbol'); > Symbol() === Symbol() false const sym = Symbol(); assert.equal(typeof sym, 'symbol');
- 307. const obj = { [sym]: 123, };
- 308. 22.1 Use cases for symbols The main use cases for symbols, are: Values for constants Unique property keys 22.1.1 Symbols: values for constants Let’s assume you want to create constants representing the colors red, orange, yellow, green, blue, and violet. One simple way of doing so would be to use strings: On the plus side, logging that constant produces helpful output. On the minus side, there is a risk of mistaking an unrelated value for a color because two strings with the same content are considered equal: We can fix that problem via symbols: Let’s use symbol-valued constants to implement a function: const COLOR_BLUE = 'Blue'; const MOOD_BLUE = 'Blue'; assert.equal(COLOR_BLUE, MOOD_BLUE); const COLOR_BLUE = Symbol('Blue'); const MOOD_BLUE = Symbol('Blue'); assert.notEqual(COLOR_BLUE, MOOD_BLUE);
- 309. 22.1.2 Symbols: unique property keys The keys of properties (fields) in objects are used at two levels: The program operates at a base level. The keys at that level reflect the problem that the program solves. Libraries and ECMAScript operate at a meta-level. The keys at that level are used by services operating on base-level data and code. One such key is 'toString'. const COLOR_RED = Symbol('Red'); const COLOR_ORANGE = Symbol('Orange'); const COLOR_YELLOW = Symbol('Yellow'); const COLOR_GREEN = Symbol('Green'); const COLOR_BLUE = Symbol('Blue'); const COLOR_VIOLET = Symbol('Violet'); function getComplement(color) { switch (color) { case COLOR_RED: return COLOR_GREEN; case COLOR_ORANGE: return COLOR_BLUE; case COLOR_YELLOW: return COLOR_VIOLET; case COLOR_GREEN: return COLOR_RED; case COLOR_BLUE: return COLOR_ORANGE; case COLOR_VIOLET: return COLOR_YELLOW; default: throw new Exception('Unknown color: '+color); } } assert.equal(getComplement(COLOR_YELLOW), COLOR_VIOLET);
- 310. The following code demonstrates the difference: Properties .x and .y exist at the base level. They hold the coordinates of the point represented by pt and are used to solve a problem – computing with points. Method .toString() exists at a meta-level. It is used by JavaScript to convert this object to a string. Meta-level properties must never interfere with base-level properties. That is, their keys must never overlap. That is difficult when both language and libraries contribute to the meta-level. For example, it is now impossible to give new meta-level methods simple names, such as toString because they might clash with existing base- level names. Python’s solution to this problem is to prefix and suffix special names with two underscores: __init__, __iter__, __hash__, etc. However, even with this solution, libraries can’t have their own meta-level properties because those might be in conflict with future language properties. Symbols, used as property keys, help us here: Each symbol is unique and a symbol key never clashes with any other string or symbol key. 22.1.2.1 Example: a library with a meta-level method const pt = { x: 7, y: 4, toString() { return `(${this.x}, ${this.y})`; }, }; assert.equal(String(pt), '(7, 4)');
- 311. As an example, let’s assume we are writing a library that treats objects differently if they implement a special method. This is what defining a property key for such a method and implementing it for an object would look like: The square brackets in line A enable us to specify that the method must have the key specialMethod. More details are explained in §28.5.2 “Computed property keys”. const specialMethod = Symbol('specialMethod'); const obj = { _id: 'kf12oi', [specialMethod]() { // (A) return this._id; } }; assert.equal(obj[specialMethod](), 'kf12oi');
- 312. 22.2 Publicly known symbols Symbols that play special roles within ECMAScript are called publicly known symbols. Examples include: Symbol.iterator: makes an object iterable. It’s the key of a method that returns an iterator. For more information on this topic, see §30 “Synchronous iteration”. Symbol.hasInstance: customizes how instanceof works. If an object implements a method with that key, it can be used at the right-hand side of that operator. For example: Symbol.toStringTag: influences the default .toString() method. Note: It’s usually better to override .toString(). Exercises: Publicly known symbols Symbol.toStringTag: exercises/symbols/to_string_tag_test.mjs const PrimitiveNull = { [Symbol.hasInstance](x) { return x === null; } }; assert.equal(null instanceof PrimitiveNull, true); > String({}) '[object Object]' > String({ [Symbol.toStringTag]: 'is no money' }) '[object is no money]'
- 314. 22.3 Converting symbols What happens if we convert a symbol sym to another primitive type? Tbl. 14 has the answers. Table 14: The results of converting symbols to other primitive types. Convert to Explicit conversion Coercion (implicit conv.) boolean Boolean(sym) → OK !sym → OK number Number(sym) → TypeError sym*2 → TypeError string String(sym) → OK ''+sym → TypeError sym.toString() → OK `${sym}` → TypeError One key pitfall with symbols is how often exceptions are thrown when converting them to something else. What is the thinking behind that? First, conversion to number never makes sense and should be warned about. Second, converting a symbol to a string is indeed useful for diagnostic output. But it also makes sense to warn about accidentally turning a symbol into a string (which is a different kind of property key): The downside is that the exceptions make working with symbols more complicated. You have to explicitly convert symbols when const obj = {}; const sym = Symbol(); assert.throws( () => { obj['__'+sym+'__'] = true }, { message: 'Cannot convert a Symbol value to a string' });
- 315. assembling strings via the plus operator: Quiz See quiz app. > const mySymbol = Symbol('mySymbol'); > 'Symbol I used: ' + mySymbol TypeError: Cannot convert a Symbol value to a string > 'Symbol I used: ' + String(mySymbol) 'Symbol I used: Symbol(mySymbol)'
- 316. 23 Control flow statements 23.1 Conditions of control flow statements 23.2 Controlling loops: break and continue 23.2.1 break 23.2.2 break plus label: leaving any labeled statement 23.2.3 continue 23.3 if statements 23.3.1 The syntax of if statements 23.4 switch statements 23.4.1 A first example of a switch statement 23.4.2 Don’t forget to return or break! 23.4.3 Empty case clauses 23.4.4 Checking for illegal values via a default clause 23.5 while loops 23.5.1 Examples of while loops 23.6 do-while loops 23.7 for loops 23.7.1 Examples of for loops 23.8 for-of loops 23.8.1 const: for-of vs. for 23.8.2 Iterating over iterables 23.8.3 Iterating over [index, element] pairs of Arrays 23.9 for-await-of loops 23.10 for-in loops (avoid)
- 317. This chapter covers the following control flow statements: if statement (ES1) switch statement (ES3) while loop (ES1) do-while loop (ES3) for loop (ES1) for-of loop (ES6) for-await-of loop (ES2018) for-in loop (ES1) Before we get to the actual control flow statements, let’s take a look at two operators for controlling loops.
- 318. 23.1 Conditions of control flow statements if, while, and do-while have conditions that are, in principle, boolean. However, a condition only has to be truthy (true if coerced to boolean) in order to be accepted. In other words, the following two control flow statements are equivalent: This is a list of all falsy values: undefined, null false 0, NaN '' All other values are truthy. For more information, see §16.2 “Falsy and truthy values”. if (value) {} if (Boolean(value) === true) {}
- 319. 23.2 Controlling loops: break and continue The two operators break and continue can be used to control loops and other statements while you are inside them. 23.2.1 break There are two versions of break: one with an operand and one without an operand. The latter version works inside the following statements: while, do-while, for, for-of, for-await-of, for-in and switch. It immediately leaves the current statement: 23.2.2 break plus label: leaving any labeled statement break with an operand works everywhere. Its operand is a label. Labels can be put in front of any statement, including blocks. break foo leaves the statement whose label is foo: for (const x of ['a', 'b', 'c']) { console.log(x); if (x === 'b') break; console.log('---') } // Output: // 'a' // '---' // 'b'
- 320. In the following example, we use break with a label to leave a loop differently when we succeeded (line A). Then we skip what comes directly after the loop, which is where we end up if we failed. 23.2.3 continue foo: { // label if (condition) break foo; // labeled break // ··· } function findSuffix(stringArray, suffix) { let result; search_block: { for (const str of stringArray) { if (str.endsWith(suffix)) { // Success: result = str; break search_block; // (A) } } // for // Failure: result = '(Untitled)'; } // search_block return { suffix, result }; // Same as: {suffix: suffix, result: result} } assert.deepEqual( findSuffix(['foo.txt', 'bar.html'], '.html'), { suffix: '.html', result: 'bar.html' } ); assert.deepEqual( findSuffix(['foo.txt', 'bar.html'], '.mjs'), { suffix: '.mjs', result: '(Untitled)' } );
- 321. continue only works inside while, do-while, for, for-of, for-await- of, and for-in. It immediately leaves the current loop iteration and continues with the next one – for example: const lines = [ 'Normal line', '# Comment', 'Another normal line', ]; for (const line of lines) { if (line.startsWith('#')) continue; console.log(line); } // Output: // 'Normal line' // 'Another normal line'
- 322. 23.3 if statements These are two simple if statements: one with just a “then” branch and one with both a “then” branch and an “else” branch: Instead of the block, else can also be followed by another if statement: You can continue this chain with more else ifs. 23.3.1 The syntax of if statements if (cond) { // then branch } if (cond) { // then branch } else { // else branch } if (cond1) { // ··· } else if (cond2) { // ··· } if (cond1) { // ··· } else if (cond2) { // ··· } else { // ··· }
- 323. The general syntax of if statements is: So far, the then_statement has always been a block, but we can use any statement. That statement must be terminated with a semicolon: That means that else if is not its own construct; it’s simply an if statement whose else_statement is another if statement. if (cond) «then_statement» else «else_statement» if (true) console.log('Yes'); else console.log('No');
- 324. 23.4 switch statements A switch statement looks as follows: switch («switch_expression») { «switch_body» } The body of switch consists of zero or more case clauses: case «case_expression»: «statements» And, optionally, a default clause: default: «statements» A switch is executed as follows: It evaluates the switch expression. It jumps to the first case clause whose expression has the same result as the switch expression. Otherwise, if there is no such clause, it jumps to the default clause. Otherwise, if there is no default clause, it does nothing. 23.4.1 A first example of a switch statement Let’s look at an example: The following function converts a number from 1–7 to the name of a weekday.
- 325. 23.4.2 Don’t forget to return or break! At the end of a case clause, execution continues with the next case clause, unless you return or break – for example: function dayOfTheWeek(num) { switch (num) { case 1: return 'Monday'; case 2: return 'Tuesday'; case 3: return 'Wednesday'; case 4: return 'Thursday'; case 5: return 'Friday'; case 6: return 'Saturday'; case 7: return 'Sunday'; } } assert.equal(dayOfTheWeek(5), 'Friday'); function englishToFrench(english) { let french; switch (english) { case 'hello': french = 'bonjour'; case 'goodbye': french = 'au revoir'; } return french; } // The result should be 'bonjour'! assert.equal(englishToFrench('hello'), 'au revoir');
- 326. That is, our implementation of dayOfTheWeek() only worked because we used return. We can fix englishToFrench() by using break: 23.4.3 Empty case clauses The statements of a case clause can be omitted, which effectively gives us multiple case expressions per case clause: function englishToFrench(english) { let french; switch (english) { case 'hello': french = 'bonjour'; break; case 'goodbye': french = 'au revoir'; break; } return french; } assert.equal(englishToFrench('hello'), 'bonjour'); // ok function isWeekDay(name) { switch (name) { case 'Monday': case 'Tuesday': case 'Wednesday': case 'Thursday': case 'Friday': return true; case 'Saturday': case 'Sunday': return false; } } assert.equal(isWeekDay('Wednesday'), true); assert.equal(isWeekDay('Sunday'), false);
- 327. 23.4.4 Checking for illegal values via a default clause A default clause is jumped to if the switch expression has no other match. That makes it useful for error checking: Exercises: switch exercises/control-flow/number_to_month_test.mjs Bonus: exercises/control- flow/is_object_via_switch_test.mjs function isWeekDay(name) { switch (name) { case 'Monday': case 'Tuesday': case 'Wednesday': case 'Thursday': case 'Friday': return true; case 'Saturday': case 'Sunday': return false; default: throw new Error('Illegal value: '+name); } } assert.throws( () => isWeekDay('January'), {message: 'Illegal value: January'});
- 328. 23.5 while loops A while loop has the following syntax: while («condition») { «statements» } Before each loop iteration, while evaluates condition: If the result is falsy, the loop is finished. If the result is truthy, the while body is executed one more time. 23.5.1 Examples of while loops The following code uses a while loop. In each loop iteration, it removes the first element of arr via .shift() and logs it. If the condition always evaluates to true, then while is an infinite loop: const arr = ['a', 'b', 'c']; while (arr.length > 0) { const elem = arr.shift(); // remove first element console.log(elem); } // Output: // 'a' // 'b' // 'c' while (true) { if (Math.random() === 0) break; }
- 329. 23.6 do-while loops The do-while loop works much like while, but it checks its condition after each loop iteration, not before. prompt() is a global function that is available in web browsers. It prompts the user to input text and returns it. let input; do { input = prompt('Enter text:'); console.log(input); } while (input !== ':q');
- 330. 23.7 for loops A for loop has the following syntax: for («initialization»; «condition»; «post_iteration») { «statements» } The first line is the head of the loop and controls how often the body (the remainder of the loop) is executed. It has three parts and each of them is optional: initialization: sets up variables, etc. for the loop. Variables declared here via let or const only exist inside the loop. condition: This condition is checked before each loop iteration. If it is falsy, the loop stops. post_iteration: This code is executed after each loop iteration. A for loop is therefore roughly equivalent to the following while loop: «initialization» while («condition») { «statements» «post_iteration» } 23.7.1 Examples of for loops As an example, this is how to count from zero to two via a for loop:
- 331. This is how to log the contents of an Array via a for loop: If you omit all three parts of the head, you get an infinite loop: for (let i=0; i<3; i++) { console.log(i); } // Output: // 0 // 1 // 2 const arr = ['a', 'b', 'c']; for (let i=0; i<arr.length; i++) { console.log(arr[i]); } // Output: // 'a' // 'b' // 'c' for (;;) { if (Math.random() === 0) break; }
- 332. 23.8 for-of loops A for-of loop iterates over an iterable – a data container that supports the iteration protocol. Each iterated value is stored in a variable, as specified in the head: for («iteration_variable» of «iterable») { «statements» } The iteration variable is usually created via a variable declaration: But you can also use a (mutable) variable that already exists: 23.8.1 const: for-of vs. for Note that in for-of loops you can use const. The iteration variable can still be different for each iteration (it just can’t change during the iteration). Think of it as a new const declaration being executed each time in a fresh scope. const iterable = ['hello', 'world']; for (const elem of iterable) { console.log(elem); } // Output: // 'hello' // 'world' const iterable = ['hello', 'world']; let elem; for (elem of iterable) { console.log(elem); }
- 333. In contrast, in for loops you must declare variables via let or var if their values change. 23.8.2 Iterating over iterables As mentioned before, for-of works with any iterable object, not just with Arrays – for example, with Sets: 23.8.3 Iterating over [index, element] pairs of Arrays Lastly, you can also use for-of to iterate over the [index, element] entries of Arrays: With [index, element], we are using destructuring to access Array elements. Exercise: for-of exercises/control-flow/array_to_string_test.mjs const set = new Set(['hello', 'world']); for (const elem of set) { console.log(elem); } const arr = ['a', 'b', 'c']; for (const [index, elem] of arr.entries()) { console.log(`${index} -> ${elem}`); } // Output: // '0 -> a' // '1 -> b' // '2 -> c'
- 334. 23.9 for-await-of loops for-await-of is like for-of, but it works with asynchronous iterables instead of synchronous ones. And it can only be used inside async functions and async generators. for-await-of is described in detail in the chapter on asynchronous iteration. for await (const item of asyncIterable) { // ··· }
- 335. 23.10 for-in loops (avoid) Recommendation: don’t use for-in loops for-in has several pitfalls. Therefore, it is usually best to avoid it. This is an example of using for-in properly, which involves boilerplate code (line A): We can implement the same functionality without for-in, which is almost always better: function getOwnPropertyNames(obj) { const result = []; for (const key in obj) { if ({}.hasOwnProperty.call(obj, key)) { // (A) result.push(key); } } return result; } assert.deepEqual( getOwnPropertyNames({ a: 1, b:2 }), ['a', 'b']); assert.deepEqual( getOwnPropertyNames(['a', 'b']), ['0', '1']); // strings! function getOwnPropertyNames(obj) { const result = []; for (const key of Object.keys(obj)) { result.push(key); } return result; }
- 336. Quiz See quiz app.
- 337. 24 Exception handling 24.1 Motivation: throwing and catching exceptions 24.2 throw 24.2.1 Options for creating error objects 24.3 The try statement 24.3.1 The try block 24.3.2 The catch clause 24.3.3 The finally clause 24.4 Error classes 24.4.1 Properties of error objects This chapter covers how JavaScript handles exceptions. Why doesn’t JavaScript throw exceptions more often? JavaScript didn’t support exceptions until ES3. That explains why they are used sparingly by the language and its standard library.
- 338. 24.1 Motivation: throwing and catching exceptions Consider the following code. It reads profiles stored in files into an Array with instances of class Profile: Let’s examine what happens in line B: An error occurred, but the best place to handle the problem is not the current location, it’s line A. There, we can skip the current file and move on to the next one. Therefore: function readProfiles(filePaths) { const profiles = []; for (const filePath of filePaths) { try { const profile = readOneProfile(filePath); profiles.push(profile); } catch (err) { // (A) console.log('Error in: '+filePath, err); } } } function readOneProfile(filePath) { const profile = new Profile(); const file = openFile(filePath); // ··· (Read the data in `file` into `profile`) return profile; } function openFile(filePath) { if (!fs.existsSync(filePath)) { throw new Error('Could not find file '+filePath); // (B) } // ··· (Open the file whose path is `filePath`) }
- 339. In line B, we use a throw statement to indicate that there was a problem. In line A, we use a try-catch statement to handle the problem. When we throw, the following constructs are active: readProfiles(···) for (const filePath of filePaths) try readOneProfile(···) openFile(···) if (!fs.existsSync(filePath)) throw One by one, throw exits the nested constructs, until it encounters a try statement. Execution continues in the catch clause of that try statement.
- 340. 24.2 throw This is the syntax of the throw statement: Any value can be thrown, but it’s best to throw an instance of Error or its subclasses. 24.2.1 Options for creating error objects Use class Error. That is less limiting in JavaScript than in a more static language because you can add your own properties to instances: Use one of JavaScript’s subclasses of Error (which are listed later). Subclass Error yourself. throw «value»; throw new Error('Problem!'); const err = new Error('Could not find the file'); err.filePath = filePath; throw err; class MyError extends Error { } function func() { throw new MyError('Problem!'); } assert.throws( () => func(), MyError);
- 341. 24.3 The try statement The maximal version of the try statement looks as follows: You can combine these clauses as follows: try-catch try-finally try-catch-finally Since ECMAScript 2019, you can omit the catch parameter (error), if you are not interested in the value that was thrown. 24.3.1 The try block The try block can be considered the body of the statement. This is where we execute the regular code. 24.3.2 The catch clause If an exception reaches the try block, then it is assigned to the parameter of the catch clause and the code in that clause is executed. try { «try_statements» } catch (error) { «catch_statements» } finally { «finally_statements» }
- 342. Next, execution normally continues after the try statement. That may change if: There is a return, break, or throw inside the catch block. There is a finally clause (which is always executed before the try statement ends). The following code demonstrates that the value that is thrown in line A is indeed caught in line B. 24.3.3 The finally clause The code inside the finally clause is always executed at the end of a try statement – no matter what happens in the try block or the catch clause. Let’s look at a common use case for finally: You have created a resource and want to always destroy it when you are done with it, no matter what happens while working with it. You’d implement that as follows: const errorObject = new Error(); function func() { throw errorObject; // (A) } try { func(); } catch (err) { // (B) assert.equal(err, errorObject); } const resource = createResource(); try {
- 343. 24.3.3.1 finally is always executed The finally clause is always executed, even if an error is thrown (line A): And even if there is a return statement (line A): // Work with `resource`. Errors may be thrown. } finally { resource.destroy(); } let finallyWasExecuted = false; assert.throws( () => { try { throw new Error(); // (A) } finally { finallyWasExecuted = true; } }, Error ); assert.equal(finallyWasExecuted, true); let finallyWasExecuted = false; function func() { try { return; // (A) } finally { finallyWasExecuted = true; } } func(); assert.equal(finallyWasExecuted, true);
- 344. 24.4 Error classes Error is the common superclass of all built-in error classes. It has the following subclasses (I’m quoting the ECMAScript specification): RangeError: Indicates a value that is not in the set or range of allowable values. ReferenceError: Indicate that an invalid reference value has been detected. SyntaxError: Indicates that a parsing error has occurred. TypeError: is used to indicate an unsuccessful operation when none of the other NativeError objects are an appropriate indication of the failure cause. URIError: Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition. 24.4.1 Properties of error objects Consider err, an instance of Error: Two properties of err are especially useful: .message: contains just the error message. const err = new Error('Hello!'); assert.equal(String(err), 'Error: Hello!'); assert.equal(err.message, 'Hello!');
- 345. .stack: contains a stack trace. It is supported by all mainstream browsers. Exercise: Exception handling exercises/exception-handling/call_function_test.mjs Quiz See quiz app. assert.equal( err.stack, ` Error: Hello! at ch_exception-handling.mjs:1:13 `.trim());
- 346. 25 Callable values 25.1 Kinds of functions 25.2 Ordinary functions 25.2.1 Parts of a function declaration 25.2.2 Roles played by ordinary functions 25.2.3 Names of ordinary functions 25.3 Specialized functions 25.3.1 Specialized functions are still functions 25.3.2 Recommendation: prefer specialized functions 25.3.3 Arrow functions 25.4 More kinds of functions and methods 25.5 Returning values from functions and methods 25.6 Parameter handling 25.6.1 Terminology: parameters vs. arguments 25.6.2 Terminology: callback 25.6.3 Too many or not enough arguments 25.6.4 Parameter default values 25.6.5 Rest parameters 25.6.6 Named parameters 25.6.7 Simulating named parameters 25.6.8 Spreading (...) into function calls 25.7 Dynamically evaluating code: eval(), new Function() (advanced) 25.7.1 eval() 25.7.2 new Function()
- 348. 25.1 Kinds of functions JavaScript has two categories of functions: An ordinary function can play several roles: Real function Method Constructor function A specialized function can only play one of those roles – for example: An arrow function can only be a real function. A method can only be a method. A class can only be a constructor function. The next two sections explain what all of those things mean.
- 349. 25.2 Ordinary functions The following code shows three ways of doing (roughly) the same thing: creating an ordinary function. As we have seen in §12.8 “Declarations: scope and activation”, function declarations are activated early, while variable declarations (e.g., via const) are not. The syntax of function declarations and function expressions is very similar. The context determines which is which. For more information on this kind of syntactic ambiguity, consult §8.5 “Ambiguous syntax”. 25.2.1 Parts of a function declaration Let’s examine the parts of a function declaration via an example: // Function declaration (a statement) function ordinary1(a, b, c) { // ··· } // const plus anonymous function expression const ordinary2 = function (a, b, c) { // ··· }; // const plus named function expression const ordinary3 = function myName(a, b, c) { // `myName` is only accessible in here };
- 350. add is the name of the function declaration. add(x, y) is the head of the function declaration. x and y are the parameters. The curly braces ({ and }) and everything between them are the body of the function declaration. The return statement explicitly returns a value from the function. 25.2.2 Roles played by ordinary functions Consider the following function declaration from the previous section: This function declaration creates an ordinary function whose name is add. As an ordinary function, add() can play three roles: Real function: invoked via a function call. Method: stored in property, invoked via a method call. function add(x, y) { return x + y; } function add(x, y) { return x + y; } assert.equal(add(2, 1), 3); const obj = { addAsMethod: add }; assert.equal(obj.addAsMethod(2, 4), 6); // (A)
- 351. In line A, obj is called the receiver of the method call. It can be accessed via this inside the method. Constructor function/class: invoked via new. (As an aside, the names of classes normally start with capital letters.) Ordinary function vs. real function In JavaScript, we distinguish: The entity ordinary function The role real function, as played by an ordinary function In many other programming languages, the entity function only plays one role – function. Therefore, the same name function can be used for both. 25.2.3 Names of ordinary functions The name of a function expression is only accessible inside the function, where the function can use it to refer to itself (e.g., for self- recursion): const inst = new add(); assert.equal(inst instanceof add, true); const func = function funcExpr() { return funcExpr }; assert.equal(func(), func); // The name `funcExpr` only exists inside the function: assert.throws(() => funcExpr(), ReferenceError);
- 352. In contrast, the name of a function declaration is accessible inside the current scope: function funcDecl() { return funcDecl } // The name `funcDecl` exists in the current scope assert.equal(funcDecl(), funcDecl);
- 353. 25.3 Specialized functions Specialized functions are single-purpose versions of ordinary functions. Each one of them specializes in a single role: The purpose of an arrow function is to be a real function: The purpose of a method is to be a method: The purpose of a class is to be a constructor function: Apart from nicer syntax, each kind of specialized function also supports new features, making them better at their jobs than ordinary functions. Arrow functions are explained later in this chapter. Methods are explained in the chapter on single objects. Classes are explained in the chapter on classes. Tbl. 15 lists the capabilities of ordinary and specialized functions. Table 15: Capabilities of four kinds of functions. “Lexical this” means that this is defined by the surroundings of an arrow function, not by method calls. const arrow = () => { return 123 }; assert.equal(arrow(), 123); const obj = { method() { return 'abc' } }; assert.equal(obj.method(), 'abc'); class MyClass { /* ··· */ } const inst = new MyClass();
- 354. Function call Method call Constructor call Function call Method call Constructor call Ordinary function (this === undefined) ✔ ✔ Arrow function ✔ (lexical this) ✘ Method (this === undefined) ✔ ✘ Class ✘ ✘ ✔ 25.3.1 Specialized functions are still functions It’s important to note that arrow functions, methods, and classes are still categorized as functions: 25.3.2 Recommendation: prefer specialized functions Normally, you should prefer specialized functions over ordinary functions, especially classes and methods. The choice between an arrow function and an ordinary function is less clear-cut, though: > (() => {}) instanceof Function true > ({ method() {} }.method) instanceof Function true > (class SomeClass {}) instanceof Function true
- 355. On one hand, an ordinary function has this as an implicit parameter. That parameter is set to undefined during function calls – which is not what you want. An arrow function treats this like any other variable. For details, see §28.4.6 “Avoiding the pitfalls of this”. On the other hand, I like the syntax of a function declaration (which produces an ordinary function). If you don’t use this inside it, it is mostly equivalent to const plus arrow function: 25.3.3 Arrow functions Arrow functions were added to JavaScript for two reasons: 1. To provide a more concise way for creating functions. 2. To make working with real functions easier: You can’t refer to the this of the surrounding scope inside an ordinary function. Next, we’ll first look at the syntax of arrow functions and then how they help with this. 25.3.3.1 The syntax of arrow functions Let’s review the syntax of an anonymous function expression: function funcDecl(x, y) { return x * y; } const arrowFunc = (x, y) => { return x * y; }; const f = function (x, y, z) { return 123 };
- 356. The (roughly) equivalent arrow function looks as follows. Arrow functions are expressions. Here, the body of the arrow function is a block. But it can also be an expression. The following arrow function works exactly like the previous one. If an arrow function has only a single parameter and that parameter is an identifier (not a destructuring pattern) then you can omit the parentheses around the parameter: That is convenient when passing arrow functions as parameters to other functions or methods: This previous example demonstrates one benefit of arrow functions – conciseness. If we perform the same task with a function expression, our code is more verbose: 25.3.3.2 Arrow functions: lexical this Ordinary functions can be both methods and real functions. Alas, the two roles are in conflict: const f = (x, y, z) => { return 123 }; const f = (x, y, z) => 123; const id = x => x; > [1,2,3].map(x => x+1) [ 2, 3, 4 ] [1,2,3].map(function (x) { return x+1 });
- 357. As each ordinary function can be a method, it has its own this. The own this makes it impossible to access the this of the surrounding scope from inside an ordinary function. And that is inconvenient for real functions. The following code demonstrates the issue: In this code, we can observe two ways of handling this: Dynamic this: In line A, we try to access the this of .someMethod() from an ordinary function. There, it is shadowed by the function’s own this, which is undefined (as filled in by the function call). Given that ordinary functions receive their this via (dynamic) function or method calls, their this is called dynamic. Lexical this: In line B, we again try to access the this of .someMethod(). This time, we succeed because the arrow function const person = { name: 'Jill', someMethod() { const ordinaryFunc = function () { assert.throws( () => this.name, // (A) /^TypeError: Cannot read property 'name' of undefined$/) }; const arrowFunc = () => { assert.equal(this.name, 'Jill'); // (B) }; ordinaryFunc(); arrowFunc(); }, }
- 358. does not have its own this. this is resolved lexically, just like any other variable. That’s why the this of arrow functions is called lexical. 25.3.3.3 Syntax pitfall: returning an object literal from an arrow function If you want the expression body of an arrow function to be an object literal, you must put the literal in parentheses: If you don’t, JavaScript thinks, the arrow function has a block body (that doesn’t return anything): {a: 1} is interpreted as a block with the label a: and the expression statement 1. Without an explicit return statement, the block body returns undefined. This pitfall is caused by syntactic ambiguity: object literals and code blocks have the same syntax. We use the parentheses to tell JavaScript that the body is an expression (an object literal) and not a statement (a block). For more information on shadowing this, consult §28.4.5 “this pitfall: accidentally shadowing this”. const func1 = () => ({a: 1}); assert.deepEqual(func1(), { a: 1 }); const func2 = () => {a: 1}; assert.deepEqual(func2(), undefined);
- 359. 25.4 More kinds of functions and methods This section is a summary of upcoming content This section mainly serves as a reference for the current and upcoming chapters. Don’t worry if you don’t understand everything. So far, all (real) functions and methods, that we have seen, were: Single-result Synchronous Later chapters will cover other modes of programming: Iteration treats objects as containers of data (so-called iterables) and provides a standardized way for retrieving what is inside them. If a function or a method returns an iterable, it returns multiple values. Asynchronous programming deals with handling a long- running computation. You are notified when the computation is finished and can do something else in between. The standard pattern for asynchronously delivering single results is called Promise. These modes can be combined – for example, there are synchronous iterables and asynchronous iterables.
- 360. Several new kinds of functions and methods help with some of the mode combinations: Async functions help implement functions that return Promises. There are also async methods. Synchronous generator functions help implement functions that return synchronous iterables. There are also synchronous generator methods. Asynchronous generator functions help implement functions that return asynchronous iterables. There are also asynchronous generator methods. That leaves us with 4 kinds (2 × 2) of functions and methods: Synchronous vs. asynchronous Generator vs. single-result Tbl. 16 gives an overview of the syntax for creating these 4 kinds of functions and methods. Table 16: Syntax for creating functions and methods. The last column specifies how many values are produced by an entity. Result Values Sync function Sync method function f() {} { m() {} } value 1 f = function () {} f = () => {} Sync generator function Sync gen. method function* f() {} { * m() {} } iterable 0+
- 361. Result Values f = function* () {} Async function Async method async function f() {} { async m() {} } Promise 1 f = async function () {} f = async () => {} Async generator function Async gen. method async function* f() {} { async * m() {} } async iterable 0+ f = async function* () {}
- 362. 25.5 Returning values from functions and methods (Everything mentioned in this section applies to both functions and methods.) The return statement explicitly returns a value from a function: Another example: If, at the end of a function, you haven’t returned anything explicitly, JavaScript returns undefined for you: function func() { return 123; } assert.equal(func(), 123); function boolToYesNo(bool) { if (bool) { return 'Yes'; } else { return 'No'; } } assert.equal(boolToYesNo(true), 'Yes'); assert.equal(boolToYesNo(false), 'No'); function noReturn() { // No explicit return } assert.equal(noReturn(), undefined);
- 363. 25.6 Parameter handling Once again, I am only mentioning functions in this section, but everything also applies to methods. 25.6.1 Terminology: parameters vs. arguments The term parameter and the term argument basically mean the same thing. If you want to, you can make the following distinction: Parameters are part of a function definition. They are also called formal parameters and formal arguments. Arguments are part of a function call. They are also called actual parameters and actual arguments. 25.6.2 Terminology: callback A callback or callback function is a function that is an argument of a function or method call. The following is an example of a callback: const myArray = ['a', 'b']; const callback = (x) => console.log(x); myArray.forEach(callback); // Output: // 'a' // 'b'
- 364. JavaScript uses the term callback broadly In other programming languages, the term callback often has a narrower meaning: it refers to a pattern for delivering results asynchronously, via a function-valued parameter. In this meaning, the callback (or continuation) is invoked after a function has completely finished its computation. Callbacks as an asynchronous pattern, are described in the chapter on asynchronous programming. 25.6.3 Too many or not enough arguments JavaScript does not complain if a function call provides a different number of arguments than expected by the function definition: Extra arguments are ignored. Missing parameters are set to undefined. For example: function foo(x, y) { return [x, y]; } // Too many arguments: assert.deepEqual(foo('a', 'b', 'c'), ['a', 'b']); // The expected number of arguments: assert.deepEqual(foo('a', 'b'), ['a', 'b']);
- 365. 25.6.4 Parameter default values Parameter default values specify the value to use if a parameter has not been provided – for example: undefined also triggers the default value: 25.6.5 Rest parameters A rest parameter is declared by prefixing an identifier with three dots (...). During a function or method call, it receives an Array with all remaining arguments. If there are no extra arguments at the end, it is an empty Array – for example: // Not enough arguments: assert.deepEqual(foo('a'), ['a', undefined]); function f(x, y=0) { return [x, y]; } assert.deepEqual(f(1), [1, 0]); assert.deepEqual(f(), [undefined, 0]); assert.deepEqual( f(undefined, undefined), [undefined, 0]); function f(x, ...y) { return [x, y]; } assert.deepEqual( f('a', 'b', 'c'), ['a', ['b', 'c']]); assert.deepEqual(
- 366. 25.6.5.1 Enforcing a certain number of arguments via a rest parameter You can use a rest parameter to enforce a certain number of arguments. Take, for example, the following function: This is how we force callers to always provide two arguments: In line A, we access the elements of args via destructuring. 25.6.6 Named parameters When someone calls a function, the arguments provided by the caller are assigned to the parameters received by the callee. Two common ways of performing the mapping are: 1. Positional parameters: An argument is assigned to a parameter if they have the same position. A function call with only positional arguments looks as follows. f(), [undefined, []]); function createPoint(x, y) { return {x, y}; // same as {x: x, y: y} } function createPoint(...args) { if (args.length !== 2) { throw new Error('Please provide exactly 2 arguments!'); } const [x, y] = args; // (A) return {x, y}; }
- 367. 2. Named parameters: An argument is assigned to a parameter if they have the same name. JavaScript doesn’t have named parameters, but you can simulate them. For example, this is a function call with only (simulated) named arguments: Named parameters have several benefits: They lead to more self-explanatory code because each argument has a descriptive label. Just compare the two versions of selectEntries(): with the second one, it is much easier to see what happens. The order of the arguments doesn’t matter (as long as the names are correct). Handling more than one optional parameter is more convenient: callers can easily provide any subset of all optional parameters and don’t have to be aware of the ones they omit (with positional parameters, you have to fill in preceding optional parameters, with undefined). 25.6.7 Simulating named parameters JavaScript doesn’t have real named parameters. The official way of simulating them is via object literals: selectEntries(3, 20, 2) selectEntries({start: 3, end: 20, step: 2}) function selectEntries({start=0, end=-1, step=1}) { return {start, end, step};
- 368. This function uses destructuring to access the properties of its single parameter. The pattern it uses is an abbreviation for the following pattern: This destructuring pattern works for empty object literals: But it does not work if you call the function without any parameters: You can fix this by providing a default value for the whole pattern. This default value works the same as default values for simpler parameter definitions: if the parameter is missing, the default is used. 25.6.8 Spreading (...) into function calls If you put three dots (...) in front of the argument of a function call, then you spread it. That means that the argument must be an } {start: start=0, end: end=-1, step: step=1} > selectEntries({}) { start: 0, end: -1, step: 1 } > selectEntries() TypeError: Cannot destructure property `start` of 'undefined' or function selectEntries({start=0, end=-1, step=1} = {}) { return {start, end, step}; } assert.deepEqual( selectEntries(), { start: 0, end: -1, step: 1 });
- 369. iterable object and the iterated values all become arguments. In other words, a single argument is expanded into multiple arguments – for example: Spreading and rest parameters use the same syntax (...), but they serve opposite purposes: Rest parameters are used when defining functions or methods. They collect arguments into Arrays. Spread arguments are used when calling functions or methods. They turn iterable objects into arguments. 25.6.8.1 Example: spreading into Math.max() Math.max() returns the largest one of its zero or more arguments. Alas, it can’t be used for Arrays, but spreading gives us a way out: function func(x, y) { console.log(x); console.log(y); } const someIterable = ['a', 'b']; func(...someIterable); // same as func('a', 'b') // Output: // 'a' // 'b' > Math.max(-1, 5, 11, 3) 11 > Math.max(...[-1, 5, 11, 3]) 11 > Math.max(-1, ...[-5, 11], 3) 11
- 370. 25.6.8.2 Example: spreading into Array.prototype.push() Similarly, the Array method .push() destructively adds its zero or more parameters to the end of its Array. JavaScript has no method for destructively appending an Array to another one. Once again, we are saved by spreading: Exercises: Parameter handling Positional parameters: exercises/callables/positional_parameters_test.mjs Named parameters: exercises/callables/named_parameters_test.mjs const arr1 = ['a', 'b']; const arr2 = ['c', 'd']; arr1.push(...arr2); assert.deepEqual(arr1, ['a', 'b', 'c', 'd']);
- 371. 25.7 Dynamically evaluating code: eval(), new Function() (advanced) Next, we’ll look at two ways of evaluating code dynamically: eval() and new Function(). 25.7.1 eval() Given a string str with JavaScript code, eval(str) evaluates that code and returns the result: There are two ways of invoking eval(): Directly, via a function call. Then the code in its argument is evaluated inside the current scope. Indirectly, not via a function call. Then it evaluates its code in global scope. “Not via a function call” means “anything that looks different than eval(···)”: eval.call(undefined, '···') (0, eval)('···') (uses the comma operator) globalThis.eval('···') const e = eval; e('···') Etc. > eval('2 ** 4') 16
- 372. The following code illustrates the difference: Evaluating code in global context is safer because the code has access to fewer internals. 25.7.2 new Function() new Function() creates a function object and is invoked as follows: The previous statement is equivalent to the next statement. Note that «param_1», etc., are not inside string literals, anymore. In the next example, we create the same function twice, first via new Function(), then via a function expression: new Function() creates non-strict mode functions globalThis.myVariable = 'global'; function func() { const myVariable = 'local'; // Direct eval assert.equal(eval('myVariable'), 'local'); // Indirect eval assert.equal(eval.call(undefined, 'myVariable'), 'global'); } const func = new Function('«param_1»', ···, '«param_n»', '«func_ const func = function («param_1», ···, «param_n») { «func_body» }; const times1 = new Function('a', 'b', 'return a * b'); const times2 = function (a, b) { return a * b };
- 373. Functions created via new Function() are sloppy. 25.7.3 Recommendations Avoid dynamic evaluation of code as much as you can: It’s a security risk because it may enable an attacker to execute arbitrary code with the privileges of your code. It may be switched off – for example, in browsers, via a Content Security Policy. Very often, JavaScript is dynamic enough so that you don’t need eval() or similar. In the following example, what we are doing with eval() (line A) can be achieved just as well without it (line B). If you have to dynamically evaluate code: Prefer new Function() over eval(): it always executes its code in global context and a function provides a clean interface to the evaluated code. Prefer indirect eval over direct eval: evaluating code in global context is safer. Quiz See quiz app. const obj = {a: 1, b: 2}; const propKey = 'b'; assert.equal(eval('obj.' + propKey), 2); // (A) assert.equal(obj[propKey], 2); // (B)
- 374. 26 Environments: under the hood of variables (bonus) 26.1 Environment: data structure for managing variables 26.2 Recursion via environments 26.2.1 Executing the code 26.3 Nested scopes via environments 26.3.1 Executing the code 26.4 Closures and environments In this chapter, we take a closer look at how the ECMAScript language specification handles variables.
- 375. 26.1 Environment: data structure for managing variables An environment is the data structure that the ECMAScript specification uses to manage variables. It is a dictionary whose keys are variable names and whose values are the values of those variables. Each scope has its associated environment. Environments must be able to support the following phenomena related to variables: Recursion Nested scopes Closures We’ll use examples to illustrate how that is done for each phenomenon.
- 376. 26.2 Recursion via environments We’ll tackle recursion first. Consider the following code: For each function call, you need fresh storage space for the variables (parameters and local variables) of the called function. This is managed via a stack of so-called execution contexts, which are references to environments (for the purpose of this chapter). Environments themselves are stored on the heap. That is necessary because they occasionally live on after execution has left their scopes (we’ll see that when exploring closures). Therefore, they themselves can’t be managed via a stack. 26.2.1 Executing the code While executing the code, we make the following pauses: function f(x) { return x * 2; } function g(y) { const tmp = y + 1; return f(tmp); } assert.equal(g(3), 8); function f(x) { // Pause 3 return x * 2; } function g(y) { const tmp = y + 1; // Pause 2 return f(tmp);
- 377. This is what happens: Pause 1 – before calling g() (fig. 7). Pause 2 – while executing g() (fig. 8). Pause 3 – while executing f() (fig. 9). Remaining steps: Every time there is a return, one execution context is removed from the stack. 0 g function (y) { … } function (x) { … } f Lexical environments Execution contexts Figure 7: Recursion, pause 1 – before calling g(): The execution context stack has one entry, which points to the top-level environment. In that environment, there are two entries; one for f() and one for g(). 1 0 g function (y) { … } function (x) { … } f Lexical environments Execution contexts tmp 4 3 y Figure 8: Recursion, pause 2 – while executing g(): The top of the execution context stack points to the environment that was created } // Pause 1 assert.equal(g(3), 8);
- 378. for g(). That environment contains entries for the argument y and for the local variable tmp. 2 1 0 g function (y) { … } function (x) { … } f Lexical environments Execution contexts tmp 4 3 y x 4 Figure 9: Recursion, pause 3 – while executing f(): The top execution context now points to the environment for f().
- 379. 26.3 Nested scopes via environments We use the following code to explore how nested scopes are implemented via environments. Here, we have three nested scopes: The top-level scope, the scope of f(), and the scope of square(). Observations: The scopes are connected. An inner scope “inherits” all the variables of an outer scope (minus the ones it shadows). Nesting scopes as a mechanism is independent of recursion. The latter is best managed by a stack of independent environments. The former is a relationship that each environment has with the environment “in which” it is created. Therefore, the environment of each scope points to the environment of the surrounding scope via a field called outer. When we are looking up the value of a variable, we first search for its name in the current environment, then in the outer environment, then in the outer environment’s outer environment, etc. The whole chain of function f(x) { function square() { const result = x * x; return result; } return square(); } assert.equal(f(6), 36);
- 380. outer environments contains all variables that can currently be accessed (minus shadowed variables). When you make a function call, you create a new environment. The outer environment of that environment is the environment in which the function was created. To help set up the field outer of environments created via function calls, each function has an internal property named [[Scope]] that points to its “birth environment”. 26.3.1 Executing the code These are the pauses we are making while executing the code: This is what happens: Pause 1 – before calling f() (fig. 10). Pause 2 – while executing f() (fig. 11). Pause 3 – while executing square() (fig. 12). After that, return statements pop execution entries off the stack. function f(x) { function square() { const result = x * x; // Pause 3 return result; } // Pause 2 return square(); } // Pause 1 assert.equal(f(6), 36);
- 381. 0 f Lexical environments Execution contexts Functions [[Scope]] Figure 10: Nested scopes, pause 1 – before calling f(): The top-level environment has a single entry, for f(). The birth environment of f() is the top-level environment. Therefore, f’s [[Scope]] points to it. 1 0 f square 6 x outer Lexical environments Execution contexts Functions [[Scope]] [[Scope]] f(6) Figure 11: Nested scopes, pause 2 – while executing f(): There is now an environment for the function call f(6). The outer environment of that environment is the birth environment of f() (the top-level environment at index 0). We can see that the field outer was set to the value of f’s [[Scope]]. Furthermore, the [[Scope]] of the new function square() is the environment that was just created. 2 1 0 f square 6 x outer Lexical environments Execution contexts Functions [[Scope]] [[Scope]] result 36 outer f(6) square() Figure 12: Nested scopes, pause 3 – while executing square(): The previous pattern was repeated: the outer of the most recent environment was set up via the [[Scope]] of the function that we just called. The chain of scopes created via outer, contains all variables that are active right now. For example, we can access result, square,
- 382. and f if we want to. Environments reflect two aspects of variables. First, the chain of outer environments reflects the nested static scopes. Second, the stack of execution contexts reflects what function calls were made, dynamically.
- 383. 26.4 Closures and environments To see how environments are used to implement closures, we are using the following example: What is going on here? add() is a function that returns a function. When we make the nested function call add(3)(1) in line B, the first parameter is for add(), the second parameter is for the function it returns. This works because the function created in line A does not lose the connection to its birth scope when it leaves that scope. The associated environment is kept alive by that connection and the function still has access to variable x in that environment (x is free inside the function). This nested way of calling add() has an advantage: if you only make the first function call, you get a version of add() whose parameter x is already filled in: Converting a function with two parameters into two nested functions with one parameter each, is called currying. add() is a curried function. function add(x) { return (y) => { // (A) return x + y; }; } assert.equal(add(3)(1), 4); // (B) const plus2 = add(2); assert.equal(plus2(5), 7);
- 384. Only filling in some of the parameters of a function is called partial application (the function has not been fully applied yet). Method .bind() of functions performs partial application. In the previous example, we can see that partial application is simple if a function is curried. 26.4.0.1 Executing the code As we are executing the following code, we are making three pauses: This is what happens: Pause 1 – during the execution of add(2) (fig. 13). Pause 2 – after the execution of add(2) (fig. 14). Pause 3 – while executing plus2(5) (fig. 15). [[Scope]] 1 0 ( uninit.) plus2 add 2 x outer Lexical environments Execution contexts Functions [[Scope]] add(2) Figure 13: Closures, pause 1 – during the execution of add(2): We can see that the function returned by add() already exists (see bottom function add(x) { return (y) => { // Pause 3: plus2(5) return x + y; }; // Pause 1: add(2) } const plus2 = add(2); // Pause 2 assert.equal(plus2(5), 7);
- 385. right corner) and that it points to its birth environment via its internal property [[Scope]]. Note that plus2 is still in its temporal dead zone and uninitialized. plus2 add 0 2 x outer Lexical environments Execution contexts Functions [[Scope]] [[Scope]] Kept alive by closure add(2) Figure 14: Closures, pause 2 – after the execution of add(2): plus2 now points to the function returned by add(2). That function keeps its birth environment (the environment of add(2)) alive via its [[Scope]]. plus2 add 1 0 2 x outer Lexical environments Execution contexts Functions [[Scope]] [[Scope]] 5 y outer add(2) plus2(5) Figure 15: Closures, pause 3 – while executing plus2(5): The [[Scope]] of plus2 is used to set up the outer of the new environment. That’s how the current function gets access to x.
- 386. 27 Modules 27.1 Overview: syntax of ECMAScript modules 27.1.1 Exporting 27.1.2 Importing 27.2 JavaScript source code formats 27.2.1 Code before built-in modules was written in ECMAScript 5 27.3 Before we had modules, we had scripts 27.4 Module systems created prior to ES6 27.4.1 Server side: CommonJS modules 27.4.2 Client side: AMD (Asynchronous Module Definition) modules 27.4.3 Characteristics of JavaScript modules 27.5 ECMAScript modules 27.5.1 ES modules: syntax, semantics, loader API 27.6 Named exports and imports 27.6.1 Named exports 27.6.2 Named imports 27.6.3 Namespace imports 27.6.4 Named exporting styles: inline versus clause (advanced) 27.7 Default exports and imports 27.7.1 The two styles of default-exporting 27.7.2 The default export as a named export (advanced) 27.8 More details on exporting and importing
- 387. 27.8.1 Imports are read-only views on exports 27.8.2 ESM’s transparent support for cyclic imports (advanced) 27.9 npm packages 27.9.1 Packages are installed inside a directory node_modules/ 27.9.2 Why can npm be used to install frontend libraries? 27.10 Naming modules 27.11 Module specifiers 27.11.1 Categories of module specifiers 27.11.2 ES module specifiers in browsers 27.11.3 ES module specifiers on Node.js 27.12 Loading modules dynamically via import() 27.12.1 Example: loading a module dynamically 27.12.2 Use cases for import() 27.13 Preview: import.meta.url 27.13.1 import.meta.url and class URL 27.13.2 import.meta.url on Node.js 27.14 Polyfills: emulating native web platform features (advanced) 27.14.1 Sources of this section
- 388. 27.1 Overview: syntax of ECMAScript modules 27.1.1 Exporting 27.1.2 Importing // Named exports export function f() {} export const one = 1; export {foo, b as bar}; // Default exports export default function f() {} // declaration with optional name // Replacement for `const` (there must be exactly one value) export default 123; // Re-exporting from another module export * from './some-module.mjs'; export {foo, b as bar} from './some-module.mjs'; // Named imports import {foo, bar as b} from './some-module.mjs'; // Namespace import import * as someModule from './some-module.mjs'; // Default import import someModule from './some-module.mjs'; // Combinations: import someModule, * as someModule from './some-module.mjs'; import someModule, {foo, bar as b} from './some-module.mjs'; // Empty import (for modules with side effects) import './some-module.mjs';
- 389. 27.2 JavaScript source code formats The current landscape of JavaScript modules is quite diverse: ES6 brought built-in modules, but the source code formats that came before them, are still around, too. Understanding the latter helps understand the former, so let’s investigate. The next sections describe the following ways of delivering JavaScript source code: Scripts are code fragments that browsers run in global scope. They are precursors of modules. CommonJS modules are a module format that is mainly used on servers (e.g., via Node.js). AMD modules are a module format that is mainly used in browsers. ECMAScript modules are JavaScript’s built-in module format. It supersedes all previous formats. Tbl. 17 gives an overview of these code formats. Note that for CommonJS modules and ECMAScript modules, two filename extensions are commonly used. Which one is appropriate depends on how you want to use a file. Details are given later in this chapter. Table 17: Ways of delivering JavaScript source code. Runs on Loaded Filename ext. Script browsers async .js
- 390. Runs on Loaded Filename ext. CommonJS module servers sync .js .cjs AMD module browsers async .js ECMAScript module browsers and servers async .js .mjs 27.2.1 Code before built-in modules was written in ECMAScript 5 Before we get to built-in modules (which were introduced with ES6), all code that you’ll see, will be written in ES5. Among other things: ES5 did not have const and let, only var. ES5 did not have arrow functions, only function expressions.
- 391. 27.3 Before we had modules, we had scripts Initially, browsers only had scripts – pieces of code that were executed in global scope. As an example, consider an HTML file that loads script files via the following HTML: The main file is my-module.js, where we simulate a module: myModule is a global variable that is assigned the result of immediately invoking a function expression. The function expression <script src="other-module1.js"></script> <script src="other-module2.js"></script> <script src="my-module.js"></script> var myModule = (function () { // Open IIFE // Imports (via global variables) var importedFunc1 = otherModule1.importedFunc1; var importedFunc2 = otherModule2.importedFunc2; // Body function internalFunc() { // ··· } function exportedFunc() { importedFunc1(); importedFunc2(); internalFunc(); } // Exports (assigned to global variable `myModule`) return { exportedFunc: exportedFunc, }; })(); // Close IIFE
- 392. starts in the first line. It is invoked in the last line. This way of wrapping a code fragment is called immediately invoked function expression (IIFE, coined by Ben Alman). What do we gain from an IIFE? var is not block-scoped (like const and let), it is function-scoped: the only way to create new scopes for var-declared variables is via functions or methods (with const and let, you can use either functions, methods, or blocks {}). Therefore, the IIFE in the example hides all of the following variables from global scope and minimizes name clashes: importedFunc1, importedFunc2, internalFunc, exportedFunc. Note that we are using an IIFE in a particular manner: at the end, we pick what we want to export and return it via an object literal. That is called the revealing module pattern (coined by Christian Heilmann). This way of simulating modules, has several issues: Libraries in script files export and import functionality via global variables, which risks name clashes. Dependencies are not stated explicitly, and there is no built-in way for a script to load the scripts it depends on. Therefore, the web page has to load not just the scripts that are needed by the page but also the dependencies of those scripts, the dependencies’ dependencies, etc. And it has to do so in the right order!
- 393. 27.4 Module systems created prior to ES6 Prior to ECMAScript 6, JavaScript did not have built-in modules. Therefore, the flexible syntax of the language was used to implement custom module systems within the language. Two popular ones are: CommonJS (targeting the server side) AMD (Asynchronous Module Definition, targeting the client side) 27.4.1 Server side: CommonJS modules The original CommonJS standard for modules was created for server and desktop platforms. It was the foundation of the original Node.js module system, where it achieved enormous popularity. Contributing to that popularity were the npm package manager for Node and tools that enabled using Node modules on the client side (browserify, webpack, and others). From now on, CommonJS module means the Node.js version of this standard (which has a few additional features). This is an example of a CommonJS module: // Imports var importedFunc1 = require('./other-module1.js').importedFunc1; var importedFunc2 = require('./other-module2.js').importedFunc2; // Body function internalFunc() {
- 394. CommonJS can be characterized as follows: Designed for servers. Modules are meant to be loaded synchronously (the importer waits while the imported module is loaded and executed). Compact syntax. 27.4.2 Client side: AMD (Asynchronous Module Definition) modules The AMD module format was created to be easier to use in browsers than the CommonJS format. Its most popular implementation is RequireJS. The following is an example of an AMD module. // ··· } function exportedFunc() { importedFunc1(); importedFunc2(); internalFunc(); } // Exports module.exports = { exportedFunc: exportedFunc, }; define(['./other-module1.js', './other-module2.js'], function (otherModule1, otherModule2) { var importedFunc1 = otherModule1.importedFunc1; var importedFunc2 = otherModule2.importedFunc2; function internalFunc() { // ··· }
- 395. AMD can be characterized as follows: Designed for browsers. Modules are meant to be loaded asynchronously. That’s a crucial requirement for browsers, where code can’t wait until a module has finished downloading. It has to be notified once the module is available. The syntax is slightly more complicated. On the plus side, AMD modules can be executed directly. In contrast, CommonJS modules must either be compiled before deployment or custom source code must be generated and evaluated dynamically (think eval()). That isn’t always permitted on the web. 27.4.3 Characteristics of JavaScript modules Looking at CommonJS and AMD, similarities between JavaScript module systems emerge: There is one module per file. Such a file is basically a piece of code that is executed: function exportedFunc() { importedFunc1(); importedFunc2(); internalFunc(); } return { exportedFunc: exportedFunc, }; });
- 396. Local scope: The code is executed in a local “module scope”. Therefore, by default, all of the variables, functions, and classes declared in it are internal and not global. Exports: If you want any declared entity to be exported, you must explicitly mark it as an export. Imports: Each module can import exported entities from other modules. Those other modules are identified via module specifiers (usually paths, occasionally full URLs). Modules are singletons: Even if a module is imported multiple times, only a single “instance” of it exists. No global variables are used. Instead, module specifiers serve as global IDs.
- 397. 27.5 ECMAScript modules ECMAScript modules (ES modules or ESM) were introduced with ES6. They continue the tradition of JavaScript modules and have all of their aforementioned characteristics. Additionally: With CommonJS, ES modules share the compact syntax and support for cyclic dependencies. With AMD, ES modules share being designed for asynchronous loading. ES modules also have new benefits: The syntax is even more compact than CommonJS’s. Modules have static structures (which can’t be changed at runtime). That helps with static checking, optimized access of imports, dead code elimination, and more. Support for cyclic imports is completely transparent. This is an example of ES module syntax: import {importedFunc1} from './other-module1.mjs'; import {importedFunc2} from './other-module2.mjs'; function internalFunc() { ··· } export function exportedFunc() { importedFunc1(); importedFunc2();
- 398. From now on, “module” means “ECMAScript module”. 27.5.1 ES modules: syntax, semantics, loader API The full standard of ES modules comprises the following parts: 1. Syntax (how code is written): What is a module? How are imports and exports declared? Etc. 2. Semantics (how code is executed): How are variable bindings exported? How are imports connected with exports? Etc. 3. A programmatic loader API for configuring module loading. Parts 1 and 2 were introduced with ES6. Work on part 3 is ongoing. internalFunc(); }
- 399. 27.6 Named exports and imports 27.6.1 Named exports Each module can have zero or more named exports. As an example, consider the following two files: lib/my-math.mjs main.mjs Module my-math.mjs has two named exports: square and LIGHTSPEED. To export something, we put the keyword export in front of a declaration. Entities that are not exported are private to a module and can’t be accessed from outside. 27.6.2 Named imports Module main.mjs has a single named import, square: // Not exported, private to module function times(a, b) { return a * b; } export function square(x) { return times(x, x); } export const LIGHTSPEED = 299792458; import {square} from './lib/my-math.mjs'; assert.equal(square(3), 9);
- 400. It can also rename its import: 27.6.2.1 Syntactic pitfall: named importing is not destructuring Both named importing and destructuring look similar: But they are quite different: Imports remain connected with their exports. You can destructure again inside a destructuring pattern, but the {} in an import statement can’t be nested. The syntax for renaming is different: Rationale: Destructuring is reminiscent of an object literal (including nesting), while importing evokes the idea of renaming. Exercise: Named exports exercises/modules/export_named_test.mjs 27.6.3 Namespace imports import {square as sq} from './lib/my-math.mjs'; assert.equal(sq(3), 9); import {foo} from './bar.mjs'; // import const {foo} = require('./bar.mjs'); // destructuring import {foo as f} from './bar.mjs'; // importing const {foo: f} = require('./bar.mjs'); // destructuring
- 401. Namespace imports are an alternative to named imports. If we namespace-import a module, it becomes an object whose properties are the named exports. This is what main.mjs looks like if we use a namespace import: 27.6.4 Named exporting styles: inline versus clause (advanced) The named export style we have seen so far was inline: We exported entities by prefixing them with the keyword export. But we can also use separate export clauses. For example, this is what lib/my-math.mjs looks like with an export clause: With an export clause, we can rename before exporting and use different names internally: import * as myMath from './lib/my-math.mjs'; assert.equal(myMath.square(3), 9); assert.deepEqual( Object.keys(myMath), ['LIGHTSPEED', 'square']); function times(a, b) { return a * b; } function square(x) { return times(x, x); } const LIGHTSPEED = 299792458; export { square, LIGHTSPEED }; // semicolon! function times(a, b) { return a * b;
- 402. } function sq(x) { return times(x, x); } const LS = 299792458; export { sq as square, LS as LIGHTSPEED, // trailing comma is optional };
- 403. 27.7 Default exports and imports Each module can have at most one default export. The idea is that the module is the default-exported value. Avoid mixing named exports and default exports A module can have both named exports and a default export, but it’s usually better to stick to one export style per module. As an example for default exports, consider the following two files: my-func.mjs main.mjs Module my-func.mjs has a default export: Module main.mjs default-imports the exported function: Note the syntactic difference: the curly braces around named imports indicate that we are reaching into the module, while a default import is the module. What are use cases for default exports? const GREETING = 'Hello!'; export default function () { return GREETING; } import myFunc from './my-func.mjs'; assert.equal(myFunc(), 'Hello!');
- 404. The most common use case for a default export is a module that contains a single function or a single class. 27.7.1 The two styles of default-exporting There are two styles of doing default exports. First, you can label existing declarations with export default: Second, you can directly default-export values. In that style, export default is itself much like a declaration. 27.7.1.1 Why are there two default export styles? The reason is that export default can’t be used to label const: const may define multiple values, but export default needs exactly one value. Consider the following hypothetical code: With this code, you don’t know which one of the three values is the default export. Exercise: Default exports export default function foo() {} // no semicolon! export default class Bar {} // no semicolon! export default 'abc'; export default foo(); export default /^xyz$/; export default 5 * 7; export default { no: false, yes: true }; // Not legal JavaScript! export default const foo = 1, bar = 2, baz = 3;
- 405. exercises/modules/export_default_test.mjs 27.7.2 The default export as a named export (advanced) Internally, a default export is simply a named export whose name is default. As an example, consider the previous module my-func.mjs with a default export: The following module my-func2.mjs is equivalent to that module: For importing, we can use a normal default import: Or we can use a named import: const GREETING = 'Hello!'; export default function () { return GREETING; } const GREETING = 'Hello!'; function greet() { return GREETING; } export { greet as default, }; import myFunc from './my-func2.mjs'; assert.equal(myFunc(), 'Hello!'); import {default as myFunc} from './my-func2.mjs'; assert.equal(myFunc(), 'Hello!');
- 406. The default export is also available via property .default of namespace imports: Isn’t default illegal as a variable name? default can’t be a variable name, but it can be an export name and it can be a property name: import * as mf from './my-func2.mjs'; assert.equal(mf.default(), 'Hello!'); const obj = { default: 123, }; assert.equal(obj.default, 123);
- 407. 27.8 More details on exporting and importing 27.8.1 Imports are read-only views on exports So far, we have used imports and exports intuitively, and everything seems to have worked as expected. But now it is time to take a closer look at how imports and exports are really related. Consider the following two modules: counter.mjs main.mjs counter.mjs exports a (mutable!) variable and a function: main.mjs name-imports both exports. When we use incCounter(), we discover that the connection to counter is live – we can always access the live state of that variable: export let counter = 3; export function incCounter() { counter++; } import { counter, incCounter } from './counter.mjs'; // The imported value `counter` is live assert.equal(counter, 3); incCounter(); assert.equal(counter, 4);
- 408. Note that while the connection is live and we can read counter, we cannot change this variable (e.g., via counter++). There are two benefits to handling imports this way: It is easier to split modules because previously shared variables can become exports. This behavior is crucial for supporting transparent cyclic imports. Read on for more information. 27.8.2 ESM’s transparent support for cyclic imports (advanced) ESM supports cyclic imports transparently. To understand how that is achieved, consider the following example: fig. 16 shows a directed graph of modules importing other modules. P importing M is the cycle in this case. Figure 16: A directed graph of modules importing modules: M imports N and O, N imports P and Q, etc. After parsing, these modules are set up in two phases: Instantiation: Every module is visited and its imports are connected to its exports. Before a parent can be instantiated, all
- 409. of its children must be instantiated. Evaluation: The bodies of the modules are executed. Once again, children are evaluated before parents. This approach handles cyclic imports correctly, due to two features of ES modules: Due to the static structure of ES modules, the exports are already known after parsing. That makes it possible to instantiate P before its child M: P can already look up M’s exports. When P is evaluated, M hasn’t been evaluated, yet. However, entities in P can already mention imports from M. They just can’t use them, yet, because the imported values are filled in later. For example, a function in P can access an import from M. The only limitation is that we must wait until after the evaluation of M, before calling that function. Imports being filled in later is enabled by them being “live immutable views” on exports.
- 410. 27.9 npm packages The npm software registry is the dominant way of distributing JavaScript libraries and apps for Node.js and web browsers. It is managed via the npm package manager (short: npm). Software is distributed as so-called packages. A package is a directory containing arbitrary files and a file package.json at the top level that describes the package. For example, when npm creates an empty package inside a directory foo/, you get this package.json: Some of these properties contain simple metadata: name specifies the name of this package. Once it is uploaded to the npm registry, it can be installed via npm install foo. version is used for version management and follows semantic versioning, with three numbers: Major version: is incremented when incompatible API changes are made. { "name": "foo", "version": "1.0.0", "description": "", "main": "index.js", "scripts": { "test": "echo "Error: no test specified" && exit 1" }, "keywords": [], "author": "", "license": "ISC" }
- 411. Minor version: is incremented when functionality is added in a backward compatible manner. Patch version: is incremented when backward compatible changes are made. description, keywords, author make it easier to find packages. license clarifies how you can use this package. Other properties enable advanced configuration: main: specifies the module that “is” the package (explained later in this chapter). scripts: are commands that you can execute via npm run. For example, the script test can be executed via npm run test. For more information on package.json, consult the npm documentation. 27.9.1 Packages are installed inside a directory node_modules/ npm always installs packages inside a directory node_modules. There are usually many of these directories. Which one npm uses, depends on the directory where one currently is. For example, if we are inside a directory /tmp/a/b/, npm tries to find a node_modules in the current directory, its parent directory, the parent directory of the parent, etc. In other words, it searches the following chain of locations: /tmp/a/b/node_modules /tmp/a/node_modules /tmp/node_modules
- 412. When installing a package foo, npm uses the closest node_modules. If, for example, we are inside /tmp/a/b/ and there is a node_modules in that directory, then npm puts the package inside the directory: /tmp/a/b/node_modules/foo/ When importing a module, we can use a special module specifier to tell Node.js that we want to import it from an installed package. How exactly that works, is explained later. For now, consider the following example: To find the-module.mjs (Node.js prefers the filename extension .mjs for ES modules), Node.js walks up the node_module chain and searches the following locations: /home/jane/proj/node_modules/the-package/the-module.mjs /home/jane/node_modules/the-package/the-module.mjs /home/node_modules/the-package/the-module.mjs 27.9.2 Why can npm be used to install frontend libraries? Finding installed modules in node_modules directories is only supported on Node.js. So why can we also use npm to install libraries for browsers? That is enabled via bundling tools, such as webpack, that compile and optimize code before it is deployed online. During this // /home/jane/proj/main.mjs import * as theModule from 'the-package/the-module.mjs';
- 413. compilation process, the code in npm packages is adapted so that it works in browsers.
- 414. 27.10 Naming modules There are no established best practices for naming module files and the variables they are imported into. In this chapter, I’m using the following naming style: The names of module files are dash-cased and start with lowercase letters: ./my-module.mjs ./some-func.mjs The names of namespace imports are lowercased and camel- cased: The names of default imports are lowercased and camel-cased: What are the rationales behind this style? npm doesn’t allow uppercase letters in package names (source). Thus, we avoid camel case, so that “local” files have names that are consistent with those of npm packages. Using only lowercase letters also minimizes conflicts between file systems that are case-sensitive and file systems that aren’t: the former distinguish files whose names have the same letters, but with different cases; the latter don’t. import * as myModule from './my-module.mjs'; import someFunc from './some-func.mjs';
- 415. There are clear rules for translating dash-cased file names to camel-cased JavaScript variable names. Due to how we name namespace imports, these rules work for both namespace imports and default imports. I also like underscore-cased module file names because you can directly use these names for namespace imports (without any translation): But that style does not work for default imports: I like underscore- casing for namespace objects, but it is not a good choice for functions, etc. import * as my_module from './my_module.mjs';
- 416. 27.11 Module specifiers Module specifiers are the strings that identify modules. They work slightly differently in browsers and Node.js. Before we can look at the differences, we need to learn about the different categories of module specifiers. 27.11.1 Categories of module specifiers In ES modules, we distinguish the following categories of specifiers. These categories originated with CommonJS modules. Relative path: starts with a dot. Examples: './some/other/module.mjs' '../../lib/counter.mjs' Absolute path: starts with a slash. Example: '/home/jane/file-tools.mjs' URL: includes a protocol (technically, paths are URLs, too). Examples: 'https://example.com/some-module.mjs' 'file:///home/john/tmp/main.mjs' Bare path: does not start with a dot, a slash or a protocol, and consists of a single filename without an extension. Examples: 'lodash' 'the-package'
- 417. Deep import path: starts with a bare path and has at least one slash. Example: 'the-package/dist/the-module.mjs' 27.11.2 ES module specifiers in browsers Browsers handle module specifiers as follows: Relative paths, absolute paths, and URLs work as expected. They all must point to real files (in contrast to CommonJS, which lets you omit filename extensions and more). The file name extensions of modules don’t matter, as long as they are served with the content type text/javascript. How bare paths will end up being handled is not yet clear. You will probably eventually be able to map them to other specifiers via lookup tables. Note that bundling tools such as webpack, which combine modules into fewer files, are often less strict with specifiers than browsers. That’s because they operate at build/compile time (not at runtime) and can search for files by traversing the file system. 27.11.3 ES module specifiers on Node.js Support for ES modules on Node.js is still new You may have to switch it on via a command line flag. See the Node.js documentation for details.
- 418. Node.js handles module specifiers as follows: Relative paths are resolved as they are in web browsers – relative to the path of the current module. Absolute paths are currently not supported. As a workaround, you can use URLs that start with file:///. You can create such URLs via url.pathToFileURL(). Only file: is supported as a protocol for URL specifiers. A bare path is interpreted as a package name and resolved relative to the closest node_modules directory. What module should be loaded, is determined by looking at property "main" of the package’s package.json (similarly to CommonJS). Deep import paths are also resolved relatively to the closest node_modules directory. They contain file names, so it is always clear which module is meant. All specifiers, except bare paths, must refer to actual files. That is, ESM does not support the following CommonJS features: CommonJS automatically adds missing filename extensions. CommonJS can import a directory foo if there is a foo/package.json with a "main" property. CommonJS can import a directory foo if there is a module foo/index.js.
- 419. All built-in Node.js modules are available via bare paths and have named ESM exports – for example: 27.11.3.1 Filename extensions on Node.js Node.js supports the following default filename extensions: .mjs for ES modules .cjs for CommonJS modules The filename extension .js stands for either ESM or CommonJS. Which one it is is configured via the “closest” package.json (in the current directory, the parent directory, etc.). Using package.json in this manner is independent of packages. In that package.json, there is a property "type", which has two settings: "commonjs" (the default): files with the extension .js or without an extension are interpreted as CommonJS modules. "module": files with the extension .js or without an extension are interpreted as ESM modules. 27.11.3.2 Interpreting non-file source code as either CommonJS or ESM import * as path from 'path'; import {strict as assert} from 'assert'; assert.equal( path.join('a/b/c', '../d'), 'a/b/d');
- 420. Not all source code executed by Node.js comes from files. You can also send it code via stdin, --eval, and --print. The command line option --input-type lets you specify how such code is interpreted: As CommonJS (the default): --input-type=commonjs As ESM: --input-type=module
- 421. 27.12 Loading modules dynamically via import() So far, the only way to import a module has been via an import statement. That statement has several limitations: You must use it at the top level of a module. That is, you can’t, for example, import something when you are inside a block. The module specifier is always fixed. That is, you can’t change what you import depending on a condition. And you can’t assemble a specifier dynamically. The import() operator changes that. Let’s look at an example of it being used. 27.12.1 Example: loading a module dynamically Consider the following files: lib/my-math.mjs main1.mjs main2.mjs We have already seen module my-math.mjs: // Not exported, private to module function times(a, b) { return a * b; } export function square(x) {
- 422. This is what using import() looks like in main1.mjs: Method .then() is part of Promises, a mechanism for handling asynchronous results, which is covered later in this book. Two things in this code weren’t possible before: We are importing inside a function (not at the top level). The module specifier comes from a variable. Next, we’ll implement the exact same functionality in main2.mjs but via a so-called async function, which provides nicer syntax for Promises. return times(x, x); } export const LIGHTSPEED = 299792458; const dir = './lib/'; const moduleSpecifier = dir + 'my-math.mjs'; function loadConstant() { return import(moduleSpecifier) .then(myMath => { const result = myMath.LIGHTSPEED; assert.equal(result, 299792458); return result; }); } const dir = './lib/'; const moduleSpecifier = dir + 'my-math.mjs'; async function loadConstant() { const myMath = await import(moduleSpecifier); const result = myMath.LIGHTSPEED; assert.equal(result, 299792458);
- 423. Why is import() an operator and not a function? Even though it works much like a function, import() is an operator: in order to resolve module specifiers relatively to the current module, it needs to know from which module it is invoked. A normal function cannot receive this information as implicitly as an operator can. It would need, for example, a parameter. 27.12.2 Use cases for import() 27.12.2.1 Loading code on demand Some functionality of web apps doesn’t have to be present when they start, it can be loaded on demand. Then import() helps because you can put such functionality into modules – for example: 27.12.2.2 Conditional loading of modules We may want to load a module depending on whether a condition is true. For example, a module with a polyfill that makes a new feature return result; } button.addEventListener('click', event => { import('./dialogBox.mjs') .then(dialogBox => { dialogBox.open(); }) .catch(error => { /* Error handling */ }) });
- 424. available on legacy platforms: 27.12.2.3 Computed module specifiers For applications such as internationalization, it helps if you can dynamically compute module specifiers: if (isLegacyPlatform()) { import('./my-polyfill.mjs') .then(···); } import(`messages_${getLocale()}.mjs`) .then(···);
- 425. 27.13 Preview: import.meta.url “import.meta” is an ECMAScript feature proposed by Domenic Denicola. The object import.meta holds metadata for the current module. Its most important property is import.meta.url, which contains a string with the URL of the current module file. For example: 'https://example.com/code/main.mjs' 27.13.1 import.meta.url and class URL Class URL is available via a global variable in browsers and on Node.js. You can look up its full functionality in the Node.js documentation. When working with import.meta.url, its constructor is especially useful: Parameter input contains the URL to be parsed. It can be relative if the second parameter, base, is provided. In other words, this constructor lets us resolve a relative path against a base URL: new URL(input: string, base?: string|URL) > new URL('other.mjs', 'https://example.com/code/main.mjs').href 'https://example.com/code/other.mjs' > new URL('../other.mjs', 'https://example.com/code/main.mjs').h 'https://example.com/other.mjs'
- 426. This is how we get a URL instance that points to a file data.txt that sits next to the current module: 27.13.2 import.meta.url on Node.js On Node.js, import.meta.url is always a string with a file: URL – for example: 'file:///Users/rauschma/my-module.mjs' 27.13.2.1 Example: reading a sibling file of a module Many Node.js file system operations accept either strings with paths or instances of URL. That enables us to read a sibling file data.txt of the current module: main() is an async function, as explained in §41 “Async functions”. fs.promises contains a Promise-based version of the fs API, which can be used with async functions. 27.13.2.2 Converting between file: URLs and paths const urlOfData = new URL('data.txt', import.meta.url); import {promises as fs} from 'fs'; async function main() { const urlOfData = new URL('data.txt', import.meta.url); const str = await fs.readFile(urlOfData, {encoding: 'UTF-8'}); assert.equal(str, 'This is textual data.n'); } main();
- 427. The Node.js module url has two functions for converting between file: URLs and paths: fileURLToPath(url: URL|string): string Converts a file: URL to a path. pathToFileURL(path: string): URL Converts a path to a file: URL. If you need a path that can be used in the local file system, then property .pathname of URL instances does not always work: Therefore, it is better to use fileURLToPath(): Similarly, pathToFileURL() does more than just prepend 'file://' to an absolute path. assert.equal( new URL('file:///tmp/with%20space.txt').pathname, '/tmp/with%20space.txt'); import * as url from 'url'; assert.equal( url.fileURLToPath('file:///tmp/with%20space.txt'), '/tmp/with space.txt'); // result on Unix
- 428. 27.14 Polyfills: emulating native web platform features (advanced) Backends have polyfills, too This section is about frontend development and web browsers, but similar ideas apply to backend development. Polyfills help with a conflict that we are facing when developing a web application in JavaScript: On one hand, we want to use modern web platform features that make the app better and/or development easier. On the other hand, the app should run on as many browsers as possible. Given a web platform feature X: A polyfill for X is a piece of code. If it is executed on a platform that already has built-in support for X, it does nothing. Otherwise, it makes the feature available on the platform. In the latter case, the polyfilled feature is (mostly) indistinguishable from a native implementation. In order to achieve that, the polyfill usually makes global changes. For example, it may modify global data or configure a global module loader. Polyfills are often packaged as modules. The term polyfill was coined by Remy Sharp.
- 429. A speculative polyfill is a polyfill for a proposed web platform feature (that is not standardized, yet). Alternative term: prollyfill A replica of X is a library that reproduces the API and functionality of X locally. Such a library exists independently of a native (and global) implementation of X. Replica is a new term introduced in this section. Alternative term: ponyfill There is also the term shim, but it doesn’t have a universally agreed upon definition. It often means roughly the same as polyfill. Every time our web applications starts, it must first execute all polyfills for features that may not be available everywhere. Afterwards, we can be sure that those features are available natively. 27.14.1 Sources of this section “What is a Polyfill?” by Remy Sharp Inspiration for the term replica: The Eiffel Tower in Las Vegas Useful clarification of “polyfill” and related terms: “Polyfills and the evolution of the Web”. Edited by Andrew Betts. Quiz See quiz app.
- 430. 28 Single objects 28.1 What is an object? 28.1.1 Roles of objects: record vs. dictionary 28.2 Objects as records 28.2.1 Object literals: properties 28.2.2 Object literals: property value shorthands 28.2.3 Getting properties 28.2.4 Setting properties 28.2.5 Object literals: methods 28.2.6 Object literals: accessors 28.3 Spreading into object literals (...) 28.3.1 Use case for spreading: copying objects 28.3.2 Use case for spreading: default values for missing properties 28.3.3 Use case for spreading: non-destructively changing properties 28.4 Methods 28.4.1 Methods are properties whose values are functions 28.4.2 .call(): specifying this via a parameter 28.4.3 .bind(): pre-filling this and parameters of functions 28.4.4 this pitfall: extracting methods 28.4.5 this pitfall: accidentally shadowing this 28.4.6 Avoiding the pitfalls of this 28.4.7 The value of this in various contexts 28.5 Objects as dictionaries (advanced) 28.5.1 Arbitrary fixed strings as property keys 28.5.2 Computed property keys 28.5.3 The in operator: is there a property with a given key? 28.5.4 Deleting properties 28.5.5 Listing property keys 28.5.6 Listing property values via Object.values() 28.5.7 Listing property entries via Object.entries()
- 431. 28.5.8 Properties are listed deterministically 28.5.9 Assembling objects via Object.fromEntries() 28.5.10 The pitfalls of using an object as a dictionary 28.6 Standard methods (advanced) 28.6.1 .toString() 28.6.2 .valueOf() 28.7 Advanced topics 28.7.1 Object.assign() 28.7.2 Freezing objects 28.7.3 Property attributes and property descriptors In this book, JavaScript’s style of object-oriented programming (OOP) is introduced in four steps. This chapter covers step 1; the next chapter covers steps 2–4. The steps are (fig. 17): 1. Single objects (this chapter): How do objects, JavaScript’s basic OOP building blocks, work in isolation? 2. Prototype chains (next chapter): Each object has a chain of zero or more prototype objects. Prototypes are JavaScript’s core inheritance mechanism. 3. Classes (next chapter): JavaScript’s classes are factories for objects. The relationship between a class and its instances is based on prototypal inheritance. 4. Subclassing (next chapter): The relationship between a subclass and its superclass is also based on prototypal inheritance. ƒ mthd data __proto__ 4 ƒ data mthd 4 MyClass data mthd SubClass subData subMthd SuperClass superData superMthd 1. Single objects 2. Prototype chains 3. Classes 4. Subclassing Figure 17: This book introduces object-oriented programming in JavaScript in four steps.
- 432. 28.1 What is an object? In JavaScript: An object is a set of properties (key-value entries). A property key can only be a string or a symbol. 28.1.1 Roles of objects: record vs. dictionary Objects play two roles in JavaScript: Records: Objects-as-records have a fixed number of properties, whose keys are known at development time. Their values can have different types. Dictionaries: Objects-as-dictionaries have a variable number of properties, whose keys are not known at development time. All of their values have the same type. These roles influence how objects are explained in this chapter: First, we’ll explore objects-as-records. Even though property keys are strings or symbols under the hood, they will appear as fixed identifiers to us, in this part of the chapter. Later, we’ll explore objects-as-dictionaries. Note that Maps are usually better dictionaries than objects. However, some of the operations that we’ll encounter, can also be useful for objects-as-records.
- 433. 28.2 Objects as records Let’s first explore the role record of objects. 28.2.1 Object literals: properties Object literals are one way of creating objects-as-records. They are a stand-out feature of JavaScript: you can directly create objects – no need for classes! This is an example: In the example, we created an object via an object literal, which starts and ends with curly braces {}. Inside it, we defined two properties (key-value entries): The first property has the key first and the value 'Jane'. The second property has the key last and the value 'Doe'. We will later see other ways of specifying property keys, but with this way of specifying them, they must follow the rules of JavaScript variable names. For example, you can use first_name as a property key, but not first-name). However, reserved words are allowed: In order to check the effects of various operations on objects, we’ll occasionally use Object.keys() in this part of the chapter. It lists property keys: 28.2.2 Object literals: property value shorthands const jane = { first: 'Jane', last: 'Doe', // optional trailing comma }; const obj = { if: true, const: true, }; > Object.keys({a:1, b:2}) [ 'a', 'b' ]
- 434. Whenever the value of a property is defined via a variable name and that name is the same as the key, you can omit the key. 28.2.3 Getting properties This is how you get (read) a property (line A): Getting an unknown property produces undefined: 28.2.4 Setting properties This is how you set (write to) a property: We just changed an existing property via setting. If we set an unknown property, we create a new entry: function createPoint(x, y) { return {x, y}; } assert.deepEqual( createPoint(9, 2), { x: 9, y: 2 } ); const jane = { first: 'Jane', last: 'Doe', }; // Get property .first assert.equal(jane.first, 'Jane'); // (A) assert.equal(jane.unknownProperty, undefined); const obj = { prop: 1, }; assert.equal(obj.prop, 1); obj.prop = 2; // (A) assert.equal(obj.prop, 2); const obj = {}; // empty object assert.deepEqual(
- 435. 28.2.5 Object literals: methods The following code shows how to create the method .says() via an object literal: During the method call jane.says('hello'), jane is called the receiver of the method call and assigned to the special variable this. That enables method .says() to access the sibling property .first in line A. 28.2.6 Object literals: accessors There are two kinds of accessors in JavaScript: A getter is a method-like entity that is invoked by getting a property. A setter is a method-like entity that is invoked by setting a property. 28.2.6.1 Getters A getter is created by prefixing a method definition with the modifier get: Object.keys(obj), []); obj.unknownProperty = 'abc'; assert.deepEqual( Object.keys(obj), ['unknownProperty']); const jane = { first: 'Jane', // data property says(text) { // method return `${this.first} says “${text}”`; // (A) }, // comma as separator (optional at end) }; assert.equal(jane.says('hello'), 'Jane says “hello”'); const jane = { first: 'Jane', last: 'Doe', get full() { return `${this.first} ${this.last}`; }, }; assert.equal(jane.full, 'Jane Doe');
- 436. 28.2.6.2 Setters A setter is created by prefixing a method definition with the modifier set: Exercise: Creating an object via an object literal exercises/single-objects/color_point_object_test.mjs jane.first = 'John'; assert.equal(jane.full, 'John Doe'); const jane = { first: 'Jane', last: 'Doe', set full(fullName) { const parts = fullName.split(' '); this.first = parts[0]; this.last = parts[1]; }, }; jane.full = 'Richard Roe'; assert.equal(jane.first, 'Richard'); assert.equal(jane.last, 'Roe');
- 437. 28.3 Spreading into object literals (...) Inside a function call, spreading (...) turns the iterated values of an iterable object into arguments. Inside an object literal, a spread property adds the properties of another object to the current one: If property keys clash, the property that is mentioned last “wins”: All values are spreadable, even undefined and null: Property .length of strings and of Arrays is hidden from this kind of operation (it is not enumerable; see §28.7.3 “Property attributes and property descriptors” for more information). 28.3.1 Use case for spreading: copying objects You can use spreading to create a copy of an object original: > const obj = {foo: 1, bar: 2}; > {...obj, baz: 3} { foo: 1, bar: 2, baz: 3 } > const obj = {foo: 1, bar: 2, baz: 3}; > {...obj, foo: true} { foo: true, bar: 2, baz: 3 } > {foo: true, ...obj} { foo: 1, bar: 2, baz: 3 } > {...undefined} {} > {...null} {} > {...123} {} > {...'abc'} { '0': 'a', '1': 'b', '2': 'c' } > {...['a', 'b']} { '0': 'a', '1': 'b' }
- 438. Caveat – copying is shallow: copy is a fresh object with duplicates of all properties (key-value entries) of original. But if property values are objects, then those are not copied themselves; they are shared between original and copy. Let’s look at an example: The first level of copy is really a copy: If you change any properties at that level, it does not affect the original: However, deeper levels are not copied. For example, the value of .b is shared between original and copy. Changing .b in the copy also changes it in the original. JavaScript doesn’t have built-in support for deep copying Deep copies of objects (where all levels are copied) are notoriously difficult to do generically. Therefore, JavaScript does not have a built-in operation for them (for now). If you need such an operation, you have to implement it yourself. 28.3.2 Use case for spreading: default values for missing properties If one of the inputs of your code is an object with data, you can make properties optional by specifying default values that are used if those properties are missing. One technique for doing so is via an object whose properties contain the default values. In the following example, that object is DEFAULTS: const copy = {...original}; const original = { a: 1, b: {foo: true} }; const copy = {...original}; copy.a = 2; assert.deepEqual( original, { a: 1, b: {foo: true} }); // no change copy.b.foo = false; assert.deepEqual( original, { a: 1, b: {foo: false} });
- 439. The result, the object allData, is created by copying DEFAULTS and overriding its properties with those of providedData. But you don’t need an object to specify the default values; you can also specify them inside the object literal, individually: 28.3.3 Use case for spreading: non-destructively changing properties So far, we have encountered one way of changing a property .foo of an object: We set it (line A) and mutate the object. That is, this way of changing a property is destructive. With spreading, we can change .foo non-destructively – we make a copy of obj where .foo has a different value: Exercise: Non-destructively updating a property via spreading (fixed key) exercises/single-objects/update_name_test.mjs const DEFAULTS = {foo: 'a', bar: 'b'}; const providedData = {foo: 1}; const allData = {...DEFAULTS, ...providedData}; assert.deepEqual(allData, {foo: 1, bar: 'b'}); const providedData = {foo: 1}; const allData = {foo: 'a', bar: 'b', ...providedData}; assert.deepEqual(allData, {foo: 1, bar: 'b'}); const obj = {foo: 'a', bar: 'b'}; obj.foo = 1; // (A) assert.deepEqual(obj, {foo: 1, bar: 'b'}); const obj = {foo: 'a', bar: 'b'}; const updatedObj = {...obj, foo: 1}; assert.deepEqual(updatedObj, {foo: 1, bar: 'b'});
- 440. 28.4 Methods 28.4.1 Methods are properties whose values are functions Let’s revisit the example that was used to introduce methods: Somewhat surprisingly, methods are functions: Why is that? We learned in the chapter on callable values, that ordinary functions play several roles. Method is one of those roles. Therefore, under the hood, jane roughly looks as follows. 28.4.2 .call(): specifying this via a parameter Remember that each function someFunc is also an object and therefore has methods. One such method is .call() – it lets you call a function while specifying this via a parameter: 28.4.2.1 Methods and .call() const jane = { first: 'Jane', says(text) { return `${this.first} says “${text}”`; }, }; assert.equal(typeof jane.says, 'function'); const jane = { first: 'Jane', says: function (text) { return `${this.first} says “${text}”`; }, }; someFunc.call(thisValue, arg1, arg2, arg3);
- 441. If you make a method call, this is an implicit parameter that is filled in via the receiver of the call: The method call in the last line sets up this as follows: As an aside, that means that there are actually two different dot operators: 1. One for accessing properties: obj.prop 2. One for making method calls: obj.prop() They are different in that (2) is not just (1) followed by the function call operator (). Instead, (2) additionally specifies a value for this. 28.4.2.2 Functions and .call() If you function-call an ordinary function, its implicit parameter this is also provided – it is implicitly set to undefined: The method call in the last line sets up this as follows: this being set to undefined during a function call, indicates that it is a feature that is only needed during a method call. const obj = { method(x) { assert.equal(this, obj); // implicit parameter assert.equal(x, 'a'); }, }; obj.method('a'); // receiver is `obj` obj.method.call(obj, 'a'); function func(x) { assert.equal(this, undefined); // implicit parameter assert.equal(x, 'a'); } func('a'); func.call(undefined, 'a');
- 442. Next, we’ll examine the pitfalls of using this. Before we can do that, we need one more tool: method .bind() of functions. 28.4.3 .bind(): pre-filling this and parameters of functions .bind() is another method of function objects. This method is invoked as follows: .bind() returns a new function boundFunc(). Calling that function invokes someFunc() with this set to thisValue and these parameters: arg1, arg2, followed by the parameters of boundFunc(). That is, the following two function calls are equivalent: 28.4.3.1 An alternative to .bind() Another way of pre-filling this and parameters is via an arrow function: 28.4.3.2 An implementation of .bind() Considering the previous section, .bind() can be implemented as a real function as follows: 28.4.3.3 Example: binding a real function const boundFunc = someFunc.bind(thisValue, arg1, arg2); boundFunc('a', 'b') someFunc.call(thisValue, arg1, arg2, 'a', 'b') const boundFunc2 = (...args) => someFunc.call(thisValue, arg1, arg2, ...args); function bind(func, thisValue, ...boundArgs) { return (...args) => func.call(thisValue, ...boundArgs, ...args); }
- 443. Using .bind() for real functions is somewhat unintuitive because you have to provide a value for this. Given that it is undefined during function calls, it is usually set to undefined or null. In the following example, we create add8(), a function that has one parameter, by binding the first parameter of add() to 8. 28.4.3.4 Example: binding a method In the following code, we turn method .says() into the stand-alone function func(): Setting this to jane via .bind() is crucial here. Otherwise, func() wouldn’t work properly because this is used in line A. 28.4.4 this pitfall: extracting methods We now know quite a bit about functions and methods and are ready to take a look at the biggest pitfall involving methods and this: function-calling a method extracted from an object can fail if you are not careful. In the following example, we fail when we extract method jane.says(), store it in the variable func, and function-call func(). function add(x, y) { return x + y; } const add8 = add.bind(undefined, 8); assert.equal(add8(1), 9); const jane = { first: 'Jane', says(text) { return `${this.first} says “${text}”`; // (A) }, }; const func = jane.says.bind(jane, 'hello'); assert.equal(func(), 'Jane says “hello”');
- 444. The function call in line A is equivalent to: So how do we fix this? We need to use .bind() to extract method .says(): The .bind() ensures that this is always jane when we call func(). You can also use arrow functions to extract methods: 28.4.4.1 Example: extracting a method The following is a simplified version of code that you may see in actual web development: const jane = { first: 'Jane', says(text) { return `${this.first} says “${text}”`; }, }; const func = jane.says; // extract the method assert.throws( () => func('hello'), // (A) { name: 'TypeError', message: "Cannot read property 'first' of undefined", }); assert.throws( () => jane.says.call(undefined, 'hello'), // `this` is undefined! { name: 'TypeError', message: "Cannot read property 'first' of undefined", }); const func2 = jane.says.bind(jane); assert.equal(func2('hello'), 'Jane says “hello”'); const func3 = text => jane.says(text); assert.equal(func3('hello'), 'Jane says “hello”'); class ClickHandler { constructor(id, elem) { this.id = id; elem.addEventListener('click', this.handleClick); // (A)
- 445. In line A, we don’t extract the method .handleClick() properly. Instead, we should do: Exercise: Extracting a method exercises/single-objects/method_extraction_exrc.mjs 28.4.5 this pitfall: accidentally shadowing this Accidentally shadowing this is only an issue with ordinary functions Arrow functions don’t shadow this. Consider the following problem: when you are inside an ordinary function, you can’t access the this of the surrounding scope because the ordinary function has its own this. In other words, a variable in an inner scope hides a variable in an outer scope. That is called shadowing. The following code is an example: In line A, we want to access the this of .prefixStringArray(). But we can’t since the surrounding ordinary function has its own this that shadows (blocks access } handleClick(event) { alert('Clicked ' + this.id); } } elem.addEventListener('click', this.handleClick.bind(this)); const prefixer = { prefix: '==> ', prefixStringArray(stringArray) { return stringArray.map( function (x) { return this.prefix + x; // (A) }); }, }; assert.throws( () => prefixer.prefixStringArray(['a', 'b']), /^TypeError: Cannot read property 'prefix' of undefined$/);
- 446. to) the this of the method. The value of the former this is undefined due to the callback being function-called. That explains the error message. The simplest way to fix this problem is via an arrow function, which doesn’t have its own this and therefore doesn’t shadow anything: We can also store this in a different variable (line A), so that it doesn’t get shadowed: Another option is to specify a fixed this for the callback via .bind() (line A): Lastly, .map() lets us specify a value for this (line A) that it uses when invoking the callback: const prefixer = { prefix: '==> ', prefixStringArray(stringArray) { return stringArray.map( (x) => { return this.prefix + x; }); }, }; assert.deepEqual( prefixer.prefixStringArray(['a', 'b']), ['==> a', '==> b']); prefixStringArray(stringArray) { const that = this; // (A) return stringArray.map( function (x) { return that.prefix + x; }); }, prefixStringArray(stringArray) { return stringArray.map( function (x) { return this.prefix + x; }.bind(this)); // (A) }, prefixStringArray(stringArray) { return stringArray.map( function (x) { return this.prefix + x;
- 447. 28.4.6 Avoiding the pitfalls of this We have seen two big this-related pitfalls: 1. Extracting methods 2. Accidentally shadowing this One simple rule helps avoid the second pitfall: “Avoid the keyword function”: Never use ordinary functions, only arrow functions (for real functions) and method definitions. Following this rule has two benefits: It prevents the second pitfall because ordinary functions are never used as real functions. this becomes easier to understand because it will only appear inside methods (never inside ordinary functions). That makes it clear that this is an OOP feature. However, even though I don’t use (ordinary) function expressions anymore, I do like function declarations syntactically. You can use them safely if you don’t refer to this inside them. The static checking tool ESLint can warn you during development when you do this wrong via a built-in rule. Alas, there is no simple way around the first pitfall: whenever you extract a method, you have to be careful and do it properly – for example, by binding this. 28.4.7 The value of this in various contexts What is the value of this in various contexts? }, this); // (A) },
- 448. Inside a callable entity, the value of this depends on how the callable entity is invoked and what kind of callable entity it is: Function call: Ordinary functions: this === undefined (in strict mode) Arrow functions: this is same as in surrounding scope (lexical this) Method call: this is receiver of call new: this refers to newly created instance You can also access this in all common top-level scopes: <script> element: this === globalThis ECMAScript modules: this === undefined CommonJS modules: this === module.exports However, I like to pretend that you can’t access this in top-level scopes because top-level this is confusing and rarely useful.
- 449. 28.5 Objects as dictionaries (advanced) Objects work best as records. But before ES6, JavaScript did not have a data structure for dictionaries (ES6 brought Maps). Therefore, objects had to be used as dictionaries, which imposed a signficant constraint: keys had to be strings (symbols were also introduced with ES6). We first look at features of objects that are related to dictionaries but also useful for objects-as-records. This section concludes with tips for actually using objects as dictionaries (spoiler: use Maps if you can). 28.5.1 Arbitrary fixed strings as property keys So far, we have always used objects as records. Property keys were fixed tokens that had to be valid identifiers and internally became strings: As a next step, we’ll go beyond this limitation for property keys: In this section, we’ll use arbitrary fixed strings as keys. In the next subsection, we’ll dynamically compute keys. Two techniques allow us to use arbitrary strings as property keys. First, when creating property keys via object literals, we can quote property keys (with single or double quotes): const obj = { mustBeAnIdentifier: 123, }; // Get property assert.equal(obj.mustBeAnIdentifier, 123); // Set property obj.mustBeAnIdentifier = 'abc'; assert.equal(obj.mustBeAnIdentifier, 'abc'); const obj = { 'Can be any string!': 123,
- 450. Second, when getting or setting properties, we can use square brackets with strings inside them: You can also use these techniques for methods: 28.5.2 Computed property keys So far, property keys were always fixed strings inside object literals. In this section we learn how to dynamically compute property keys. That enables us to use either arbitrary strings or symbols. The syntax of dynamically computed property keys in object literals is inspired by dynamically accessing properties. That is, we can use square brackets to wrap expressions: The main use case for computed keys is having symbols as property keys (line A). }; // Get property assert.equal(obj['Can be any string!'], 123); // Set property obj['Can be any string!'] = 'abc'; assert.equal(obj['Can be any string!'], 'abc'); const obj = { 'A nice method'() { return 'Yes!'; }, }; assert.equal(obj['A nice method'](), 'Yes!'); const obj = { ['Hello world!']: true, ['f'+'o'+'o']: 123, [Symbol.toStringTag]: 'Goodbye', // (A) }; assert.equal(obj['Hello world!'], true); assert.equal(obj.foo, 123); assert.equal(obj[Symbol.toStringTag], 'Goodbye');
- 451. Note that the square brackets operator for getting and setting properties works with arbitrary expressions: Methods can have computed property keys, too: For the remainder of this chapter, we’ll mostly use fixed property keys again (because they are syntactically more convenient). But all features are also available for arbitrary strings and symbols. Exercise: Non-destructively updating a property via spreading (computed key) exercises/single-objects/update_property_test.mjs 28.5.3 The in operator: is there a property with a given key? The in operator checks if an object has a property with a given key: 28.5.3.1 Checking if a property exists via truthiness assert.equal(obj['f'+'o'+'o'], 123); assert.equal(obj['==> foo'.slice(-3)], 123); const methodKey = Symbol(); const obj = { [methodKey]() { return 'Yes!'; }, }; assert.equal(obj[methodKey](), 'Yes!'); const obj = { foo: 'abc', bar: false, }; assert.equal('foo' in obj, true); assert.equal('unknownKey' in obj, false);
- 452. You can also use a truthiness check to determine if a property exists: The previous checks work because obj.foo is truthy and because reading a missing property returns undefined (which is falsy). There is, however, one important caveat: truthiness checks fail if the property exists, but has a falsy value (undefined, null, false, 0, "", etc.): 28.5.4 Deleting properties You can delete properties via the delete operator: 28.5.5 Listing property keys Table 18: Standard library methods for listing own (non-inherited) property keys. All of them return Arrays with strings and/or symbols. enumerable non- e. string symbol Object.keys() ✔ ✔ Object.getOwnPropertyNames() ✔ ✔ ✔ Object.getOwnPropertySymbols() ✔ ✔ ✔ assert.equal( obj.foo ? 'exists' : 'does not exist', 'exists'); assert.equal( obj.unknownKey ? 'exists' : 'does not exist', 'does not exist'); assert.equal( obj.bar ? 'exists' : 'does not exist', 'does not exist'); // should be: 'exists' const obj = { foo: 123, }; assert.deepEqual(Object.keys(obj), ['foo']); delete obj.foo; assert.deepEqual(Object.keys(obj), []);
- 453. enumerable non- e. string symbol Reflect.ownKeys() ✔ ✔ ✔ ✔ Each of the methods in tbl. 18 returns an Array with the own property keys of the parameter. In the names of the methods, you can see that the following distinction is made: A property key can be either a string or a symbol. A property name is a property key whose value is a string. A property symbol is a property key whose value is a symbol. The next section describes the term enumerable and demonstrates each of the methods. 28.5.5.1 Enumerability Enumerability is an attribute of a property. Non-enumerable properties are ignored by some operations – for example, by Object.keys() (see tbl. 18) and by spread properties. By default, most properties are enumerable. The next example shows how to change that. It also demonstrates the various ways of listing property keys. const enumerableSymbolKey = Symbol('enumerableSymbolKey'); const nonEnumSymbolKey = Symbol('nonEnumSymbolKey'); // We create enumerable properties via an object literal const obj = { enumerableStringKey: 1, [enumerableSymbolKey]: 2, } // For non-enumerable properties, we need a more powerful tool Object.defineProperties(obj, { nonEnumStringKey: { value: 3, enumerable: false, }, [nonEnumSymbolKey]: { value: 4,
- 454. Object.defineProperties() is explained later in this chapter. 28.5.6 Listing property values via Object.values() Object.values() lists the values of all enumerable properties of an object: 28.5.7 Listing property entries via Object.entries() Object.entries() lists key-value pairs of enumerable properties. Each pair is encoded as a two-element Array: enumerable: false, }, }); assert.deepEqual( Object.keys(obj), [ 'enumerableStringKey' ]); assert.deepEqual( Object.getOwnPropertyNames(obj), [ 'enumerableStringKey', 'nonEnumStringKey' ]); assert.deepEqual( Object.getOwnPropertySymbols(obj), [ enumerableSymbolKey, nonEnumSymbolKey ]); assert.deepEqual( Reflect.ownKeys(obj), [ 'enumerableStringKey', 'nonEnumStringKey', enumerableSymbolKey, nonEnumSymbolKey, ]); const obj = {foo: 1, bar: 2}; assert.deepEqual( Object.values(obj), [1, 2]); const obj = {foo: 1, bar: 2}; assert.deepEqual( Object.entries(obj), [ ['foo', 1], ['bar', 2], ]);
- 455. Exercise: Object.entries() exercises/single-objects/find_key_test.mjs 28.5.8 Properties are listed deterministically Own (non-inherited) properties of objects are always listed in the following order: 1. Properties with string keys that contain integer indices (that includes Array indices): In ascending numeric order 2. Remaining properties with string keys: In the order in which they were added 3. Properties with symbol keys: In the order in which they were added The following example demonstrates how property keys are sorted according to these rules: The order of properties The ECMAScript specification describes in more detail how properties are ordered. 28.5.9 Assembling objects via Object.fromEntries() Given an iterable over [key, value] pairs, Object.fromEntries() creates an object: > Object.keys({b:0,a:0, 10:0,2:0}) [ '2', '10', 'b', 'a' ] assert.deepEqual( Object.fromEntries([['foo',1], ['bar',2]]), { foo: 1, bar: 2, } );
- 456. Object.fromEntries() does the opposite of Object.entries(). To demonstrate both, we’ll use them to implement two tool functions from the library Underscore in the next subsubsections. 28.5.9.1 Example: pick(object, ...keys) pick returns a copy of object that only has those properties whose keys are mentioned as arguments: We can implement pick() as follows: 28.5.9.2 Example: invert(object) invert returns a copy of object where the keys and values of all properties are swapped: const address = { street: 'Evergreen Terrace', number: '742', city: 'Springfield', state: 'NT', zip: '49007', }; assert.deepEqual( pick(address, 'street', 'number'), { street: 'Evergreen Terrace', number: '742', } ); function pick(object, ...keys) { const filteredEntries = Object.entries(object) .filter(([key, _value]) => keys.includes(key)); return Object.fromEntries(filteredEntries); } assert.deepEqual( invert({a: 1, b: 2, c: 3}), {1: 'a', 2: 'b', 3: 'c'} );
- 457. We can implement invert() like this: 28.5.9.3 A simple implementation of Object.fromEntries() The following function is a simplified version of Object.fromEntries(): 28.5.9.4 A polyfill for Object.fromEntries() The npm package object.fromentries is a polyfill for Object.entries(): it installs its own implementation if that method doesn’t exist on the current platform. Exercise: Object.entries() and Object.fromEntries() exercises/single-objects/omit_properties_test.mjs 28.5.10 The pitfalls of using an object as a dictionary If you use plain objects (created via object literals) as dictionaries, you have to look out for two pitfalls. function invert(object) { const mappedEntries = Object.entries(object) .map(([key, value]) => [value, key]); return Object.fromEntries(mappedEntries); } function fromEntries(iterable) { const result = {}; for (const [key, value] of iterable) { let coercedKey; if (typeof key === 'string' || typeof key === 'symbol') { coercedKey = key; } else { coercedKey = String(key); } result[coercedKey] = value; } return result; }
- 458. The first pitfall is that the in operator also finds inherited properties: We want dict to be treated as empty, but the in operator detects the properties it inherits from its prototype, Object.prototype. The second pitfall is that you can’t use the property key __proto__ because it has special powers (it sets the prototype of the object): So how do we avoid these pitfalls? Whenever you can, use Maps. They are the best solution for dictionaries. If you can’t, use a library for objects-as-dictionaries that does everything safely. If you can’t, use an object without a prototype. The following code demonstrates using objects without prototypes as dictionaries: We avoided both pitfalls: First, a property without a prototype does not inherit any properties (line A). Second, in modern JavaScript, __proto__ is implemented via Object.prototype. That means that it is switched off if Object.prototype is not in the prototype chain. Exercise: Using an object as a dictionary const dict = {}; assert.equal('toString' in dict, true); const dict = {}; dict['__proto__'] = 123; // No property was added to dict: assert.deepEqual(Object.keys(dict), []); const dict = Object.create(null); // no prototype assert.equal('toString' in dict, false); // (A) dict['__proto__'] = 123; assert.deepEqual(Object.keys(dict), ['__proto__']);
- 460. 28.6 Standard methods (advanced) Object.prototype defines several standard methods that can be overridden to configure how an object is treated by the language. Two important ones are: .toString() .valueOf() 28.6.1 .toString() .toString() determines how objects are converted to strings: 28.6.2 .valueOf() .valueOf() determines how objects are converted to numbers: > String({toString() { return 'Hello!' }}) 'Hello!' > String({}) '[object Object]' > Number({valueOf() { return 123 }}) 123 > Number({}) NaN
- 461. 28.7 Advanced topics The following subsections give brief overviews of a few advanced topics. 28.7.1 Object.assign() Object.assign() is a tool method: This expression assigns all properties of source_1 to target, then all properties of source_2, etc. At the end, it returns target – for example: The use cases for Object.assign() are similar to those for spread properties. In a way, it spreads destructively. 28.7.2 Freezing objects Object.freeze(obj) makes obj completely immutable: You can’t change properties, add properties, or change its prototype – for example: Object.assign(target, source_1, source_2, ···) const target = { foo: 1 }; const result = Object.assign( target, {bar: 2}, {baz: 3, bar: 4}); assert.deepEqual( result, { foo: 1, bar: 4, baz: 3 }); // target was modified and returned: assert.equal(result, target); const frozen = Object.freeze({ x: 2, y: 5 }); assert.throws( () => { frozen.x = 7 }, { name: 'TypeError', message: /^Cannot assign to read only property 'x'/, });
- 462. There is one caveat: Object.freeze(obj) freezes shallowly. That is, only the properties of obj are frozen but not objects stored in properties. 28.7.3 Property attributes and property descriptors Just as objects are composed of properties, properties are composed of attributes. The value of a property is only one of several attributes. Others include: writable: Is it possible to change the value of the property? enumerable: Is the property considered by Object.keys(), spreading, etc.? When you are using one of the operations for handling property attributes, attributes are specified via property descriptors: objects where each property represents one attribute. For example, this is how you read the attributes of a property obj.foo: And this is how you set the attributes of a property obj.bar: const obj = { foo: 123 }; assert.deepEqual( Object.getOwnPropertyDescriptor(obj, 'foo'), { value: 123, writable: true, enumerable: true, configurable: true, }); const obj = { foo: 1, bar: 2, }; assert.deepEqual(Object.keys(obj), ['foo', 'bar']); // Hide property `bar` from Object.keys() Object.defineProperty(obj, 'bar', { enumerable: false, });
- 463. Enumerability is covered in greater detail earlier in this chapter. For more information on property attributes and property descriptors, consult Speaking JavaScript. Quiz See quiz app. assert.deepEqual(Object.keys(obj), ['foo']);
- 464. 29 Prototype chains and classes 29.1 Prototype chains 29.1.1 JavaScript’s operations: all properties vs. own properties 29.1.2 Pitfall: only the first member of a prototype chain is mutated 29.1.3 Tips for working with prototypes (advanced) 29.1.4 Sharing data via prototypes 29.2 Classes 29.2.1 A class for persons 29.2.2 Classes under the hood 29.2.3 Class definitions: prototype properties 29.2.4 Class definitions: static properties 29.2.5 The instanceof operator 29.2.6 Why I recommend classes 29.3 Private data for classes 29.3.1 Private data: naming convention 29.3.2 Private data: WeakMaps 29.3.3 More techniques for private data 29.4 Subclassing 29.4.1 Subclasses under the hood (advanced) 29.4.2 instanceof in more detail (advanced) 29.4.3 Prototype chains of built-in objects (advanced) 29.4.4 Dispatched vs. direct method calls (advanced)
- 465. 29.4.5 Mixin classes (advanced) 29.5 FAQ: objects 29.5.1 Why do objects preserve the insertion order of properties? In this book, JavaScript’s style of object-oriented programming (OOP) is introduced in four steps. This chapter covers steps 2–4, the previous chapter covers step 1. The steps are (fig. 18): 1. Single objects (previous chapter): How do objects, JavaScript’s basic OOP building blocks, work in isolation? 2. Prototype chains (this chapter): Each object has a chain of zero or more prototype objects. Prototypes are JavaScript’s core inheritance mechanism. 3. Classes (this chapter): JavaScript’s classes are factories for objects. The relationship between a class and its instances is based on prototypal inheritance. 4. Subclassing (this chapter): The relationship between a subclass and its superclass is also based on prototypal inheritance. ƒ mthd data __proto__ 4 ƒ data mthd 4 MyClass data mthd SubClass subData subMthd SuperClass superData superMthd 1. Single objects 2. Prototype chains 3. Classes 4. Subclassing Figure 18: This book introduces object-oriented programming in JavaScript in four steps.
- 466. 29.1 Prototype chains Prototypes are JavaScript’s only inheritance mechanism: each object has a prototype that is either null or an object. In the latter case, the object inherits all of the prototype’s properties. In an object literal, you can set the prototype via the special property __proto__: Given that a prototype object can have a prototype itself, we get a chain of objects – the so-called prototype chain. That means that inheritance gives us the impression that we are dealing with single objects, but we are actually dealing with chains of objects. Fig. 19 shows what the prototype chain of obj looks like. const proto = { protoProp: 'a', }; const obj = { __proto__: proto, objProp: 'b', }; // obj inherits .protoProp: assert.equal(obj.protoProp, 'a'); assert.equal('protoProp' in obj, true);
- 467. __proto__ protoProp 'a' proto . . . objProp __proto__ 'b' obj Figure 19: obj starts a chain of objects that continues with proto and other objects. Non-inherited properties are called own properties. obj has one own property, .objProp. 29.1.1 JavaScript’s operations: all properties vs. own properties Some operations consider all properties (own and inherited) – for example, getting properties: Other operations only consider own properties – for example, Object.keys(): > const obj = { foo: 1 }; > typeof obj.foo // own 'number' > typeof obj.toString // inherited 'function' > Object.keys(obj) [ 'foo' ]
- 468. Read on for another operation that also only considers own properties: setting properties. 29.1.2 Pitfall: only the first member of a prototype chain is mutated One aspect of prototype chains that may be counter-intuitive is that setting any property via an object – even an inherited one – only changes that very object – never one of the prototypes. Consider the following object obj: In the next code snippet, we set the inherited property obj.protoProp (line A). That “changes” it by creating an own property: When reading obj.protoProp, the own property is found first and its value overrides the value of the inherited property. const proto = { protoProp: 'a', }; const obj = { __proto__: proto, objProp: 'b', }; // In the beginning, obj has one own property assert.deepEqual(Object.keys(obj), ['objProp']); obj.protoProp = 'x'; // (A) // We created a new own property: assert.deepEqual(Object.keys(obj), ['objProp', 'protoProp']); // The inherited property itself is unchanged: assert.equal(proto.protoProp, 'a');
- 469. The prototype chain of obj is depicted in fig. 20. protoProp 'a' __proto__ proto . . . 'b' __proto__ objProp protoProp 'x' obj Figure 20: The own property .protoProp of obj overrides the property inherited from proto. 29.1.3 Tips for working with prototypes (advanced) 29.1.3.1 Best practice: avoid __proto__, except in object literals I recommend to avoid the pseudo-property __proto__: As we will see later, not all objects have it. However, __proto__ in object literals is different. There, it is a built- in feature and always available. The recommended ways of getting and setting prototypes are: // The own property overrides the inherited property: assert.equal(obj.protoProp, 'x');
- 470. The best way to get a prototype is via the following method: The best way to set a prototype is when creating an object – via __proto__ in an object literal or via: If you have to, you can use Object.setPrototypeOf() to change the prototype of an existing object. But that may affect performance negatively. This is how these features are used: 29.1.3.2 Check: is an object a prototype of another one? So far, “p is a prototype of o” always meant “p is a direct prototype of o”. But it can also be used more loosely and mean that p is in the prototype chain of o. That looser relationship can be checked via: For example: Object.getPrototypeOf(obj: Object) : Object Object.create(proto: Object) : Object const proto1 = {}; const proto2 = {}; const obj = Object.create(proto1); assert.equal(Object.getPrototypeOf(obj), proto1); Object.setPrototypeOf(obj, proto2); assert.equal(Object.getPrototypeOf(obj), proto2); p.isPrototypeOf(o) const a = {}; const b = {__proto__: a};
- 471. 29.1.4 Sharing data via prototypes Consider the following code: We have two objects that are very similar. Both have two properties whose names are .name and .describe. Additionally, method .describe() is the same. How can we avoid duplicating that method? We can move it to an object PersonProto and make that object a prototype of both jane and tarzan: const c = {__proto__: b}; assert.equal(a.isPrototypeOf(b), true); assert.equal(a.isPrototypeOf(c), true); assert.equal(a.isPrototypeOf(a), false); assert.equal(c.isPrototypeOf(a), false); const jane = { name: 'Jane', describe() { return 'Person named '+this.name; }, }; const tarzan = { name: 'Tarzan', describe() { return 'Person named '+this.name; }, }; assert.equal(jane.describe(), 'Person named Jane'); assert.equal(tarzan.describe(), 'Person named Tarzan'); const PersonProto = { describe() {
- 472. The name of the prototype reflects that both jane and tarzan are persons. __proto__ name 'Jane' name __proto__ 'Tarzan' describe function() {···} jane tarzan PersonProto Figure 21: Objects jane and tarzan share method .describe(), via their common prototype PersonProto. Fig. 21 illustrates how the three objects are connected: The objects at the bottom now contain the properties that are specific to jane and tarzan. The object at the top contains the properties that are shared between them. When you make the method call jane.describe(), this points to the receiver of that method call, jane (in the bottom-left corner of the diagram). That’s why the method still works. tarzan.describe() works similarly. return 'Person named ' + this.name; }, }; const jane = { __proto__: PersonProto, name: 'Jane', }; const tarzan = { __proto__: PersonProto, name: 'Tarzan', };
- 473. assert.equal(jane.describe(), 'Person named Jane'); assert.equal(tarzan.describe(), 'Person named Tarzan');
- 474. 29.2 Classes We are now ready to take on classes, which are basically a compact syntax for setting up prototype chains. Under the hood, JavaScript’s classes are unconventional. But that is something you rarely see when working with them. They should normally feel familiar to people who have used other object-oriented programming languages. 29.2.1 A class for persons We have previously worked with jane and tarzan, single objects representing persons. Let’s use a class declaration to implement a factory for person objects: jane and tarzan can now be created via new Person(): class Person { constructor(name) { this.name = name; } describe() { return 'Person named '+this.name; } } const jane = new Person('Jane'); assert.equal(jane.name, 'Jane'); assert.equal(jane.describe(), 'Person named Jane'); const tarzan = new Person('Tarzan'); assert.equal(tarzan.name, 'Tarzan'); assert.equal(tarzan.describe(), 'Person named Tarzan');
- 475. Class Person has two methods: The normal method .describe() The special method .constructor() which is called directly after a new instance has been created and initializes that instance. It receives the arguments that are passed to the new operator (after the class name). If you don’t need any arguments to set up a new instance, you can omit the constructor. 29.2.1.1 Class expressions There are two kinds of class definitions (ways of defining classes): Class declarations, which we have seen in the previous section. Class expressions, which we’ll see next. Class expressions can be anonymous and named: The name of a named class expression works similarly to the name of a named function expression. This was a first look at classes. We’ll explore more features soon, but first we need to learn the internals of classes. 29.2.2 Classes under the hood // Anonymous class expression const Person = class { ··· }; // Named class expression const Person = class MyClass { ··· };
- 476. There is a lot going on under the hood of classes. Let’s look at the diagram for jane (fig. 22). __proto__ name 'Jane' describe function() {...} constructor jane Person.prototype prototype Person Figure 22: The class Person has the property .prototype that points to an object that is the prototype of all instances of Person. jane is one such instance. The main purpose of class Person is to set up the prototype chain on the right (jane, followed by Person.prototype). It is interesting to note that both constructs inside class Person (.constructor and .describe()) created properties for Person.prototype, not for Person. The reason for this slightly odd approach is backward compatibility: prior to classes, constructor functions (ordinary functions, invoked via the new operator) were often used as factories for objects. Classes are mostly better syntax for constructor functions and therefore remain compatible with old code. That explains why classes are functions: > typeof Person 'function'
- 477. In this book, I use the terms constructor (function) and class interchangeably. It is easy to confuse .__proto__ and .prototype. Hopefully, fig. 22 makes it clear how they differ: .__proto__ is a pseudo-property for accessing the prototype of an object. .prototype is a normal property that is only special due to how the new operator uses it. The name is not ideal: Person.prototype does not point to the prototype of Person, it points to the prototype of all instances of Person. 29.2.2.1 Person.prototype.constructor (advanced) There is one detail in fig. 22 that we haven’t looked at, yet: Person.prototype.constructor points back to Person: This setup also exists due to backward compatibility. But it has two additional benefits. First, each instance of a class inherits property .constructor. Therefore, given an instance, you can make “similar” objects using it: > Person.prototype.constructor === Person true const jane = new Person('Jane'); const cheeta = new jane.constructor('Cheeta'); // cheeta is also an instance of Person // (the instanceof operator is explained later) assert.equal(cheeta instanceof Person, true);
- 478. Second, you can get the name of the class that created a given instance: 29.2.3 Class definitions: prototype properties All constructs in the body of the following class declaration create properties of Foo.prototype. Let’s examine them in order: .constructor() is called after creating a new instance of Foo to set up that instance. .protoMethod() is a normal method. It is stored in Foo.prototype. .protoGetter is a getter that is stored in Foo.prototype. The following interaction uses class Foo: const tarzan = new Person('Tarzan'); assert.equal(tarzan.constructor.name, 'Person'); class Foo { constructor(prop) { this.prop = prop; } protoMethod() { return 'protoMethod'; } get protoGetter() { return 'protoGetter'; } }
- 479. 29.2.4 Class definitions: static properties All constructs in the body of the following class declaration create so- called static properties – properties of Bar itself. The static method and the static getter are used as follows: 29.2.5 The instanceof operator The instanceof operator tells you if a value is an instance of a given class: > const foo = new Foo(123); > foo.prop 123 > foo.protoMethod() 'protoMethod' > foo.protoGetter 'protoGetter' class Bar { static staticMethod() { return 'staticMethod'; } static get staticGetter() { return 'staticGetter'; } } > Bar.staticMethod() 'staticMethod' > Bar.staticGetter 'staticGetter' > new Person('Jane') instanceof Person true
- 480. We’ll explore the instanceof operator in more detail later, after we have looked at subclassing. 29.2.6 Why I recommend classes I recommend using classes for the following reasons: Classes are a common standard for object creation and inheritance that is now widely supported across frameworks (React, Angular, Ember, etc.). This is an improvement to how things were before, when almost every framework had its own inheritance library. They help tools such as IDEs and type checkers with their work and enable new features there. If you come from another language to JavaScript and are used to classes, then you can get started more quickly. JavaScript engines optimize them. That is, code that uses classes is almost always faster than code that uses a custom inheritance library. You can subclass built-in constructor functions such as Error. That doesn’t mean that classes are perfect: > ({}) instanceof Person false > ({}) instanceof Object true > [] instanceof Array true
- 481. There is a risk of overdoing inheritance. There is a risk of putting too much functionality in classes (when some of it is often better put in functions). How they work superficially and under the hood is quite different. In other words, there is a disconnect between syntax and semantics. Two examples are: A method definition inside a class C creates a method in the object C.prototype. Classes are functions. The motivation for the disconnect is backward compatibility. Thankfully, the disconnect causes few problems in practice; you are usually OK if you go along with what classes pretend to be. Exercise: Writing a class exercises/proto-chains-classes/point_class_test.mjs
- 482. 29.3 Private data for classes This section describes techniques for hiding some of the data of an object from the outside. We discuss them in the context of classes, but they also work for objects created directly, e.g., via object literals. 29.3.1 Private data: naming convention The first technique makes a property private by prefixing its name with an underscore. This doesn’t protect the property in any way; it merely signals to the outside: “You don’t need to know about this property.” In the following code, the properties ._counter and ._action are private. class Countdown { constructor(counter, action) { this._counter = counter; this._action = action; } dec() { this._counter--; if (this._counter === 0) { this._action(); } } } // The two properties aren’t really private: assert.deepEqual( Object.keys(new Countdown()), ['_counter', '_action']);
- 483. With this technique, you don’t get any protection and private names can clash. On the plus side, it is easy to use. 29.3.2 Private data: WeakMaps Another technique is to use WeakMaps. How exactly that works is explained in the chapter on WeakMaps. This is a preview: This technique offers you considerable protection from outside access and there can’t be any name clashes. But it is also more complicated to use. 29.3.3 More techniques for private data const _counter = new WeakMap(); const _action = new WeakMap(); class Countdown { constructor(counter, action) { _counter.set(this, counter); _action.set(this, action); } dec() { let counter = _counter.get(this); counter--; _counter.set(this, counter); if (counter === 0) { _action.get(this)(); } } } // The two pseudo-properties are truly private: assert.deepEqual( Object.keys(new Countdown()), []);
- 484. This book explains the most important techniques for private data in classes. There will also probably soon be built-in support for it. Consult the ECMAScript proposal “Class Public Instance Fields & Private Instance Fields” for details. A few additional techniques are explained in Exploring ES6.
- 485. 29.4 Subclassing Classes can also subclass (“extend”) existing classes. As an example, the following class Employee subclasses Person: Two comments: class Person { constructor(name) { this.name = name; } describe() { return `Person named ${this.name}`; } static logNames(persons) { for (const person of persons) { console.log(person.name); } } } class Employee extends Person { constructor(name, title) { super(name); this.title = title; } describe() { return super.describe() + ` (${this.title})`; } } const jane = new Employee('Jane', 'CTO'); assert.equal( jane.describe(), 'Person named Jane (CTO)');
- 486. Inside a .constructor() method, you must call the super- constructor via super() before you can access this. That’s because this doesn’t exist before the super-constructor is called (this phenomenon is specific to classes). Static methods are also inherited. For example, Employee inherits the static method .logNames(): Exercise: Subclassing exercises/proto-chains-classes/color_point_class_test.mjs 29.4.1 Subclasses under the hood (advanced) Person Person.prototype Employee Employee.prototype jane __proto__ __proto__ prototype prototype Object.prototype __proto__ __proto__ Function.prototype __proto__ Figure 23: These are the objects that make up class Person and its subclass, Employee. The left column is about classes. The right column is about the Employee instance jane and its prototype chain. > 'logNames' in Employee true
- 487. The classes Person and Employee from the previous section are made up of several objects (fig. 23). One key insight for understanding how these objects are related is that there are two prototype chains: The instance prototype chain, on the right. The class prototype chain, on the left. 29.4.1.1 The instance prototype chain (right column) The instance prototype chain starts with jane and continues with Employee.prototype and Person.prototype. In principle, the prototype chain ends at this point, but we get one more object: Object.prototype. This prototype provides services to virtually all objects, which is why it is included here, too: 29.4.1.2 The class prototype chain (left column) In the class prototype chain, Employee comes first, Person next. Afterward, the chain continues with Function.prototype, which is only there because Person is a function and functions need the services of Function.prototype. 29.4.2 instanceof in more detail (advanced) > Object.getPrototypeOf(Person.prototype) === Object.prototype true > Object.getPrototypeOf(Person) === Function.prototype true
- 488. We have not yet seen how instanceof really works. Given the expression: How does instanceof determine if x is an instance of C (or a subclass of C)? It does so by checking if C.prototype is in the prototype chain of x. That is, the following expression is equivalent: If we go back to fig. 23, we can confirm that the prototype chain does lead us to the following correct answers: 29.4.3 Prototype chains of built-in objects (advanced) Next, we’ll use our knowledge of subclassing to understand the prototype chains of a few built-in objects. The following tool function p() helps us with our explorations. We extracted method .getPrototypeOf() of Object and assigned it to p. 29.4.3.1 The prototype chain of {} x instanceof C C.prototype.isPrototypeOf(x) > jane instanceof Employee true > jane instanceof Person true > jane instanceof Object true const p = Object.getPrototypeOf.bind(Object);
- 489. Let’s start by examining plain objects: Object.prototype {} __proto__ null __proto__ Figure 24: The prototype chain of an object created via an object literal starts with that object, continues with Object.prototype, and ends with null. Fig. 24 shows a diagram for this prototype chain. We can see that {} really is an instance of Object – Object.prototype is in its prototype chain. 29.4.3.2 The prototype chain of [] What does the prototype chain of an Array look like? > p({}) === Object.prototype true > p(p({})) === null true > p([]) === Array.prototype true > p(p([])) === Object.prototype true > p(p(p([]))) === null true
- 490. Object.prototype Array.prototype [] __proto__ __proto__ null __proto__ Figure 25: The prototype chain of an Array has these members: the Array instance, Array.prototype, Object.prototype, null. This prototype chain (visualized in fig. 25) tells us that an Array object is an instance of Array, which is a subclass of Object. 29.4.3.3 The prototype chain of function () {} Lastly, the prototype chain of an ordinary function tells us that all functions are objects: 29.4.3.4 Objects that aren’t instances of Object An object is only an instance of Object if Object.prototype is in its prototype chain. Most objects created via various literals are instances of Object: > p(function () {}) === Function.prototype true > p(p(function () {})) === Object.prototype true
- 491. Objects that don’t have prototypes are not instances of Object: Object.prototype ends most prototype chains. Its prototype is null, which means it isn’t an instance of Object either: 29.4.3.5 How exactly does the pseudo-property .__proto__ work? The pseudo-property .__proto__ is implemented by class Object via a getter and a setter. It could be implemented like this: That means that you can switch .__proto__ off by creating an object that doesn’t have Object.prototype in its prototype chain (see the previous section): > ({}) instanceof Object true > (() => {}) instanceof Object true > /abc/ug instanceof Object true > ({ __proto__: null }) instanceof Object false > Object.prototype instanceof Object false class Object { get __proto__() { return Object.getPrototypeOf(this); } set __proto__(other) { Object.setPrototypeOf(this, other); } // ··· }
- 492. 29.4.4 Dispatched vs. direct method calls (advanced) Let’s examine how method calls work with classes. We are revisiting jane from earlier: Fig. 26 has a diagram with jane’s prototype chain. __proto__ describe function() {···} Person.prototype . . . name __proto__ 'Jane' jane Figure 26: The prototype chain of jane starts with jane and continues with Person.prototype. > '__proto__' in {} true > '__proto__' in { __proto__: null } false class Person { constructor(name) { this.name = name; } describe() { return 'Person named '+this.name; } } const jane = new Person('Jane');
- 493. Normal method calls are dispatched – the method call jane.describe() happens in two steps: Dispatch: In the prototype chain of jane, find the first property whose key is 'describe' and retrieve its value. Call: Call the value, while setting this to jane. This way of dynamically looking for a method and invoking it is called dynamic dispatch. You can make the same method call directly, without dispatching: This time, we directly point to the method via Person.prototype.describe and don’t search for it in the prototype chain. We also specify this differently via .call(). Note that this always points to the beginning of a prototype chain. That enables .describe() to access .name. 29.4.4.1 Borrowing methods Direct method calls become useful when you are working with methods of Object.prototype. For example, Object.prototype.hasOwnProperty(k) checks if this has a non- inherited property whose key is k: const func = jane.describe; func.call(jane); Person.prototype.describe.call(jane)
- 494. However, in the prototype chain of an object, there may be another property with the key 'hasOwnProperty' that overrides the method in Object.prototype. Then a dispatched method call doesn’t work: The workaround is to use a direct method call: This kind of direct method call is often abbreviated as follows: This pattern may seem inefficient, but most engines optimize this pattern, so performance should not be an issue. 29.4.5 Mixin classes (advanced) JavaScript’s class system only supports single inheritance. That is, each class can have at most one superclass. One way around this limitation is via a technique called mixin classes (short: mixins). > const obj = { foo: 123 }; > obj.hasOwnProperty('foo') true > obj.hasOwnProperty('bar') false > const obj = { hasOwnProperty: true }; > obj.hasOwnProperty('bar') TypeError: obj.hasOwnProperty is not a function > Object.prototype.hasOwnProperty.call(obj, 'bar') false > Object.prototype.hasOwnProperty.call(obj, 'hasOwnProperty') true > ({}).hasOwnProperty.call(obj, 'bar') false > ({}).hasOwnProperty.call(obj, 'hasOwnProperty') true
- 495. The idea is as follows: Let’s say we want a class C to inherit from two superclasses S1 and S2. That would be multiple inheritance, which JavaScript doesn’t support. Our workaround is to turn S1 and S2 into mixins, factories for subclasses: Each of these two functions returns a class that extends a given superclass Sup. We create class C as follows: We now have a class C that extends a class S2 that extends a class S1 that extends Object (which most classes do implicitly). 29.4.5.1 Example: a mixin for brand management We implement a mixin Branded that has helper methods for setting and getting the brand of an object: We use this mixin to implement brand management for a class Car: const S1 = (Sup) => class extends Sup { /*···*/ }; const S2 = (Sup) => class extends Sup { /*···*/ }; class C extends S2(S1(Object)) { /*···*/ } const Branded = (Sup) => class extends Sup { setBrand(brand) { this._brand = brand; return this; } getBrand() { return this._brand; } };
- 496. The following code confirms that the mixin worked: Car has method .setBrand() of Branded. 29.4.5.2 The benefits of mixins Mixins free us from the constraints of single inheritance: The same class can extend a single superclass and zero or more mixins. The same mixin can be used by multiple classes. class Car extends Branded(Object) { constructor(model) { super(); this._model = model; } toString() { return `${this.getBrand()} ${this._model}`; } } const modelT = new Car('Model T').setBrand('Ford'); assert.equal(modelT.toString(), 'Ford Model T');
- 497. 29.5 FAQ: objects 29.5.1 Why do objects preserve the insertion order of properties? In principle, objects are unordered. The main reason for ordering properties is so that operations that list entries, keys, or values are deterministic. That helps, e.g., with testing. Quiz See quiz app.
- 498. 30 Synchronous iteration 30.1 What is synchronous iteration about? 30.2 Core iteration constructs: iterables and iterators 30.3 Iterating manually 30.3.1 Iterating over an iterable via while 30.4 Iteration in practice 30.4.1 Iterating over Arrays 30.4.2 Iterating over Sets 30.5 Quick reference: synchronous iteration 30.5.1 Iterable data sources 30.5.2 Iterating constructs
- 499. 30.1 What is synchronous iteration about? Synchronous iteration is a protocol (interfaces plus rules for using them) that connects two groups of entities in JavaScript: Data sources: On one hand, data comes in all shapes and sizes. In JavaScript’s standard library, you have the linear data structure Array, the ordered collection Set (elements are ordered by time of addition), the ordered dictionary Map (entries are ordered by time of addition), and more. In libraries, you may find tree-shaped data structures and more. Data consumers: On the other hand, you have a whole class of constructs and algorithms that only need to access their input sequentially: one value at a time, until all values were visited. Examples include the for-of loop and spreading into function calls (via ...). The iteration protocol connects these two groups via the interface Iterable: data sources deliver their contents sequentially “through it”; data consumers get their input via it. Data consumers Interface for-of loop Iterable spreading Data sources Arrays Maps Strings
- 500. Figure 27: Data consumers such as the for-of loop use the interface Iterable. Data sources such as Arrays implement that interface. Fig. 27 illustrates how iteration works: data consumers use the interface Iterable; data sources implement it. The JavaScript way of implementing interfaces In JavaScript, an object implements an interface if it has all the methods that it describes. The interfaces mentioned in this chapter only exist in the ECMAScript specification. Both sources and consumers of data profit from this arrangement: If you develop a new data structure, you only need to implement Iterable and a raft of tools can immediately be applied to it. If you write code that uses iteration, it automatically works with many sources of data.
- 501. 30.2 Core iteration constructs: iterables and iterators Two roles (described by interfaces) form the core of iteration (fig. 28): An iterable is an object whose contents can be traversed sequentially. An iterator is the pointer used for the traversal. [Symbol.iterator]() ··· Iterable: traversable data structure next() Iterator: pointer for traversing iterable returns Figure 28: Iteration has two main interfaces: Iterable and Iterator. The former has a method that returns the latter. These are type definitions (in TypeScript’s notation) for the interfaces of the iteration protocol: interface Iterable<T> { [Symbol.iterator]() : Iterator<T>; } interface Iterator<T> { next() : IteratorResult<T>; } interface IteratorResult<T> { value: T; done: boolean; }
- 502. The interfaces are used as follows: You ask an Iterable for an iterator via the method whose key is Symbol.iterator. The Iterator returns the iterated values via its method .next(). The values are not returned directly, but wrapped in objects with two properties: .value is the iterated value. .done indicates if the end of the iteration has been reached yet. It is true after the last iterated value and false beforehand.
- 503. 30.3 Iterating manually This is an example of using the iteration protocol: 30.3.1 Iterating over an iterable via while The following code demonstrates how to use a while loop to iterate over an iterable: const iterable = ['a', 'b']; // The iterable is a factory for iterators: const iterator = iterable[Symbol.iterator](); // Call .next() until .done is true: assert.deepEqual( iterator.next(), { value: 'a', done: false }); assert.deepEqual( iterator.next(), { value: 'b', done: false }); assert.deepEqual( iterator.next(), { value: undefined, done: true }); function logAll(iterable) { const iterator = iterable[Symbol.iterator](); while (true) { const {value, done} = iterator.next(); if (done) break; console.log(value); } } logAll(['a', 'b']); // Output: // 'a' // 'b'
- 504. Exercise: Using sync iteration manually exercises/sync-iteration-use/sync_iteration_manually_exrc.mjs
- 505. 30.4 Iteration in practice We have seen how to use the iteration protocol manually, and it is relatively cumbersome. But the protocol is not meant to be used directly – it is meant to be used via higher-level language constructs built on top of it. This section shows what that looks like. 30.4.1 Iterating over Arrays JavaScript’s Arrays are iterable. That enables us to use the for-of loop: Destructuring via Array patterns (explained later) also uses iteration under the hood: 30.4.2 Iterating over Sets JavaScript’s Set data structure is iterable. That means for-of works: const myArray = ['a', 'b', 'c']; for (const x of myArray) { console.log(x); } // Output: // 'a' // 'b' // 'c' const [first, second] = myArray; assert.equal(first, 'a'); assert.equal(second, 'b');
- 506. As does Array-destructuring: const mySet = new Set().add('a').add('b').add('c'); for (const x of mySet) { console.log(x); } // Output: // 'a' // 'b' // 'c' const [first, second] = mySet; assert.equal(first, 'a'); assert.equal(second, 'b');
- 507. 30.5 Quick reference: synchronous iteration 30.5.1 Iterable data sources The following built-in data sources are iterable: Arrays Strings Maps Sets (Browsers: DOM data structures) To iterate over the properties of objects, you need helpers such as Object.keys() and Object.entries(). That is necessary because properties exist at a different level that is independent of the level of data structures. 30.5.2 Iterating constructs The following constructs are based on iteration: Destructuring via an Array pattern: The for-of loop: const [x,y] = iterable; for (const x of iterable) { /*···*/ }
- 508. Array.from(): Spreading (via ...) into function calls and Array literals: new Map() and new Set(): Promise.all() and Promise.race(): yield*: Quiz See quiz app. const arr = Array.from(iterable); func(...iterable); const arr = [...iterable]; const m = new Map(iterableOverKeyValuePairs); const s = new Set(iterableOverElements); const promise1 = Promise.all(iterableOverPromises); const promise2 = Promise.race(iterableOverPromises); function* generatorFunction() { yield* iterable; }
- 509. 31 Arrays (Array) 31.1 The two roles of Arrays in JavaScript 31.2 Basic Array operations 31.2.1 Creating, reading, writing Arrays 31.2.2 The .length of an Array 31.2.3 Clearing an Array 31.2.4 Spreading into Array literals 31.2.5 Arrays: listing indices and entries 31.2.6 Is a value an Array? 31.3 for-of and Arrays 31.3.1 for-of: iterating over elements 31.3.2 for-of: iterating over [index, element] pairs 31.4 Array-like objects 31.5 Converting iterable and Array-like values to Arrays 31.5.1 Converting iterables to Arrays via spreading (...) 31.5.2 Converting iterables and Array-like objects to Arrays via Array.from() (advanced) 31.6 Creating and filling Arrays with arbitrary lengths 31.6.1 Do you need to create an empty Array that you’ll fill completely later on? 31.6.2 Do you need to create an Array filled with a primitive value? 31.6.3 Do you need to create an Array filled with objects? 31.6.4 Do you need to create a range of integers?
- 510. 31.6.5 Use a Typed Array if the elements are all integers or all floats 31.7 Multidimensional Arrays 31.8 More Array features (advanced) 31.8.1 Array indices are (slightly special) property keys 31.8.2 Arrays are dictionaries and can have holes 31.9 Adding and removing elements (destructively and non- destructively) 31.9.1 Prepending elements and Arrays 31.9.2 Appending elements and Arrays 31.9.3 Removing elements 31.10 Methods: iteration and transformation (.find(), .map(), .filter(), etc.) 31.10.1 Callbacks for iteration and transformation methods 31.10.2 Searching elements: .find(), .findIndex() 31.10.3 .map(): copy while giving elements new values 31.10.4 .flatMap(): mapping to zero or more values 31.10.5 .filter(): only keep some of the elements 31.10.6 .reduce(): deriving a value from an Array (advanced) 31.11 .sort(): sorting Arrays 31.11.1 Customizing the sort order 31.11.2 Sorting numbers 31.11.3 Sorting objects 31.12 Quick reference: Array<T> 31.12.1 new Array() 31.12.2 Static methods of Array 31.12.3 Methods of Array<T>.prototype
- 511. 31.12.4 Sources
- 512. 31.1 The two roles of Arrays in JavaScript Arrays play two roles in JavaScript: Tuples: Arrays-as-tuples have a fixed number of indexed elements. Each of those elements can have a different type. Sequences: Arrays-as-sequences have a variable number of indexed elements. Each of those elements has the same type. In practice, these two roles are often mixed. Notably, Arrays-as-sequences are so flexible that you can use them as (traditional) arrays, stacks, and queues (see exercise later in this chapter).
- 513. 31.2 Basic Array operations 31.2.1 Creating, reading, writing Arrays The best way to create an Array is via an Array literal: The Array literal starts and ends with square brackets []. It creates an Array with three elements: 'a', 'b', and 'c'. To read an Array element, you put an index in square brackets (indices start at zero): To change an Array element, you assign to an Array with an index: The range of Array indices is 32 bits (excluding the maximum length): [0, 232−1) 31.2.2 The .length of an Array Every Array has a property .length that can be used to both read and change(!) the number of elements in an Array. The length of an Array is always the highest index plus one: const arr = ['a', 'b', 'c']; assert.equal(arr[0], 'a'); arr[0] = 'x'; assert.deepEqual(arr, ['x', 'b', 'c']);
- 514. If you write to the Array at the index of the length, you append an element: Another way of (destructively) appending an element is via the Array method .push(): If you set .length, you are pruning the Array by removing elements: 31.2.3 Clearing an Array To clear (empty) an Array, you can either set its .length to zero: or you can assign a new empty Array to the variable storing the Array: > const arr = ['a', 'b']; > arr.length 2 > arr[arr.length] = 'c'; > arr [ 'a', 'b', 'c' ] > arr.length 3 > arr.push('d'); > arr [ 'a', 'b', 'c', 'd' ] > arr.length = 1; > arr [ 'a' ] const arr = ['a', 'b', 'c']; arr.length = 0; assert.deepEqual(arr, []);
- 515. The latter approach has the advantage of not affecting other locations that point to the same Array. If, however, you do want to reset a shared Array for everyone, then you need the former approach. Exercise: Removing empty lines via .push() exercises/arrays/remove_empty_lines_push_test.mjs 31.2.4 Spreading into Array literals Inside an Array literal, a spread element consists of three dots (...) followed by an expression. It results in the expression being evaluated and then iterated over. Each iterated value becomes an additional Array element – for example: That means that we can use spreading to create a copy of an Array: Spreading is also convenient for concatenating Arrays (and other iterables) into Arrays: let arr = ['a', 'b', 'c']; arr = []; assert.deepEqual(arr, []); > const iterable = ['b', 'c']; > ['a', ...iterable, 'd'] [ 'a', 'b', 'c', 'd' ] const original = ['a', 'b', 'c']; const copy = [...original]; const arr1 = ['a', 'b']; const arr2 = ['c', 'd'];
- 516. Due to spreading using iteration, it only works if the value is iterable: Spreading into Array literals is shallow Similar to spreading into object literals, spreading into Array literals creates shallow copies. That is, nested Arrays are not copied. 31.2.5 Arrays: listing indices and entries Method .keys() lists the indices of an Array: .keys() returns an iterable. In line A, we spread to obtain an Array. Listing Array indices is different from listing properties. The former produces numbers; the latter produces stringified numbers (in addition to non-index property keys): const concatenated = [...arr1, ...arr2, 'e']; assert.deepEqual( concatenated, ['a', 'b', 'c', 'd', 'e']); > [...'abc'] // strings are iterable [ 'a', 'b', 'c' ] > [...123] TypeError: number 123 is not iterable > [...undefined] TypeError: undefined is not iterable const arr = ['a', 'b']; assert.deepEqual( [...arr.keys()], // (A) [0, 1]);
- 517. Method .entries() lists the contents of an Array as [index, element] pairs: 31.2.6 Is a value an Array? Following are two ways of checking if a value is an Array: instanceof is usually fine. You need Array.isArray() if a value may come from another realm. Roughly, a realm is an instance of JavaScript’s global scope. Some realms are isolated from each other (e.g., Web Workers in browsers), but there are also realms between which you can move data – for example, same-origin iframes in browsers. x instanceof Array checks the prototype chain of x and therefore returns false if x is an Array from another realm. typeof categorizes Arrays as objects: const arr = ['a', 'b']; arr.prop = true; assert.deepEqual( Object.keys(arr), ['0', '1', 'prop']); const arr = ['a', 'b']; assert.deepEqual( [...arr.entries()], [[0, 'a'], [1, 'b']]); > [] instanceof Array true > Array.isArray([]) true
- 518. > typeof [] 'object'
- 519. 31.3 for-of and Arrays We have already encountered the for-of loop. This section briefly recaps how to use it for Arrays. 31.3.1 for-of: iterating over elements The following for-of loop iterates over the elements of an Array. 31.3.2 for-of: iterating over [index, element] pairs The following for-of loop iterates over [index, element] pairs. Destructuring (described later), gives us convenient syntax for setting up index and element in the head of for-of. for (const element of ['a', 'b']) { console.log(element); } // Output: // 'a' // 'b' for (const [index, element] of ['a', 'b'].entries()) { console.log(index, element); } // Output: // 0, 'a' // 1, 'b'
- 520. 31.4 Array-like objects Some operations that work with Arrays require only the bare minimum: values must only be Array-like. An Array-like value is an object with the following properties: .length: holds the length of the Array-like object. [0]: holds the element at index 0 (etc.). Note that if you use numbers as property names, they are always coerced to strings. Therefore, [0] retrieves the value of the property whose key is '0'. For example, Array.from() accepts Array-like objects and converts them to Arrays: The TypeScript interface for Array-like objects is: Array-like objects are relatively rare in modern JavaScript // If you omit .length, it is interpreted as 0 assert.deepEqual( Array.from({}), []); assert.deepEqual( Array.from({length:2, 0:'a', 1:'b'}), [ 'a', 'b' ]); interface ArrayLike<T> { length: number; [n: number]: T; }
- 521. Array-like objects used to be common before ES6; now you don’t see them very often.
- 522. 31.5 Converting iterable and Array- like values to Arrays There are two common ways of converting iterable and Array-like values to Arrays: spreading and Array.from(). 31.5.1 Converting iterables to Arrays via spreading (...) Inside an Array literal, spreading via ... converts any iterable object into a series of Array elements. For example: The conversion works because the DOM collection is iterable. 31.5.2 Converting iterables and Array- like objects to Arrays via Array.from() (advanced) Array.from() can be used in two modes. // Get an Array-like collection from a web browser’s DOM const domCollection = document.querySelectorAll('a'); // Alas, the collection is missing many Array methods assert.equal('map' in domCollection, false); // Solution: convert it to an Array const arr = [...domCollection]; assert.deepEqual( arr.map(x => x.href), ['https://2ality.com', 'https://exploringjs.com']);
- 523. 31.5.2.1 Mode 1 of Array.from(): converting The first mode has the following type signature: Interface Iterable is shown in the chapter on synchronous iteration. Interface ArrayLike appeared earlier in this chapter. With a single parameter, Array.from() converts anything iterable or Array-like to an Array: 31.5.2.2 Mode 2 of Array.from(): converting and mapping The second mode of Array.from() involves two parameters: In this mode, Array.from() does several things: It iterates over iterable. It calls mapFunc with each iterated value. The optional parameter thisArg specifies a this for mapFunc. It applies mapFunc to each iterated value. It collects the results in a new Array and returns it. .from<T>(iterable: Iterable<T> | ArrayLike<T>): T[] > Array.from(new Set(['a', 'b'])) [ 'a', 'b' ] > Array.from({length: 2, 0:'a', 1:'b'}) [ 'a', 'b' ] .from<T, U>( iterable: Iterable<T> | ArrayLike<T>, mapFunc: (v: T, i: number) => U, thisArg?: any) : U[]
- 524. In other words: we are going from an iterable with elements of type T to an Array with elements of type U. This is an example: > Array.from(new Set(['a', 'b']), x => x + x) [ 'aa', 'bb' ]
- 525. 31.6 Creating and filling Arrays with arbitrary lengths The best way of creating an Array is via an Array literal. However, you can’t always use one: The Array may be too large, you may not know its length during development, or you may want to keep its length flexible. Then I recommend the following techniques for creating, and possibly filling, Arrays. 31.6.1 Do you need to create an empty Array that you’ll fill completely later on? Note that the result has three holes (empty slots) – the last comma in an Array literal is always ignored. 31.6.2 Do you need to create an Array filled with a primitive value? Caveat: If you use .fill() with an object, then each Array element will refer to this object (sharing it). > new Array(3) [ , , ,] > new Array(3).fill(0) [0, 0, 0] const arr = new Array(3).fill({}); arr[0].prop = true; assert.deepEqual(
- 526. The next subsection explains how to fix this. 31.6.3 Do you need to create an Array filled with objects? 31.6.4 Do you need to create a range of integers? Here is an alternative, slightly hacky technique for creating integer ranges that start at zero: arr, [ {prop: true}, {prop: true}, {prop: true}, ]); > Array.from({length: 3}, () => ({})) [{}, {}, {}] function createRange(start, end) { return Array.from({length: end-start}, (_, i) => i+start); } assert.deepEqual( createRange(2, 5), [2, 3, 4]); /** Returns an iterable */ function createRange(end) { return new Array(end).keys(); } assert.deepEqual( [...createRange(4)], [0, 1, 2, 3]);
- 527. This works because .keys() treats holes like undefined elements and lists their indices. 31.6.5 Use a Typed Array if the elements are all integers or all floats If you are dealing with Arrays of integers or floats, consider Typed Arrays, which were created for this purpose.
- 528. 31.7 Multidimensional Arrays JavaScript does not have real multidimensional Arrays; you need to resort to Arrays whose elements are Arrays: function initMultiArray(...dimensions) { function initMultiArrayRec(dimIndex) { if (dimIndex >= dimensions.length) { return 0; } else { const dim = dimensions[dimIndex]; const arr = []; for (let i=0; i<dim; i++) { arr.push(initMultiArrayRec(dimIndex+1)); } return arr; } } return initMultiArrayRec(0); } const arr = initMultiArray(4, 3, 2); arr[3][2][1] = 'X'; // last in each dimension assert.deepEqual(arr, [ [ [ 0, 0 ], [ 0, 0 ], [ 0, 0 ] ], [ [ 0, 0 ], [ 0, 0 ], [ 0, 0 ] ], [ [ 0, 0 ], [ 0, 0 ], [ 0, 0 ] ], [ [ 0, 0 ], [ 0, 0 ], [ 0, 'X' ] ], ]);
- 529. 31.8 More Array features (advanced) In this section, we look at phenomena you don’t encounter often when working with Arrays. 31.8.1 Array indices are (slightly special) property keys You’d think that Array elements are special because you are accessing them via numbers. But the square brackets operator [] for doing so is the same operator that is used for accessing properties. It coerces any value (that is not a symbol) to a string. Therefore, Array elements are (almost) normal properties (line A) and it doesn’t matter if you use numbers or strings as indices (lines B and C): To make matters even more confusing, this is only how the language specification defines things (the theory of JavaScript, if you will). Most JavaScript engines optimize under the hood and do use actual integers to access Array elements (the practice of JavaScript, if you will). const arr = ['a', 'b']; arr.prop = 123; assert.deepEqual( Object.keys(arr), ['0', '1', 'prop']); // (A) assert.equal(arr[0], 'a'); // (B) assert.equal(arr['0'], 'a'); // (C)
- 530. Property keys (strings!) that are used for Array elements are called indices. A string str is an index if converting it to a 32-bit unsigned integer and back results in the original value. Written as a formula: ToString(ToUint32(str)) === str 31.8.1.1 Listing indices When listing property keys, indices are treated specially – they always come first and are sorted like numbers ('2' comes before '10'): Note that .length, .entries() and .keys() treat Array indices as numbers and ignore non-index properties: We used a spread element (...) to convert the iterables returned by .keys() and .entries() to Arrays. 31.8.2 Arrays are dictionaries and can have holes const arr = []; arr.prop = true; arr[1] = 'b'; arr[0] = 'a'; assert.deepEqual( Object.keys(arr), ['0', '1', 'prop']); assert.equal(arr.length, 2); assert.deepEqual( [...arr.keys()], [0, 1]); assert.deepEqual( [...arr.entries()], [[0, 'a'], [1, 'b']]);
- 531. We distinguish two kinds of Arrays in JavaScript: An Array arr is dense if all indices i, with 0 ≤ i < arr.length, exist. That is, the indices form a contiguous range. An Array is sparse if the range of indices has holes in it. That is, some indices are missing. Arrays can be sparse in JavaScript because Arrays are actually dictionaries from indices to values. Recommendation: avoid holes So far, we have only seen dense Arrays and it’s indeed recommended to avoid holes: They make your code more complicated and are not handled consistently by Array methods. Additionally, JavaScript engines optimize dense Arrays, making them faster. 31.8.2.1 Creating holes You can create holes by skipping indices when assigning elements: In line A, we are using Object.keys() because arr.keys() treats holes as if they were undefined elements and does not reveal them. const arr = []; arr[0] = 'a'; arr[2] = 'c'; assert.deepEqual(Object.keys(arr), ['0', '2']); // (A) assert.equal(0 in arr, true); // element assert.equal(1 in arr, false); // hole
- 532. Another way of creating holes is to skip elements in Array literals: You can also delete Array elements: 31.8.2.2 How do Array operations treat holes? Alas, there are many different ways in which Array operations treat holes. Some Array operations remove holes: Some Array operations ignore holes: Some Array operations ignore but preserve holes: Some Array operations treat holes as undefined elements: const arr = ['a', , 'c']; assert.deepEqual(Object.keys(arr), ['0', '2']); const arr = ['a', 'b', 'c']; assert.deepEqual(Object.keys(arr), ['0', '1', '2']); delete arr[1]; assert.deepEqual(Object.keys(arr), ['0', '2']); > ['a',,'b'].filter(x => true) [ 'a', 'b' ] > ['a', ,'a'].every(x => x === 'a') true > ['a',,'b'].map(x => 'c') [ 'c', , 'c' ] > Array.from(['a',,'b'], x => x) [ 'a', undefined, 'b' ]
- 533. Object.keys() works differently than .keys() (strings vs. numbers, holes don’t have keys): There is no rule to remember here. If it ever matters how an Array operation treats holes, the best approach is to do a quick test in a console. > [...['a',,'b'].entries()] [[0, 'a'], [1, undefined], [2, 'b']] > [...['a',,'b'].keys()] [ 0, 1, 2 ] > Object.keys(['a',,'b']) [ '0', '2' ]
- 534. 31.9 Adding and removing elements (destructively and non- destructively) JavaScript’s Array is quite flexible and more like a combination of array, stack, and queue. This section explores ways of adding and removing Array elements. Most operations can be performed both destructively (modifying the Array) and non-destructively (producing a modified copy). 31.9.1 Prepending elements and Arrays In the following code, we destructively prepend single elements to arr1 and an Array to arr2: Spreading lets us unshift an Array into arr2. Non-destructive prepending is done via spread elements: const arr1 = ['a', 'b']; arr1.unshift('x', 'y'); // prepend single elements assert.deepEqual(arr1, ['x', 'y', 'a', 'b']); const arr2 = ['a', 'b']; arr2.unshift(...['x', 'y']); // prepend Array assert.deepEqual(arr2, ['x', 'y', 'a', 'b']); const arr1 = ['a', 'b']; assert.deepEqual( ['x', 'y', ...arr1], // prepend single elements ['x', 'y', 'a', 'b']); assert.deepEqual(arr1, ['a', 'b']); // unchanged!
- 535. 31.9.2 Appending elements and Arrays In the following code, we destructively append single elements to arr1 and an Array to arr2: Spreading lets us push an Array into arr2. Non-destructive appending is done via spread elements: 31.9.3 Removing elements const arr2 = ['a', 'b']; assert.deepEqual( [...['x', 'y'], ...arr2], // prepend Array ['x', 'y', 'a', 'b']); assert.deepEqual(arr2, ['a', 'b']); // unchanged! const arr1 = ['a', 'b']; arr1.push('x', 'y'); // append single elements assert.deepEqual(arr1, ['a', 'b', 'x', 'y']); const arr2 = ['a', 'b']; arr2.push(...['x', 'y']); // append Array assert.deepEqual(arr2, ['a', 'b', 'x', 'y']); const arr1 = ['a', 'b']; assert.deepEqual( [...arr1, 'x', 'y'], // append single elements ['a', 'b', 'x', 'y']); assert.deepEqual(arr1, ['a', 'b']); // unchanged! const arr2 = ['a', 'b']; assert.deepEqual( [...arr2, ...['x', 'y']], // append Array ['a', 'b', 'x', 'y']); assert.deepEqual(arr2, ['a', 'b']); // unchanged!
- 536. These are three destructive ways of removing Array elements: .splice() is covered in more detail in the quick reference at the end of this chapter. Destructuring via a rest element lets you non-destructively remove elements from the beginning of an Array (destructuring is covered later). Alas, a rest element must come last in an Array. Therefore, you can only use it to extract suffixes. Exercise: Implementing a queue via an Array exercises/arrays/queue_via_array_test.mjs // Destructively remove first element: const arr1 = ['a', 'b', 'c']; assert.equal(arr1.shift(), 'a'); assert.deepEqual(arr1, ['b', 'c']); // Destructively remove last element: const arr2 = ['a', 'b', 'c']; assert.equal(arr2.pop(), 'c'); assert.deepEqual(arr2, ['a', 'b']); // Remove one or more elements anywhere: const arr3 = ['a', 'b', 'c', 'd']; assert.deepEqual(arr3.splice(1, 2), ['b', 'c']); assert.deepEqual(arr3, ['a', 'd']); const arr1 = ['a', 'b', 'c']; // Ignore first element, extract remaining elements const [, ...arr2] = arr1; assert.deepEqual(arr2, ['b', 'c']); assert.deepEqual(arr1, ['a', 'b', 'c']); // unchanged!
- 537. 31.10 Methods: iteration and transformation (.find(), .map(), .filter(), etc.) In this section, we take a look at Array methods for iterating over Arrays and for transforming Arrays. 31.10.1 Callbacks for iteration and transformation methods All iteration and transformation methods use callbacks. The former feed all iterated values to their callbacks; the latter ask their callbacks how to transform Arrays. These callbacks have type signatures that look as follows: That is, the callback gets three parameters (it is free to ignore any of them): value is the most important one. This parameter holds the iterated value that is currently being processed. index can additionally tell the callback what the index of the iterated value is. array points to the current Array (the receiver of the method call). Some algorithms need to refer to the whole Array – e.g., to callback: (value: T, index: number, array: Array<T>) => boolean
- 538. search it for answers. This parameter lets you write reusable callbacks for such algorithms. What the callback is expected to return depends on the method it is passed to. Possibilities include: .map() fills its result with the values returned by its callback: .find() returns the first Array element for which its callback returns true: Both of these methods are described in more detail later. 31.10.2 Searching elements: .find(), .findIndex() .find() returns the first element for which its callback returns a truthy value (and undefined if it can’t find anything): .findIndex() returns the index of the first element for which its callback returns a truthy value (and -1 if it can’t find anything): > ['a', 'b', 'c'].map(x => x + x) [ 'aa', 'bb', 'cc' ] > ['a', 'bb', 'ccc'].find(str => str.length >= 2) 'bb' > [6, -5, 8].find(x => x < 0) -5 > [6, 5, 8].find(x => x < 0) undefined > [6, -5, 8].findIndex(x => x < 0) 1
- 539. .findIndex() can be implemented as follows: 31.10.3 .map(): copy while giving elements new values .map() returns a modified copy of the receiver. The elements of the copy are the results of applying map’s callback to the elements of the receiver. All of this is easier to understand via examples: .map() can be implemented as follows: > [6, 5, 8].findIndex(x => x < 0) -1 function findIndex(arr, callback) { for (const [i, x] of arr.entries()) { if (callback(x, i, arr)) { return i; } } return -1; } > [1, 2, 3].map(x => x * 3) [ 3, 6, 9 ] > ['how', 'are', 'you'].map(str => str.toUpperCase()) [ 'HOW', 'ARE', 'YOU' ] > [true, true, true].map((_x, index) => index) [ 0, 1, 2 ] function map(arr, mapFunc) { const result = []; for (const [i, x] of arr.entries()) { result.push(mapFunc(x, i, arr)); }
- 540. Exercise: Numbering lines via .map() exercises/arrays/number_lines_test.mjs 31.10.4 .flatMap(): mapping to zero or more values The type signature of Array<T>.prototype.flatMap() is: Both .map() and .flatMap() take a function callback as a parameter that controls how an input Array is translated to an output Array: With .map(), each input Array element is translated to exactly one output element. That is, callback returns a single value. With .flatMap(), each input Array element is translated to zero or more output elements. That is, callback returns an Array of values (it can also return non-Array values, but that is rare). This is .flatMap() in action: return result; } .flatMap<U>( callback: (value: T, index: number, array: T[]) => U|Array<U>, thisValue?: any ): U[] > ['a', 'b', 'c'].flatMap(x => [x,x]) [ 'a', 'a', 'b', 'b', 'c', 'c' ] > ['a', 'b', 'c'].flatMap(x => [x]) [ 'a', 'b', 'c' ] > ['a', 'b', 'c'].flatMap(x => []) []
- 541. 31.10.4.1 A simple implementation You could implement .flatMap() as follows. Note: This implementation is simpler than the built-in version, which, for example, performs more checks. What is .flatMap() good for? Let’s look at use cases! 31.10.4.2 Use case: filtering and mapping at the same time The result of the Array method .map() always has the same length as the Array it is invoked on. That is, its callback can’t skip Array elements it isn’t interested in. The ability of .flatMap() to do so is useful in the next example. We will use the following function processArray() to create an Array that we’ll then filter and map via .flatMap(): function flatMap(arr, mapFunc) { const result = []; for (const [index, elem] of arr.entries()) { const x = mapFunc(elem, index, arr); // We allow mapFunc() to return non-Arrays if (Array.isArray(x)) { result.push(...x); } else { result.push(x); } } return result; } function processArray(arr, callback) { return arr.map(x => { try { return { value: callback(x) };
- 542. Next, we create an Array results via processArray(): We can now use .flatMap() to extract just the values or just the errors from results: 31.10.4.3 Use case: mapping to multiple values The Array method .map() maps each input Array element to one output element. But what if we want to map it to multiple output elements? } catch (e) { return { error: e }; } }); } const results = processArray([1, -5, 6], throwIfNegative); assert.deepEqual(results, [ { value: 1 }, { error: new Error('Illegal value: -5') }, { value: 6 }, ]); function throwIfNegative(value) { if (value < 0) { throw new Error('Illegal value: '+value); } return value; } const values = results.flatMap( result => result.value ? [result.value] : []); assert.deepEqual(values, [1, 6]); const errors = results.flatMap( result => result.error ? [result.error] : []); assert.deepEqual(errors, [new Error('Illegal value: -5')]);
- 543. That becomes necessary in the following example: We want to convert an Array of strings to an Array of Unicode characters (code points). The following function achieves that via .flatMap(): Exercises: .flatMap() exercises/arrays/convert_to_numbers_test.mjs exercises/arrays/replace_objects_test.mjs 31.10.5 .filter(): only keep some of the elements The Array method .filter() returns an Array collecting all elements for which the callback returns a truthy value. For example: .filter() can be implemented as follows: > stringsToCodePoints(['many', 'a', 'moon']) ['m', 'a', 'n', 'y', 'a', 'm', 'o', 'o', 'n'] function stringsToCodePoints(strs) { return strs.flatMap(str => [...str]); } > [-1, 2, 5, -7, 6].filter(x => x >= 0) [ 2, 5, 6 ] > ['a', 'b', 'c', 'd'].filter((_x,i) => (i%2)===0) [ 'a', 'c' ]
- 544. Exercise: Removing empty lines via .filter() exercises/arrays/remove_empty_lines_filter_test.mjs 31.10.6 .reduce(): deriving a value from an Array (advanced) Method .reduce() is a powerful tool for computing a “summary” of an Array arr. A summary can be any kind of value: A number. For example, the sum of all elements of arr. An Array. For example, a copy of arr, where each element is twice the original element. Etc. reduce is also known as foldl (“fold left”) in functional programming and popular there. One caveat is that it can make code difficult to understand. .reduce() has the following type signature (inside an Array<T>): function filter(arr, filterFunc) { const result = []; for (const [i, x] of arr.entries()) { if (filterFunc(x, i, arr)) { result.push(x); } } return result; } .reduce<U>( callback: (accumulator: U, element: T, index: number, array: T init?: U) : U
- 545. T is the type of the Array elements, U is the type of the summary. The two may or may not be different. accumulator is just another name for “summary”. To compute the summary of an Array arr, .reduce() feeds all Array elements to its callback one at a time: callback combines the previously computed summary (stored in its parameter accumulator) with the current Array element and returns the next accumulator. The result of .reduce() is the final accumulator – the last result of callback after it has visited all elements. In other words: callback does most of the work; .reduce() just invokes it in a useful manner. You could say that the callback folds Array elements into the accumulator. That’s why this operation is called “fold” in functional programming. 31.10.6.1 A first example Let’s look at an example of .reduce() in action: function addAll() computes the sum of all numbers in an Array arr. const accumulator_0 = callback(init, arr[0]); const accumulator_1 = callback(accumulator_0, arr[1]); const accumulator_2 = callback(accumulator_1, arr[2]); // Etc. function addAll(arr) { const startSum = 0; const callback = (sum, element) => sum + element;
- 546. In this case, the accumulator holds the sum of all Array elements that callback has already visited. How was the result 6 derived from the Array in line A? Via the following invocations of callback: callback(0, 1) --> 1 callback(1, 2) --> 3 callback(3, 3) --> 6 Notes: The first parameters are the current accumulators (starting with parameter init of .reduce()). The second parameters are the current Array elements. The results are the next accumulators. The last result of callback is also the result of .reduce(). Alternatively, we could have implemented addAll() via a for-of loop: It’s hard to say which of the two implementations is “better”: the one based on .reduce() is a little more concise, while the one based on return arr.reduce(callback, startSum); } assert.equal(addAll([1, 2, 3]), 6); // (A) assert.equal(addAll([7, -4, 2]), 5); function addAll(arr) { let sum = 0; for (const element of arr) { sum = sum + element; } return sum; }
- 547. for-of may be a little easier to understand – especially if you are not familiar with functional programming. 31.10.6.2 Example: finding indices via .reduce() The following function is an implementation of the Array method .indexOf(). It returns the first index at which the given searchValue appears inside the Array arr: One limitation of .reduce() is that you can’t finish early (in a for-of loop, you can break). Here, we always immediately return the result once we have found it. 31.10.6.3 Example: doubling Array elements Function double(arr) returns a copy of inArr whose elements are all multiplied by 2: const NOT_FOUND = -1; function indexOf(arr, searchValue) { return arr.reduce( (result, elem, index) => { if (result !== NOT_FOUND) { // We have already found something: don’t change anythin return result; } else if (elem === searchValue) { return index; } else { return NOT_FOUND; } }, NOT_FOUND); } assert.equal(indexOf(['a', 'b', 'c'], 'b'), 1); assert.equal(indexOf(['a', 'b', 'c'], 'x'), -1);
- 548. We modify the initial value [] by pushing into it. A non-destructive, more functional version of double() looks as follows: This version is more elegant but also slower and uses more memory. Exercises: .reduce() map() via .reduce(): exercises/arrays/map_via_reduce_test.mjs filter() via .reduce(): exercises/arrays/filter_via_reduce_test.mjs countMatches() via .reduce(): exercises/arrays/count_matches_via_reduce_test.mjs function double(inArr) { return inArr.reduce( (outArr, element) => { outArr.push(element * 2); return outArr; }, []); } assert.deepEqual( double([1, 2, 3]), [2, 4, 6]); function double(inArr) { return inArr.reduce( // Don’t change `outArr`, return a fresh Array (outArr, element) => [...outArr, element * 2], []); } assert.deepEqual( double([1, 2, 3]), [2, 4, 6]);
- 549. 31.11 .sort(): sorting Arrays .sort() has the following type definition: By default, .sort() sorts string representations of the elements. These representations are compared via <. This operator compares lexicographically (the first characters are most significant). You can see that when sorting numbers: When sorting human-language strings, you need to be aware that they are compared according to their code unit values (char codes): As you can see, all unaccented uppercase letters come before all unaccented lowercase letters, which come before all accented letters. Use Intl, the JavaScript internationalization API, if you want proper sorting for human languages. Note that .sort() sorts in place; it changes and returns its receiver: sort(compareFunc?: (a: T, b: T) => number): this > [200, 3, 10].sort() [ 10, 200, 3 ] > ['pie', 'cookie', 'éclair', 'Pie', 'Cookie', 'Éclair'].sort() [ 'Cookie', 'Pie', 'cookie', 'pie', 'Éclair', 'éclair' ] > const arr = ['a', 'c', 'b']; > arr.sort() === arr true > arr [ 'a', 'b', 'c' ]
- 550. 31.11.1 Customizing the sort order You can customize the sort order via the parameter compareFunc, which must return a number that is: negative if a < b zero if a === b positive if a > b Tip for remembering these rules A negative number is less than zero (etc.). 31.11.2 Sorting numbers You can use this helper function to sort numbers: The following is a quick and dirty alternative. function compareNumbers(a, b) { if (a < b) { return -1; } else if (a === b) { return 0; } else { return 1; } } assert.deepEqual( [200, 3, 10].sort(compareNumbers), [3, 10, 200]); > [200, 3, 10].sort((a,b) => a - b) [ 3, 10, 200 ]
- 551. The downsides of this approach are: It is cryptic. There is a risk of numeric overflow or underflow, if a-b becomes a large positive or negative number. 31.11.3 Sorting objects You also need to use a compare function if you want to sort objects. As an example, the following code shows how to sort objects by age. Exercise: Sorting objects by name exercises/arrays/sort_objects_test.mjs const arr = [ {age: 200}, {age: 3}, {age: 10} ]; assert.deepEqual( arr.sort((obj1, obj2) => obj1.age - obj2.age), [{ age: 3 }, { age: 10 }, { age: 200 }] );
- 552. 31.12 Quick reference: Array<T> Legend: R: method does not change the Array (non-destructive). W: method changes the Array (destructive). 31.12.1 new Array() new Array(n) creates an Array of length n that contains n holes: new Array() creates an empty Array. However, I recommend to always use [] instead. 31.12.2 Static methods of Array Array.from<T>(iterable: Iterable<T> | ArrayLike<T>): T[] [ES6] Array.from<T,U>(iterable: Iterable<T> | ArrayLike<T>, mapFunc: (v: T, k: number) => U, thisArg?: any): U[] [ES6] Converts an iterable or an Array-like object to an Array. Optionally, the input values can be translated via mapFunc before they are added to the output Array. // Trailing commas are always ignored. // Therefore: number of commas = number of holes assert.deepEqual(new Array(3), [,,,]);
- 553. Examples: Array.of<T>(...items: T[]): T[] [ES6] This static method is mainly useful for subclasses of Array, where it serves as a custom Array literal: 31.12.3 Methods of Array<T>.prototype .concat(...items: Array<T[] | T>): T[] [R, ES3] Returns a new Array that is the concatenation of the receiver and all items. Non-Array parameters (such as 'b' in the following example) are treated as if they were Arrays with single elements. .copyWithin(target: number, start: number, end=this.length): this [W, ES6] Copies the elements whose indices range from (including) start to (excluding) end to indices starting with target. Overlapping is > Array.from(new Set(['a', 'b'])) // iterable [ 'a', 'b' ] > Array.from({length: 2, 0:'a', 1:'b'}) // Array-like object [ 'a', 'b' ] class MyArray extends Array {} assert.equal( MyArray.of('a', 'b') instanceof MyArray, true); > ['a'].concat('b', ['c', 'd']) [ 'a', 'b', 'c', 'd' ]
- 554. handled correctly. If start or end is negative, then .length is added to it. .entries(): Iterable<[number, T]> [R, ES6] Returns an iterable over [index, element] pairs. .every(callback: (value: T, index: number, array: Array<T>) => boolean, thisArg?: any): boolean [R, ES5] Returns true if callback returns a truthy value for every element. Otherwise, it returns false. It stops as soon as it receives a falsy value. This method corresponds to universal quantification (“for all”, ∀) in mathematics. Related method: .some() (“exists”). .fill(value: T, start=0, end=this.length): this [W, ES6] Assigns value to every index between (including) start and (excluding) end. > ['a', 'b', 'c', 'd'].copyWithin(0, 2, 4) [ 'c', 'd', 'c', 'd' ] > Array.from(['a', 'b'].entries()) [ [ 0, 'a' ], [ 1, 'b' ] ] > [1, 2, 3].every(x => x > 0) true > [1, -2, 3].every(x => x > 0) false > [0, 1, 2].fill('a') [ 'a', 'a', 'a' ]
- 555. Caveat: Don’t use this method to fill an Array with an object obj; then each element will refer to obj (sharing it). In this case, it’s better to use Array.from(). .filter(callback: (value: T, index: number, array: Array<T>) => any, thisArg?: any): T[] [R, ES5] Returns an Array with only those elements for which callback returns a truthy value. .find(predicate: (value: T, index: number, obj: T[]) => boolean, thisArg?: any): T | undefined [R, ES6] The result is the first element for which predicate returns a truthy value. If there is no such element, the result is undefined. .findIndex(predicate: (value: T, index: number, obj: T[]) => boolean, thisArg?: any): number [R, ES6] The result is the index of the first element for which predicate returns a truthy value. If there is no such element, the result is -1. > [1, -2, 3].filter(x => x > 0) [ 1, 3 ] > [1, -2, 3].find(x => x < 0) -2 > [1, 2, 3].find(x => x < 0) undefined > [1, -2, 3].findIndex(x => x < 0) 1 > [1, 2, 3].findIndex(x => x < 0) -1
- 556. .flat(depth = 1): any[] [R, ES2019] “Flattens” an Array: It descends into the Arrays that are nested inside the input Array and creates a copy where all values it finds at level depth or lower are moved to the top level. .flatMap<U>(callback: (value: T, index: number, array: T[]) => U|Array<U>, thisValue?: any): U[] [R, ES2019] The result is produced by invoking callback() for each element of the original Array and concatenating the Arrays it returns. .forEach(callback: (value: T, index: number, array: Array<T>) => void, thisArg?: any): void [R, ES5] Calls callback for each element. > [ 1,2, [3,4], [[5,6]] ].flat(0) // no change [ 1, 2, [3,4], [[5,6]] ] > [ 1,2, [3,4], [[5,6]] ].flat(1) [1, 2, 3, 4, [5,6]] > [ 1,2, [3,4], [[5,6]] ].flat(2) [1, 2, 3, 4, 5, 6] > ['a', 'b', 'c'].flatMap(x => [x,x]) [ 'a', 'a', 'b', 'b', 'c', 'c' ] > ['a', 'b', 'c'].flatMap(x => [x]) [ 'a', 'b', 'c' ] > ['a', 'b', 'c'].flatMap(x => []) [] ['a', 'b'].forEach((x, i) => console.log(x, i)) // Output:
- 557. A for-of loop is usually a better choice: it’s faster, supports break and can iterate over arbitrary iterables. .includes(searchElement: T, fromIndex=0): boolean [R, ES2016] Returns true if the receiver has an element whose value is searchElement and false, otherwise. Searching starts at index fromIndex. .indexOf(searchElement: T, fromIndex=0): number [R, ES5] Returns the index of the first element that is strictly equal to searchElement. Returns -1 if there is no such element. Starts searching at index fromIndex, visiting higher indices next. .join(separator = ','): string [R, ES1] Creates a string by concatenating string representations of all elements, separating them with separator. // 'a', 0 // 'b', 1 > [0, 1, 2].includes(1) true > [0, 1, 2].includes(5) false > ['a', 'b', 'a'].indexOf('a') 0 > ['a', 'b', 'a'].indexOf('a', 1) 2 > ['a', 'b', 'a'].indexOf('c') -1
- 558. .keys(): Iterable<number> [R, ES6] Returns an iterable over the keys of the receiver. .lastIndexOf(searchElement: T, fromIndex=this.length-1): number [R, ES5] Returns the index of the last element that is strictly equal to searchElement. Returns -1 if there is no such element. Starts searching at index fromIndex, visiting lower indices next. .map<U>(mapFunc: (value: T, index: number, array: Array<T>) => U, thisArg?: any): U[] [R, ES5] Returns a new Array, in which every element is the result of mapFunc being applied to the corresponding element of the receiver. > ['a', 'b', 'c'].join('##') 'a##b##c' > ['a', 'b', 'c'].join() 'a,b,c' > [...['a', 'b'].keys()] [ 0, 1 ] > ['a', 'b', 'a'].lastIndexOf('a') 2 > ['a', 'b', 'a'].lastIndexOf('a', 1) 0 > ['a', 'b', 'a'].lastIndexOf('c') -1 > [1, 2, 3].map(x => x * 2) [ 2, 4, 6 ] > ['a', 'b', 'c'].map((x, i) => i) [ 0, 1, 2 ]
- 559. .pop(): T | undefined [W, ES3] Removes and returns the last element of the receiver. That is, it treats the end of the receiver as a stack. The opposite of .push(). .push(...items: T[]): number [W, ES3] Adds zero or more items to the end of the receiver. That is, it treats the end of the receiver as a stack. The return value is the length of the receiver after the change. The opposite of .pop(). .reduce<U>(callback: (accumulator: U, element: T, index: number, array: T[]) => U, init?: U): U [R, ES5] This method produces a summary of the receiver: it feeds all Array elements to callback, which combines a current summary (in parameter accumulator) with the current Array element and returns the next accumulator: > const arr = ['a', 'b', 'c']; > arr.pop() 'c' > arr [ 'a', 'b' ] > const arr = ['a', 'b']; > arr.push('c', 'd') 4 > arr [ 'a', 'b', 'c', 'd' ] const accumulator_0 = callback(init, arr[0]); const accumulator_1 = callback(accumulator_0, arr[1]); const accumulator_2 = callback(accumulator_1, arr[2]); // Etc.
- 560. The result of .reduce() is the last result of callback after it has visited all Array elements. If no init is provided, the Array element at index 0 is used and the element at index 1 is visited first. Therefore, the Array must have at least length 1. .reduceRight<U>(callback: (accumulator: U, element: T, index: number, array: T[]) => U, init?: U): U [R, ES5] Works like .reduce(), but visits the Array elements backward, starting with the last element. .reverse(): this [W, ES1] Rearranges the elements of the receiver so that they are in reverse order and then returns the receiver. .shift(): T | undefined [W, ES3] > [1, 2, 3].reduce((accu, x) => accu + x, 0) 6 > [1, 2, 3].reduce((accu, x) => accu + String(x), '') '123' > [1, 2, 3].reduceRight((accu, x) => accu + String(x), '') '321' > const arr = ['a', 'b', 'c']; > arr.reverse() [ 'c', 'b', 'a' ] > arr [ 'c', 'b', 'a' ]
- 561. Removes and returns the first element of the receiver. The opposite of .unshift(). .slice(start=0, end=this.length): T[] [R, ES3] Returns a new Array containing the elements of the receiver whose indices are between (including) start and (excluding) end. Negative indices are allowed and added to .length: .some(callback: (value: T, index: number, array: Array<T>) => boolean, thisArg?: any): boolean [R, ES5] Returns true if callback returns a truthy value for at least one element. Otherwise, it returns false. It stops as soon as it receives a truthy value. This method corresponds to existential quantification (“exists”, ∃) in mathematics. > const arr = ['a', 'b', 'c']; > arr.shift() 'a' > arr [ 'b', 'c' ] > ['a', 'b', 'c', 'd'].slice(1, 3) [ 'b', 'c' ] > ['a', 'b'].slice() // shallow copy [ 'a', 'b' ] > ['a', 'b', 'c'].slice(-2) [ 'b', 'c' ] > [1, 2, 3].some(x => x < 0) false
- 562. Related method: .every() (“for all”). .sort(compareFunc?: (a: T, b: T) => number): this [W, ES1] Sorts the receiver and returns it. By default, it sorts string representations of the elements. It does so lexicographically and according to the code unit values (char codes) of the characters: You can customize the sort order via compareFunc, which returns a number that is: negative if a < b zero if a === b positive if a > b Trick for sorting numbers (with a risk of numeric overflow or underflow): .sort() is stable Since ECMAScript 2019, sorting is guaranteed to be stable: if elements are considered equal by sorting, then sorting does > [1, -2, 3].some(x => x < 0) true > ['pie', 'cookie', 'éclair', 'Pie', 'Cookie', 'Éclair'].sor [ 'Cookie', 'Pie', 'cookie', 'pie', 'Éclair', 'éclair' ] > [200, 3, 10].sort() [ 10, 200, 3 ] > [200, 3, 10].sort((a, b) => a - b) [ 3, 10, 200 ]
- 563. not change the order of those elements (relative to each other). .splice(start: number, deleteCount=this.length-start, ...items: T[]): T[] [W, ES3] At index start, it removes deleteCount elements and inserts the items. It returns the deleted elements. start can be negative and is added to .length if it is: .toString(): string [R, ES1] Converts all elements to strings via String(), concatenates them while separating them with commas, and returns the result. .unshift(...items: T[]): number [W, ES3] Inserts the items at the beginning of the receiver and returns its length after this modification. > const arr = ['a', 'b', 'c', 'd']; > arr.splice(1, 2, 'x', 'y') [ 'b', 'c' ] > arr [ 'a', 'x', 'y', 'd' ] > ['a', 'b', 'c'].splice(-2, 2) [ 'b', 'c' ] > [1, 2, 3].toString() '1,2,3' > ['1', '2', '3'].toString() '1,2,3' > [].toString() ''
- 564. .values(): Iterable<T> [R, ES6] Returns an iterable over the values of the receiver. 31.12.4 Sources TypeScript’s built-in typings MDN web docs for JavaScript ECMAScript language specification Quiz See quiz app. > const arr = ['c', 'd']; > arr.unshift('e', 'f') 4 > arr [ 'e', 'f', 'c', 'd' ] > [...['a', 'b'].values()] [ 'a', 'b' ]
- 565. 32 Typed Arrays: handling binary data (Advanced) 32.1 The basics of the API 32.1.1 Use cases for Typed Arrays 32.1.2 The core classes: ArrayBuffer, Typed Arrays, DataView 32.1.3 Using Typed Arrays 32.1.4 Using DataViews 32.2 Element types 32.2.1 Handling overflow and underflow 32.2.2 Endianness 32.3 More information on Typed Arrays 32.3.1 The static method «ElementType»Array.from() 32.3.2 Typed Arrays are iterable 32.3.3 Typed Arrays vs. normal Arrays 32.3.4 Converting Typed Arrays to and from normal Arrays 32.3.5 Concatenating Typed Arrays 32.4 Quick references: indices vs. offsets 32.5 Quick reference: ArrayBuffers 32.5.1 new ArrayBuffer() 32.5.2 Static methods of ArrayBuffer 32.5.3 Properties of ArrayBuffer.prototype 32.6 Quick reference: Typed Arrays 32.6.1 Static methods of TypedArray<T> 32.6.2 Properties of TypedArray<T>.prototype
- 566. 32.6.3 new «ElementType»Array() 32.6.4 Static properties of «ElementType»Array 32.6.5 Properties of «ElementType»Array.prototype 32.7 Quick reference: DataViews 32.7.1 new DataView() 32.7.2 Properties of DataView.prototype
- 567. 32.1 The basics of the API Much data on the web is text: JSON files, HTML files, CSS files, JavaScript code, etc. JavaScript handles such data well via its built-in strings. However, before 2011, it did not handle binary data well. The Typed Array Specification 1.0 was introduced on February 8, 2011 and provides tools for working with binary data. With ECMAScript 6, Typed Arrays were added to the core language and gained methods that were previously only available for normal Arrays (.map(), .filter(), etc.). 32.1.1 Use cases for Typed Arrays The main uses cases for Typed Arrays, are: Processing binary data: managing image data, manipulating binary files, handling binary network protocols, etc. Interacting with native APIs: Native APIs often receive and return data in a binary format, which you could neither store nor manipulate well in pre-ES6 JavaScript. That meant that whenever you were communicating with such an API, data had to be converted from JavaScript to binary and back for every call. Typed Arrays eliminate this bottleneck. One example of communicating with native APIs is WebGL, for which Typed Arrays were initially created. Section “History of Typed Arrays”
- 568. of the article “Typed Arrays: Binary Data in the Browser” (by Ilmari Heikkinen for HTML5 Rocks) has more information. 32.1.2 The core classes: ArrayBuffer, Typed Arrays, DataView The Typed Array API stores binary data in instances of ArrayBuffer: An ArrayBuffer itself is a black box: if you want to access its data, you must wrap it in another object – a view object. Two kinds of view objects are available: Typed Arrays: let you access the data as an indexed sequence of elements that all have the same type. Examples include: Uint8Array: Elements are unsigned 8-bit integers. Unsigned means that their ranges start at zero. Int16Array: Elements are signed 16-bit integers. Signed means that they have a sign and can be negative, zero, or positive. Float32Array: Elements are 32-bit floating point numbers. DataViews: let you interpret the data as various types (Uint8, Int16, Float32, etc.) that you can read and write at any byte offset. Fig. 29 shows a class diagram of the API. const buf = new ArrayBuffer(4); // length in bytes // buf is initialized with zeros
- 569. Figure 29: The classes of the Typed Array API. 32.1.3 Using Typed Arrays Typed Arrays are used much like normal Arrays with a few notable differences: Typed Arrays store their data in ArrayBuffers. All elements are initialized with zeros. All elements have the same type. Writing values to a Typed Array coerces them to that type. Reading values produces normal numbers. The length of a Typed Array is immutable; it can’t be changed. Typed Arrays can’t have holes. 32.1.3.1 Creating Typed Arrays The following code shows three different ways of creating the same Typed Array:
- 570. 32.1.3.2 The wrapped ArrayBuffer 32.1.3.3 Getting and setting elements 32.1.4 Using DataViews This is how DataViews are used: // Argument: Typed Array or Array-like object const ta1 = new Uint8Array([0, 1, 2]); const ta2 = Uint8Array.of(0, 1, 2); const ta3 = new Uint8Array(3); // length of Typed Array ta3[0] = 0; ta3[1] = 1; ta3[2] = 2; assert.deepEqual(ta1, ta2); assert.deepEqual(ta1, ta3); const typedArray = new Int16Array(2); // 2 elements assert.equal(typedArray.length, 2); assert.deepEqual( typedArray.buffer, new ArrayBuffer(4)); // 4 bytes const typedArray = new Int16Array(2); assert.equal(typedArray[1], 0); // initialized with 0 typedArray[1] = 72; assert.equal(typedArray[1], 72); const dataView = new DataView(new ArrayBuffer(4)); assert.equal(dataView.getInt16(0), 0); assert.equal(dataView.getUint8(0), 0); dataView.setUint8(0, 5);
- 571. 32.2 Element types Table 19: Element types supported by the Typed Array API. Element Typed Array Bytes Description Int8 Int8Array 1 8-bit signed integer ES6 Uint8 Uint8Array 1 8-bit unsigned integer ES6 Uint8C Uint8ClampedArray 1 8-bit unsigned integer ES6 (clamped conversion) ES6 Int16 Int16Array 2 16-bit signed integer ES6 Uint16 Uint16Array 2 16-bit unsigned integer ES6 Int32 Int32Array 4 32-bit signed integer ES6 Uint32 Uint32Array 4 32-bit unsigned integer ES6 Float32 Float32Array 4 32-bit floating point ES6 Float64 Float64Array 8 64-bit floating point ES6 Tbl. 19 lists the available element types. These types (e.g., Int32) show up in two locations:
- 572. They are the types of the elements of Typed Arrays. For example, all elements of a Int32Array have the type Int32. The element type is the only aspect of Typed Arrays that differs. They are the lenses through which an ArrayBuffer accesses its DataView when you use methods such as .getInt32() and .setInt32(). The element type Uint8C is special: it is not supported by DataView and only exists to enable Uint8ClampedArray. This Typed Array is used by the canvas element (where it replaces CanvasPixelArray) and should otherwise be avoided. The only difference between Uint8C and Uint8 is how overflow and underflow are handled (as explained in the next subsection). 32.2.1 Handling overflow and underflow Normally, when a value is out of the range of the element type, modulo arithmetic is used to convert it to a value within range. For signed and unsigned integers that means that: The highest value plus one is converted to the lowest value (0 for unsigned integers). The lowest value minus one is converted to the highest value. The following function helps illustrate how conversion works: function setAndGet(typedArray, value) { typedArray[0] = value; return typedArray[0]; }
- 573. Modulo conversion for unsigned 8-bit integers: Modulo conversion for signed 8-bit integers: Clamped conversion is different: All underflowing values are converted to the lowest value. All overflowing values are converted to the highest value. const uint8 = new Uint8Array(1); // Highest value of range assert.equal(setAndGet(uint8, 255), 255); // Overflow assert.equal(setAndGet(uint8, 256), 0); // Lowest value of range assert.equal(setAndGet(uint8, 0), 0); // Underflow assert.equal(setAndGet(uint8, -1), 255); const int8 = new Int8Array(1); // Highest value of range assert.equal(setAndGet(int8, 127), 127); // Overflow assert.equal(setAndGet(int8, 128), -128); // Lowest value of range assert.equal(setAndGet(int8, -128), -128); // Underflow assert.equal(setAndGet(int8, -129), 127); const uint8c = new Uint8ClampedArray(1); // Highest value of range assert.equal(setAndGet(uint8c, 255), 255); // Overflow assert.equal(setAndGet(uint8c, 256), 255);
- 574. 32.2.2 Endianness Whenever a type (such as Uint16) is stored as a sequence of multiple bytes, endianness matters: Big endian: the most significant byte comes first. For example, the Uint16 value 0x4321 is stored as two bytes – first 0x43, then 0x21. Little endian: the least significant byte comes first. For example, the Uint16 value 0x4321 is stored as two bytes – first 0x21, then 0x43. Endianness tends to be fixed per CPU architecture and consistent across native APIs. Typed Arrays are used to communicate with those APIs, which is why their endianness follows the endianness of the platform and can’t be changed. On the other hand, the endianness of protocols and binary files varies, but is fixed per format, across platforms. Therefore, we must be able to access data with either endianness. DataViews serve this use case and let you specify endianness when you get or set a value. Quoting Wikipedia on Endianness: // Lowest value of range assert.equal(setAndGet(uint8c, 0), 0); // Underflow assert.equal(setAndGet(uint8c, -1), 0);
- 575. Big-endian representation is the most common convention in data networking; fields in the protocols of the Internet protocol suite, such as IPv4, IPv6, TCP, and UDP, are transmitted in big- endian order. For this reason, big-endian byte order is also referred to as network byte order. Little-endian storage is popular for microprocessors in part due to significant historical influence on microprocessor designs by Intel Corporation. Other orderings are also possible. Those are generically called middle-endian or mixed-endian.
- 576. 32.3 More information on Typed Arrays In this section, «ElementType»Array stands for Int8Array, Uint8Array, etc. ElementType is Int8, Uint8, etc. 32.3.1 The static method «ElementType»Array.from() This method has the type signature: .from() converts source into an instance of this (a Typed Array). For example, normal Arrays are iterable and can be converted with this method: Typed Arrays are also iterable: source can also be an Array-like object: .from<S>( source: Iterable<S>|ArrayLike<S>, mapfn?: S => ElementType, thisArg?: any) : «ElementType»Array assert.deepEqual( Uint16Array.from([0, 1, 2]), Uint16Array.of(0, 1, 2)); assert.deepEqual( Uint16Array.from(Uint8Array.of(0, 1, 2)), Uint16Array.of(0, 1, 2));
- 577. The optional mapfn lets you transform the elements of source before they become elements of the result. Why perform the two steps mapping and conversion in one go? Compared to mapping separately via .map(), there are two advantages: 1. No intermediate Array or Typed Array is needed. 2. When converting between Typed Arrays with different precisions, less can go wrong. Read on for an explanation of the second advantage. 32.3.1.1 Pitfall: mapping while converting between Typed Array types The static method .from() can optionally both map and convert between Typed Array types. Less can go wrong if you use that method. To see why that is, let us first convert a Typed Array to a Typed Array with a higher precision. If we use .from() to map, the result is automatically correct. Otherwise, you must first convert and then map. assert.deepEqual( Uint16Array.from({0:0, 1:1, 2:2, length: 3}), Uint16Array.of(0, 1, 2)); const typedArray = Int8Array.of(127, 126, 125); assert.deepEqual( Int16Array.from(typedArray, x => x * 2), Int16Array.of(254, 252, 250)); assert.deepEqual(
- 578. If we go from a Typed Array to a Typed Array with a lower precision, mapping via .from() produces the correct result. Otherwise, we must first map and then convert. The problem is that if we map via .map(), then input type and output type are the same. In contrast, .from() goes from an arbitrary input type to an output type that you specify via its receiver. 32.3.2 Typed Arrays are iterable Typed Arrays are iterable. That means that you can use the for-of loop and other iteration-based mechanisms: Int16Array.from(typedArray).map(x => x * 2), Int16Array.of(254, 252, 250)); // OK assert.deepEqual( Int16Array.from(typedArray.map(x => x * 2)), Int16Array.of(-2, -4, -6)); // wrong assert.deepEqual( Int8Array.from(Int16Array.of(254, 252, 250), x => x / 2), Int8Array.of(127, 126, 125)); assert.deepEqual( Int8Array.from(Int16Array.of(254, 252, 250).map(x => x / 2)), Int8Array.of(127, 126, 125)); // OK assert.deepEqual( Int8Array.from(Int16Array.of(254, 252, 250)).map(x => x / 2), Int8Array.of(-1, -2, -3)); // wrong const ui8 = Uint8Array.of(0, 1, 2); for (const byte of ui8) { console.log(byte); } // Output: // 0
- 579. ArrayBuffers and DataViews are not iterable. 32.3.3 Typed Arrays vs. normal Arrays Typed Arrays are much like normal Arrays: they have a .length, elements can be accessed via the bracket operator [], and they have most of the standard Array methods. They differ from normal Arrays in the following ways: Typed Arrays have buffers. The elements of a Typed Array ta are not stored in ta, they are stored in an associated ArrayBuffer that can be accessed via ta.buffer: Typed Arrays are initialized with zeros: new Array(4) creates a normal Array without any elements. It only has four holes (indices less than the .length that have no associated elements). new Uint8Array(4) creates a Typed Array whose four elements are all 0. All of the elements of a Typed Array have the same type: // 1 // 2 const ta = new Uint16Array(2); // 2 elements assert.deepEqual( ta.buffer, new ArrayBuffer(4)); // 4 bytes assert.deepEqual(new Uint8Array(4), Uint8Array.of(0, 0, 0, 0
- 580. Setting elements converts values to that type. Getting elements returns numbers. The .length of a Typed Array is derived from its ArrayBuffer and never changes (unless you switch to a different ArrayBuffer). Normal Arrays can have holes; Typed Arrays can’t. 32.3.4 Converting Typed Arrays to and from normal Arrays To convert a normal Array to a Typed Array, you pass it to a Typed Array constructor (which accepts Array-like objects and Typed Arrays) or to «ElementType»Array.from() (which accepts iterables and Array-like objects). For example: To convert a Typed Array to a normal Array, you can use spreading or Array.from() (because Typed Arrays are iterable): const ta = new Uint8Array(1); ta[0] = 257; assert.equal(ta[0], 1); // 257 % 256 (overflow) ta[0] = '2'; assert.equal(ta[0], 2); const ta = new Uint8Array(1); assert.equal(ta[0], 0); assert.equal(typeof ta[0], 'number'); const ta1 = new Uint8Array([0, 1, 2]); const ta2 = Uint8Array.from([0, 1, 2]); assert.deepEqual(ta1, ta2);
- 581. 32.3.5 Concatenating Typed Arrays Typed Arrays don’t have a method .concat(), like normal Arrays do. The workaround is to use their overloaded method .set(): It copies the existing typedArray or arrayLike into the receiver, at index offset. TypedArray is a fictitious abstract superclass of all concrete Typed Array classes. The following function uses that method to copy zero or more Typed Arrays (or Array-like objects) into an instance of resultConstructor: assert.deepEqual( [...Uint8Array.of(0, 1, 2)], [0, 1, 2] ); assert.deepEqual( Array.from(Uint8Array.of(0, 1, 2)), [0, 1, 2] ); .set(typedArray: TypedArray, offset=0): void .set(arrayLike: ArrayLike<number>, offset=0): void function concatenate(resultConstructor, ...arrays) { let totalLength = 0; for (const arr of arrays) { totalLength += arr.length; } const result = new resultConstructor(totalLength); let offset = 0; for (const arr of arrays) { result.set(arr, offset); offset += arr.length; } return result; } assert.deepEqual( concatenate(Uint8Array, Uint8Array.of(1, 2), [3, 4]), Uint8Array.of(1, 2, 3, 4));
- 582. 32.4 Quick references: indices vs. offsets In preparation for the quick references on ArrayBuffers, Typed Arrays, and DataViews, we need learn the differences between indices and offsets: Indices for the bracket operator [ ]: You can only use non- negative indices (starting at 0). In normal Arrays, writing to negative indices creates properties: In Typed Arrays, writing to negative indices is ignored: Indices for methods of ArrayBuffers, Typed Arrays, and DataViews: Every index can be negative. If it is, it is added to the length of the entity to produce the actual index. Therefore, -1 refers to the last element, -2 to the second-last, etc. Methods of normal Arrays work the same way. const arr = [6, 7]; arr[-1] = 5; assert.deepEqual( Object.keys(arr), ['0', '1', '-1']); const tarr = Uint8Array.of(6, 7); tarr[-1] = 5; assert.deepEqual( Object.keys(tarr), ['0', '1']); const ui8 = Uint8Array.of(0, 1, 2); assert.deepEqual(ui8.slice(-1), Uint8Array.of(2));
- 583. Offsets passed to methods of Typed Arrays and DataViews: must be non-negative – for example: Whether a parameter is an index or an offset can only be determined by looking at documentation; there is no simple rule. const dataView = new DataView(new ArrayBuffer(4)); assert.throws( () => dataView.getUint8(-1), { name: 'RangeError', message: 'Offset is outside the bounds of the DataView', });
- 584. 32.5 Quick reference: ArrayBuffers ArrayBuffers store binary data, which is meant to be accessed via Typed Arrays and DataViews. 32.5.1 new ArrayBuffer() The type signature of the constructor is: Invoking this constructor via new creates an instance whose capacity is length bytes. Each of those bytes is initially 0. You can’t change the length of an ArrayBuffer; you can only create a new one with a different length. 32.5.2 Static methods of ArrayBuffer ArrayBuffer.isView(arg: any) Returns true if arg is an object and a view for an ArrayBuffer (i.e., if it is a Typed Array or a DataView). 32.5.3 Properties of ArrayBuffer.prototype get .byteLength(): number Returns the capacity of this ArrayBuffer in bytes. new ArrayBuffer(length: number)
- 585. .slice(startIndex: number, endIndex=this.byteLength) Creates a new ArrayBuffer that contains the bytes of this ArrayBuffer whose indices are greater than or equal to startIndex and less than endIndex. start and endIndex can be negative (see §32.4 “Quick references: indices vs. offsets”).
- 586. 32.6 Quick reference: Typed Arrays The properties of the various Typed Array objects are introduced in two steps: 1. TypedArray: First, we look at the abstract superclass of all Typed Array classes (which was shown in the class diagram at the beginning of this chapter). I’m calling that superclass TypedArray, but it is not directly accessible from JavaScript. TypedArray.prototype houses all methods of Typed Arrays. 2. «ElementType»Array: The concrete Typed Array classes are called Uint8Array, Int16Array, Float32Array, etc. These are the classes that you use via new, .of, and .from(). 32.6.1 Static methods of TypedArray<T> Both static TypedArray methods are inherited by its subclasses (Uint8Array, etc.). TypedArray is abstract. Therefore, you always use these methods via the subclasses, which are concrete and can have direct instances. .from<S>(source: Iterable<S>|ArrayLike<S>, mapfn?: S => T, thisArg?: any) : instanceof this Converts an iterable (including Arrays and Typed Arrays) or an Array-like object to an instance of this (instanceof this is my invention to express that fact).
- 587. The optional mapfn lets you transform the elements of source before they become elements of the result. .of(...items: number[]): instanceof this Creates a new instance of this whose elements are items (coerced to the element type). 32.6.2 Properties of TypedArray<T>.prototype Indices accepted by Typed Array methods can be negative (they work like traditional Array methods that way). Offsets must be non- negative. For details, see §32.4 “Quick references: indices vs. offsets”. 32.6.2.1 Properties specific to Typed Arrays The following properties are specific to Typed Arrays; normal Arrays don’t have them: get .buffer(): ArrayBuffer assert.deepEqual( Uint16Array.from([0, 1, 2]), Uint16Array.of(0, 1, 2)); assert.deepEqual( Int16Array.from(Int8Array.of(127, 126, 125), x => x * 2), Int16Array.of(254, 252, 250)); assert.deepEqual( Int16Array.of(-1234, 5, 67), new Int16Array([-1234, 5, 67]) );
- 588. Returns the buffer backing this Typed Array. get .length(): number Returns the length in elements of this Typed Array’s buffer. get .byteLength(): number Returns the size in bytes of this Typed Array’s buffer. get .byteOffset(): number Returns the offset where this Typed Array “starts” inside its ArrayBuffer. .set(typedArray: TypedArray, offset=0): void .set(arrayLike: ArrayLike<number>, offset=0): void Copies all elements of the first parameter to this Typed Array. The element at index 0 of the parameter is written to index offset of this Typed Array (etc.). For more information on Array-like objects, consult §31.4 “Array-like objects”. .subarray(startIndex=0, endIndex=this.length): TypedArray<T> Returns a new Typed Array that has the same buffer as this Typed Array, but a (generally) smaller range. If startIndex is non-negative then the first element of the resulting Typed Array is this[startIndex], the second this[startIndex+1] (etc.). If startIndex in negative, it is converted appropriately.
- 589. 32.6.2.2 Array methods The following methods are basically the same as the methods of normal Arrays: .copyWithin(target: number, start: number, end=this.length): this [W, ES6] .entries(): Iterable<[number, T]> [R, ES6] .every(callback: (value: T, index: number, array: TypedArray<T>) => boolean, thisArg?: any): boolean [R, ES5] .fill(value: T, start=0, end=this.length): this [W, ES6] .filter(callback: (value: T, index: number, array: TypedArray<T>) => any, thisArg?: any): T[] [R, ES5] .find(predicate: (value: T, index: number, obj: T[]) => boolean, thisArg?: any): T | undefined [R, ES6] .findIndex(predicate: (value: T, index: number, obj: T[]) => boolean, thisArg?: any): number [R, ES6] .forEach(callback: (value: T, index: number, array: TypedArray<T>) => void, thisArg?: any): void [R, ES5] .includes(searchElement: T, fromIndex=0): boolean [R, ES2016] .indexOf(searchElement: T, fromIndex=0): number [R, ES5] .join(separator = ','): string [R, ES1] .keys(): Iterable<number> [R, ES6] .lastIndexOf(searchElement: T, fromIndex=this.length-1): number [R, ES5] .map<U>(mapFunc: (value: T, index: number, array: TypedArray<T>) => U, thisArg?: any): U[] [R, ES5] .reduce<U>(callback: (accumulator: U, element: T, index: number, array: T[]) => U, init?: U): U [R, ES5]
- 590. .reduceRight<U>(callback: (accumulator: U, element: T, index: number, array: T[]) => U, init?: U): U [R, ES5] .reverse(): this [W, ES1] .slice(start=0, end=this.length): T[] [R, ES3] .some(callback: (value: T, index: number, array: TypedArray<T>) => boolean, thisArg?: any): boolean [R, ES5] .sort(compareFunc?: (a: T, b: T) => number): this [W, ES1] .toString(): string [R, ES1] .values(): Iterable<number> [R, ES6] For details on how these methods work, please consult §31.12.3 “Methods of Array<T>.prototype”. 32.6.3 new «ElementType»Array() Each Typed Array constructor has a name that follows the pattern «ElementType»Array, where «ElementType» is one of the element types in the table at the beginning. That means that there are nine constructors for Typed Arrays: Float32Array, Float64Array Int8Array, Int16Array, Int32Array Uint8Array, Uint8ClampedArray, Uint16Array, Uint32Array Each constructor has four overloaded versions – it behaves differently depending on how many arguments it receives and what their types are: new «ElementType»Array(buffer: ArrayBuffer, byteOffset=0, length=0)
- 591. Creates a new «ElementType»Array whose buffer is buffer. It starts accessing the buffer at the given byteOffset and will have the given length. Note that length counts elements of the Typed Array (with 1–8 bytes each), not bytes. new «ElementType»Array(length=0) Creates a new «ElementType»Array with the given length and the appropriate buffer. The buffer’s size in bytes is: new «ElementType»Array(source: TypedArray) Creates a new instance of «ElementType»Array whose elements have the same values as the elements of source, but coerced to ElementType. new «ElementType»Array(source: ArrayLike<number>) Creates a new instance of «ElementType»Array whose elements have the same values as the elements of source, but coerced to ElementType. For more information on Array-like objects, consult §31.4 “Array-like objects”. 32.6.4 Static properties of «ElementType»Array «ElementType»Array.BYTES_PER_ELEMENT: number Counts how many bytes are needed to store a single element: length * «ElementType»Array.BYTES_PER_ELEMENT
- 592. 32.6.5 Properties of «ElementType»Array.prototype .BYTES_PER_ELEMENT: number The same as «ElementType»Array.BYTES_PER_ELEMENT. > Uint8Array.BYTES_PER_ELEMENT 1 > Int16Array.BYTES_PER_ELEMENT 2 > Float64Array.BYTES_PER_ELEMENT 8
- 593. 32.7 Quick reference: DataViews 32.7.1 new DataView() new DataView(buffer: ArrayBuffer, byteOffset=0, byteLength=buffer.byteLength-byteOffset) Creates a new DataView whose data is stored in the ArrayBuffer buffer. By default, the new DataView can access all of buffer. The last two parameters allow you to change that. 32.7.2 Properties of DataView.prototype In the remainder of this section, «ElementType» refers to either: Float32, Float64 Int8, Int16, Int32 Uint8, Uint16, Uint32 These are the properties of DataView.prototype: get .buffer() Returns the ArrayBuffer of this DataView. get .byteLength() Returns how many bytes can be accessed by this DataView. get .byteOffset()
- 594. Returns at which offset this DataView starts accessing the bytes in its buffer. .get«ElementType»(byteOffset: number, littleEndian=false) Reads a value from the buffer of this DataView. .set«ElementType»(byteOffset: number, value: number, littleEndian=false) Writes value to the buffer of this DataView.
- 595. 33 Maps (Map) 33.1 Using Maps 33.1.1 Creating Maps 33.1.2 Copying Maps 33.1.3 Working with single entries 33.1.4 Determining the size of a Map and clearing it 33.1.5 Getting the keys and values of a Map 33.1.6 Getting the entries of a Map 33.1.7 Listed in insertion order: entries, keys, values 33.1.8 Converting between Maps and Objects 33.2 Example: Counting characters 33.3 A few more details about the keys of Maps (advanced) 33.3.1 What keys are considered equal? 33.4 Missing Map operations 33.4.1 Mapping and filtering Maps 33.4.2 Combining Maps 33.5 Quick reference: Map<K,V> 33.5.1 Constructor 33.5.2 Map<K,V>.prototype: handling single entries 33.5.3 Map<K,V>.prototype: handling all entries 33.5.4 Map<K,V>.prototype: iterating and looping 33.5.5 Sources of this section 33.6 FAQ: Maps 33.6.1 When should I use a Map, and when should I use an object?
- 596. 33.6.2 When would I use an object as a key in a Map? 33.6.3 Why do Maps preserve the insertion order of entries? 33.6.4 Why do Maps have a .size, while Arrays have a .length? Before ES6, JavaScript didn’t have a data structure for dictionaries and (ab)used objects as dictionaries from strings to arbitrary values. ES6 brought Maps, which are dictionaries from arbitrary values to arbitrary values.
- 597. 33.1 Using Maps An instance of Map maps keys to values. A single key-value mapping is called an entry. 33.1.1 Creating Maps There are three common ways of creating Maps. First, you can use the constructor without any parameters to create an empty Map: Second, you can pass an iterable (e.g., an Array) over key-value “pairs” (Arrays with two elements) to the constructor: Third, the .set() method adds entries to a Map and is chainable: 33.1.2 Copying Maps const emptyMap = new Map(); assert.equal(emptyMap.size, 0); const map = new Map([ [1, 'one'], [2, 'two'], [3, 'three'], // trailing comma is ignored ]); const map = new Map() .set(1, 'one') .set(2, 'two') .set(3, 'three');
- 598. As we’ll see later, Maps are also iterables over key-value pairs. Therefore, you can use the constructor to create a copy of a Map. That copy is shallow: keys and values are the same; they are not duplicated. 33.1.3 Working with single entries .set() and .get() are for writing and reading values (given keys). .has() checks if a Map has an entry with a given key. .delete() removes entries. const original = new Map() .set(false, 'no') .set(true, 'yes'); const copy = new Map(original); assert.deepEqual(original, copy); const map = new Map(); map.set('foo', 123); assert.equal(map.get('foo'), 123); // Unknown key: assert.equal(map.get('bar'), undefined); // Use the default value '' if an entry is missing: assert.equal(map.get('bar') || '', ''); const map = new Map([['foo', 123]]); assert.equal(map.has('foo'), true); assert.equal(map.delete('foo'), true) assert.equal(map.has('foo'), false)
- 599. 33.1.4 Determining the size of a Map and clearing it .size contains the number of entries in a Map. .clear() removes all entries of a Map. 33.1.5 Getting the keys and values of a Map .keys() returns an iterable over the keys of a Map: We can use spreading (...) to convert the iterable returned by .keys() to an Array: const map = new Map() .set('foo', true) .set('bar', false) ; assert.equal(map.size, 2) map.clear(); assert.equal(map.size, 0) const map = new Map() .set(false, 'no') .set(true, 'yes') ; for (const key of map.keys()) { console.log(key); } // Output: // false // true
- 600. .values() works like .keys(), but for values instead of keys. 33.1.6 Getting the entries of a Map .entries() returns an iterable over the entries of a Map: Spreading (...) converts the iterable returned by .entries() to an Array: Map instances are also iterables over entries. In the following code, we use destructuring to access the keys and values of map: assert.deepEqual( [...map.keys()], [false, true]); const map = new Map() .set(false, 'no') .set(true, 'yes') ; for (const entry of map.entries()) { console.log(entry); } // Output: // [false, 'no'] // [true, 'yes'] assert.deepEqual( [...map.entries()], [[false, 'no'], [true, 'yes']]); for (const [key, value] of map) { console.log(key, value); } // Output: // false, 'no' // true, 'yes'
- 601. 33.1.7 Listed in insertion order: entries, keys, values Maps record in which order entries were created and honor that order when listing entries, keys, or values: 33.1.8 Converting between Maps and Objects As long as a Map only uses strings and symbols as keys, you can convert it to an object (via Object.fromEntries()): You can also convert an object to a Map with string or symbol keys (via Object.entries()): const map1 = new Map([ ['a', 1], ['b', 2], ]); assert.deepEqual( [...map1.keys()], ['a', 'b']); const map2 = new Map([ ['b', 2], ['a', 1], ]); assert.deepEqual( [...map2.keys()], ['b', 'a']); const map = new Map([ ['a', 1], ['b', 2], ]); const obj = Object.fromEntries(map); assert.deepEqual( obj, {a: 1, b: 2});
- 602. const obj = { a: 1, b: 2, }; const map = new Map(Object.entries(obj)); assert.deepEqual( map, new Map([['a', 1], ['b', 2]]));
- 603. 33.2 Example: Counting characters countChars() returns a Map that maps characters to numbers of occurrences. function countChars(chars) { const charCounts = new Map(); for (let ch of chars) { ch = ch.toLowerCase(); const prevCount = charCounts.get(ch) || 0; charCounts.set(ch, prevCount+1); } return charCounts; } const result = countChars('AaBccc'); assert.deepEqual( [...result], [ ['a', 2], ['b', 1], ['c', 3], ] );
- 604. 33.3 A few more details about the keys of Maps (advanced) Any value can be a key, even an object: 33.3.1 What keys are considered equal? Most Map operations need to check whether a value is equal to one of the keys. They do so via the internal operation SameValueZero, which works like === but considers NaN to be equal to itself. As a consequence, you can use NaN as a key in Maps, just like any other value: Different objects are always considered to be different. That is something that can’t be changed (yet – configuring key equality is on const map = new Map(); const KEY1 = {}; const KEY2 = {}; map.set(KEY1, 'hello'); map.set(KEY2, 'world'); assert.equal(map.get(KEY1), 'hello'); assert.equal(map.get(KEY2), 'world'); > const map = new Map(); > map.set(NaN, 123); > map.get(NaN) 123
- 605. TC39’s long-term roadmap). > new Map().set({}, 1).set({}, 2).size 2
- 606. 33.4 Missing Map operations 33.4.1 Mapping and filtering Maps You can .map() and .filter() an Array, but there are no such operations for a Map. The solution is: 1. Convert the Map into an Array of [key, value] pairs. 2. Map or filter the Array. 3. Convert the result back to a Map. I’ll use the following Map to demonstrate how that works. Mapping originalMap: Filtering originalMap: const originalMap = new Map() .set(1, 'a') .set(2, 'b') .set(3, 'c'); const mappedMap = new Map( // step 3 [...originalMap] // step 1 .map(([k, v]) => [k * 2, '_' + v]) // step 2 ); assert.deepEqual([...mappedMap], [[2,'_a'], [4,'_b'], [6,'_c']]); const filteredMap = new Map( // step 3 [...originalMap] // step 1 .filter(([k, v]) => k < 3) // step 2 ); assert.deepEqual([...filteredMap], [[1,'a'], [2,'b']]);
- 607. Step 1 is performed by spreading (...) in the Array literal. 33.4.2 Combining Maps There are no methods for combining Maps, which is why we must use a workaround that is similar to the one from the previous section. Let’s combine the following two Maps: To combine map1 and map2, we turn them into Arrays via spreading (...) and concatenate those Arrays. Afterward, we convert the result back to a Map. All of that is done in line A. Exercise: Combining two Maps const map1 = new Map() .set(1, '1a') .set(2, '1b') .set(3, '1c') ; const map2 = new Map() .set(2, '2b') .set(3, '2c') .set(4, '2d') ; const combinedMap = new Map([...map1, ...map2]); // (A) assert.deepEqual( [...combinedMap], // convert to Array for comparison [ [ 1, '1a' ], [ 2, '2b' ], [ 3, '2c' ], [ 4, '2d' ] ] );
- 609. 33.5 Quick reference: Map<K,V> Note: For the sake of conciseness, I’m pretending that all keys have the same type K and that all values have the same type V. 33.5.1 Constructor new Map<K, V>(entries?: Iterable<[K, V]>) [ES6] If you don’t provide the parameter entries, then an empty Map is created. If you do provide an iterable over [key, value] pairs, then those pairs are added as entries to the Map. For example: 33.5.2 Map<K,V>.prototype: handling single entries .get(key: K): V [ES6] Returns the value that key is mapped to in this Map. If there is no key key in this Map, undefined is returned. const map = new Map([ [ 1, 'one' ], [ 2, 'two' ], [ 3, 'three' ], // trailing comma is ignored ]); const map = new Map([[1, 'one'], [2, 'two']]); assert.equal(map.get(1), 'one'); assert.equal(map.get(5), undefined);
- 610. .set(key: K, value: V): this [ES6] Maps the given key to the given value. If there is already an entry whose key is key, it is updated. Otherwise, a new entry is created. This method returns this, which means that you can chain it. .has(key: K): boolean [ES6] Returns whether the given key exists in this Map. .delete(key: K): boolean [ES6] If there is an entry whose key is key, it is removed and true is returned. Otherwise, nothing happens and false is returned. 33.5.3 Map<K,V>.prototype: handling all entries const map = new Map([[1, 'one'], [2, 'two']]); map.set(1, 'ONE!') .set(3, 'THREE!'); assert.deepEqual( [...map.entries()], [[1, 'ONE!'], [2, 'two'], [3, 'THREE!']]); const map = new Map([[1, 'one'], [2, 'two']]); assert.equal(map.has(1), true); // key exists assert.equal(map.has(5), false); // key does not exist const map = new Map([[1, 'one'], [2, 'two']]); assert.equal(map.delete(1), true); assert.equal(map.delete(5), false); // nothing happens assert.deepEqual( [...map.entries()], [[2, 'two']]);
- 611. get .size: number [ES6] Returns how many entries this Map has. .clear(): void [ES6] Removes all entries from this Map. 33.5.4 Map<K,V>.prototype: iterating and looping Both iterating and looping happen in the order in which entries were added to a Map. .entries(): Iterable<[K,V]> [ES6] Returns an iterable with one [key, value] pair for each entry in this Map. The pairs are Arrays of length 2. const map = new Map([[1, 'one'], [2, 'two']]); assert.equal(map.size, 2); const map = new Map([[1, 'one'], [2, 'two']]); assert.equal(map.size, 2); map.clear(); assert.equal(map.size, 0); const map = new Map([[1, 'one'], [2, 'two']]); for (const entry of map.entries()) { console.log(entry); } // Output: // [1, 'one'] // [2, 'two']
- 612. .forEach(callback: (value: V, key: K, theMap: Map<K,V>) => void, thisArg?: any): void [ES6] The first parameter is a callback that is invoked once for each entry in this Map. If thisArg is provided, this is set to it for each invocation. Otherwise, this is set to undefined. .keys(): Iterable<K> [ES6] Returns an iterable over all keys in this Map. .values(): Iterable<V> [ES6] Returns an iterable over all values in this Map. [Symbol.iterator](): Iterable<[K,V]> [ES6] const map = new Map([[1, 'one'], [2, 'two']]); map.forEach((value, key) => console.log(value, key)); // Output: // 'one', 1 // 'two', 2 const map = new Map([[1, 'one'], [2, 'two']]); for (const key of map.keys()) { console.log(key); } // Output: // 1 // 2 const map = new Map([[1, 'one'], [2, 'two']]); for (const value of map.values()) { console.log(value); } // Output: // 'one' // 'two'
- 613. The default way of iterating over Maps. Same as .entries(). 33.5.5 Sources of this section TypeScript’s built-in typings const map = new Map([[1, 'one'], [2, 'two']]); for (const [key, value] of map) { console.log(key, value); } // Output: // 1, 'one' // 2, 'two'
- 614. 33.6 FAQ: Maps 33.6.1 When should I use a Map, and when should I use an object? If you need a dictionary-like data structure with keys that are neither strings nor symbols, you have no choice: you must use a Map. If, however, your keys are either strings or symbols, you must decide whether or not to use an object. A rough general guideline is: Is there a fixed set of keys (known at development time)? Then use an object obj and access the values via fixed keys: Can the set of keys change at runtime? Then use a Map map and access the values via keys stored in variables: 33.6.2 When would I use an object as a key in a Map? You normally want Map keys to be compared by value (two keys are considered equal if they have the same content). That excludes const value = obj.key; const theKey = 123; map.get(theKey);
- 615. objects. However, there is one use case for objects as keys: externally attaching data to objects. But that use case is served better by WeakMaps, where entries don’t prevent keys from being garbage- collected (for details, consult the next chapter). 33.6.3 Why do Maps preserve the insertion order of entries? In principle, Maps are unordered. The main reason for ordering entries is so that operations that list entries, keys, or values are deterministic. That helps, for example, with testing. 33.6.4 Why do Maps have a .size, while Arrays have a .length? In JavaScript, indexable sequences (such as Arrays and strings) have a .length, while unindexed collections (such as Maps and Sets) have a .size: .length is based on indices; it is always the highest index plus one. .size counts the number of elements in a collection. Quiz See quiz app.
- 616. 34 WeakMaps (WeakMap) 34.1 WeakMaps are black boxes 34.2 The keys of a WeakMap are weakly held 34.2.1 All WeakMap keys must be objects 34.2.2 Use case: attaching values to objects 34.3 Examples 34.3.1 Caching computed results via WeakMaps 34.3.2 Keeping private data in WeakMaps 34.4 WeakMap API WeakMaps are similar to Maps, with the following differences: They are black boxes, where a value can only be accessed if you have both the WeakMap and the key. The keys of a WeakMap are weakly held: if an object is a key in a WeakMap, it can still be garbage-collected. That lets us use WeakMaps to attach data to objects. The next two sections examine in more detail what that means.
- 617. 34.1 WeakMaps are black boxes It is impossible to inspect what’s inside a WeakMap: For example, you can’t iterate or loop over keys, values or entries. And you can’t compute the size. Additionally, you can’t clear a WeakMap either – you have to create a fresh instance. These restrictions enable a security property. Quoting Mark Miller: The mapping from weakmap/key pair value can only be observed or affected by someone who has both the weakmap and the key. With clear(), someone with only the WeakMap would’ve been able to affect the WeakMap-and-key-to-value mapping.
- 618. 34.2 The keys of a WeakMap are weakly held The keys of a WeakMap are said to be weakly held: Normally if one object refers to another one, then the latter object can’t be garbage- collected as long as the former exists. With a WeakMap, that is different: If an object is a key and not referred to elsewhere, it can be garbage-collected while the WeakMap still exists. That also leads to the corresponding entry being removed (but there is no way to observe that). 34.2.1 All WeakMap keys must be objects All WeakMap keys must be objects. You get an error if you use a primitive value: With primitive values as keys, WeakMaps wouldn’t be black boxes anymore. But given that primitive values are never garbage- collected, you don’t profit from weakly held keys anyway, and can just as well use a normal Map. 34.2.2 Use case: attaching values to objects > const wm = new WeakMap(); > wm.set(123, 'test') TypeError: Invalid value used as weak map key
- 619. This is the main use case for WeakMaps: you can use them to externally attach values to objects – for example: In line A, we attach a value to obj. In line B, obj can already be garbage-collected, even though wm still exists. This technique of attaching a value to an object is equivalent to a property of that object being stored externally. If wm were a property, the previous code would look as follows: const wm = new WeakMap(); { const obj = {}; wm.set(obj, 'attachedValue'); // (A) } // (B) { const obj = {}; obj.wm = 'attachedValue'; }
- 620. 34.3 Examples 34.3.1 Caching computed results via WeakMaps With WeakMaps, you can associate previously computed results with objects without having to worry about memory management. The following function countOwnKeys() is an example: it caches previous results in the WeakMap cache. If we use this function with an object obj, you can see that the result is only computed for the first invocation, while a cached value is used for the second invocation: 34.3.2 Keeping private data in WeakMaps const cache = new WeakMap(); function countOwnKeys(obj) { if (cache.has(obj)) { return [cache.get(obj), 'cached']; } else { const count = Object.keys(obj).length; cache.set(obj, count); return [count, 'computed']; } } > const obj = { foo: 1, bar: 2}; > countOwnKeys(obj) [2, 'computed'] > countOwnKeys(obj) [2, 'cached']
- 621. In the following code, the WeakMaps _counter and _action are used to store the values of virtual properties of instances of Countdown: This is how Countdown is used: Exercise: WeakMaps for private data exercises/weakmaps/weakmaps_private_data_test.mjs const _counter = new WeakMap(); const _action = new WeakMap(); class Countdown { constructor(counter, action) { _counter.set(this, counter); _action.set(this, action); } dec() { let counter = _counter.get(this); counter--; _counter.set(this, counter); if (counter === 0) { _action.get(this)(); } } } // The two pseudo-properties are truly private: assert.deepEqual( Object.keys(new Countdown()), []); let invoked = false; const cd = new Countdown(3, () => invoked = true); cd.dec(); assert.equal(invoked, false); cd.dec(); assert.equal(invoked, false); cd.dec(); assert.equal(invoked, true);
- 622. 34.4 WeakMap API The constructor and the four methods of WeakMap work the same as their Map equivalents: new WeakMap<K, V>(entries?: Iterable<[K, V]>) [ES6] .delete(key: K) : boolean [ES6] .get(key: K) : V [ES6] .has(key: K) : boolean [ES6] .set(key: K, value: V) : this [ES6] Quiz See quiz app.
- 623. 35 Sets (Set) 35.1 Using Sets 35.1.1 Creating Sets 35.1.2 Adding, removing, checking membership 35.1.3 Determining the size of a Set and clearing it 35.1.4 Iterating over Sets 35.2 Examples of using Sets 35.2.1 Removing duplicates from an Array 35.2.2 Creating a set of Unicode characters (code points) 35.3 What Set elements are considered equal? 35.4 Missing Set operations 35.4.1 Union (a ∪ b) 35.4.2 Intersection (a ∩ b) 35.4.3 Difference (a b) 35.4.4 Mapping over Sets 35.4.5 Filtering Sets 35.5 Quick reference: Set<T> 35.5.1 Constructor 35.5.2 Set<T>.prototype: single Set elements 35.5.3 Set<T>.prototype: all Set elements 35.5.4 Set<T>.prototype: iterating and looping 35.5.5 Symmetry with Map 35.6 FAQ: Sets 35.6.1 Why do Sets have a .size, while Arrays have a .length?
- 624. Before ES6, JavaScript didn’t have a data structure for sets. Instead, two workarounds were used: The keys of an object were used as a set of strings. Arrays were used as sets of arbitrary values. The downside is that checking membership (if an Array contains a value) is slower. Since ES6, JavaScript has the data structure Set, which can contain arbitrary values and performs membership checks quickly.
- 625. 35.1 Using Sets 35.1.1 Creating Sets There are three common ways of creating Sets. First, you can use the constructor without any parameters to create an empty Set: Second, you can pass an iterable (e.g., an Array) to the constructor. The iterated values become elements of the new Set: Third, the .add() method adds elements to a Set and is chainable: 35.1.2 Adding, removing, checking membership .add() adds an element to a Set. .has() checks if an element is a member of a Set. const emptySet = new Set(); assert.equal(emptySet.size, 0); const set = new Set(['red', 'green', 'blue']); const set = new Set() .add('red') .add('green') .add('blue'); const set = new Set(); set.add('red');
- 626. .delete() removes an element from a Set. 35.1.3 Determining the size of a Set and clearing it .size contains the number of elements in a Set. .clear() removes all elements of a Set. 35.1.4 Iterating over Sets Sets are iterable and the for-of loop works as you’d expect: assert.equal(set.has('red'), true); assert.equal(set.delete('red'), true); // there was a deletion assert.equal(set.has('red'), false); const set = new Set() .add('foo') .add('bar'); assert.equal(set.size, 2) set.clear(); assert.equal(set.size, 0) const set = new Set(['red', 'green', 'blue']); for (const x of set) { console.log(x); } // Output: // 'red' // 'green' // 'blue'
- 627. As you can see, Sets preserve insertion order. That is, elements are always iterated over in the order in which they were added. Given that Sets are iterable, you can use spreading (...) to convert them to Arrays: const set = new Set(['red', 'green', 'blue']); const arr = [...set]; // ['red', 'green', 'blue']
- 628. 35.2 Examples of using Sets 35.2.1 Removing duplicates from an Array Converting an Array to a Set and back, removes duplicates from the Array: 35.2.2 Creating a set of Unicode characters (code points) Strings are iterable and can therefore be used as parameters for new Set(): assert.deepEqual( [...new Set([1, 2, 1, 2, 3, 3, 3])], [1, 2, 3]); assert.deepEqual( new Set('abc'), new Set(['a', 'b', 'c']));
- 629. 35.3 What Set elements are considered equal? As with Map keys, Set elements are compared similarly to ===, with the exception of NaN being equal to itself. As with ===, two different objects are never considered equal (and there is no way to change that at the moment): > const set = new Set([NaN, NaN, NaN]); > set.size 1 > set.has(NaN) true > const set = new Set(); > set.add({}); > set.size 1 > set.add({}); > set.size 2
- 630. 35.4 Missing Set operations Sets are missing several common operations. Such an operation can usually be implemented by: Converting the input Sets to Arrays by spreading into Array literals. Performing the operation on Arrays. Converting the result to a Set and returning it. 35.4.1 Union (a ∪ b) Computing the union of two Sets a and b means creating a Set that contains the elements of both a and b. 35.4.2 Intersection (a ∩ b) Computing the intersection of two Sets a and b means creating a Set that contains those elements of a that are also in b. const a = new Set([1,2,3]); const b = new Set([4,3,2]); // Use spreading to concatenate two iterables const union = new Set([...a, ...b]); assert.deepEqual([...union], [1, 2, 3, 4]); const a = new Set([1,2,3]); const b = new Set([4,3,2]); const intersection = new Set( [...a].filter(x => b.has(x)));
- 631. 35.4.3 Difference (a b) Computing the difference between two Sets a and b means creating a Set that contains those elements of a that are not in b. This operation is also sometimes called minus (−). 35.4.4 Mapping over Sets Sets don’t have a method .map(). But we can borrow the one that Arrays have: 35.4.5 Filtering Sets We can’t directly .filter() Sets, so we need to use the corresponding Array method: assert.deepEqual([...intersection], [2, 3]); const a = new Set([1,2,3]); const b = new Set([4,3,2]); const difference = new Set( [...a].filter(x => !b.has(x))); assert.deepEqual([...difference], [1]); const set = new Set([1, 2, 3]); const mappedSet = new Set([...set].map(x => x * 2)); // Convert mappedSet to an Array to check what’s inside it assert.deepEqual([...mappedSet], [2, 4, 6]); const set = new Set([1, 2, 3, 4, 5]); const filteredSet = new Set([...set].filter(x => (x % 2) === 0))
- 633. 35.5 Quick reference: Set<T> 35.5.1 Constructor new Set<T>(values?: Iterable<T>) [ES6] If you don’t provide the parameter values, then an empty Set is created. If you do, then the iterated values are added as elements to the Set. For example: 35.5.2 Set<T>.prototype: single Set elements .add(value: T): this [ES6] Adds value to this Set. This method returns this, which means that it can be chained. .delete(value: T): boolean [ES6] Removes value from this Set. Returns true if something was deleted and false, otherwise. const set = new Set(['red', 'green', 'blue']); const set = new Set(['red']); set.add('green').add('blue'); assert.deepEqual([...set], ['red', 'green', 'blue']); const set = new Set(['red', 'green', 'blue']); assert.equal(set.delete('red'), true); // there was a deleti assert.deepEqual([...set], ['green', 'blue']);
- 634. .has(value: T): boolean [ES6] Checks whether value is in this Set. 35.5.3 Set<T>.prototype: all Set elements get .size: number [ES6] Returns how many elements there are in this Set. .clear(): void [ES6] Removes all elements from this Set. 35.5.4 Set<T>.prototype: iterating and looping .values(): Iterable<T> [ES6] Returns an iterable over all elements of this Set. const set = new Set(['red', 'green']); assert.equal(set.has('red'), true); assert.equal(set.has('blue'), false); const set = new Set(['red', 'green', 'blue']); assert.equal(set.size, 3); const set = new Set(['red', 'green', 'blue']); assert.equal(set.size, 3); set.clear(); assert.equal(set.size, 0);
- 635. [Symbol.iterator](): Iterable<T> [ES6] Default way of iterating over Sets. Same as .values(). .forEach(callback: (value: T, key: T, theSet: Set<T>) => void, thisArg?: any): void [ES6] Feeds each element of this Set to callback(). value and key both contain the current element. This redundancy was introduced so that this callback has the same type signature as the callback of Map.prototype.forEach(). You can specify the this of callback via thisArg. If you omit it, this is undefined. const set = new Set(['red', 'green']); for (const x of set.values()) { console.log(x); } // Output: // 'red' // 'green' const set = new Set(['red', 'green']); for (const x of set) { console.log(x); } // Output: // 'red' // 'green' const set = new Set(['red', 'green']); set.forEach(x => console.log(x)); // Output: // 'red' // 'green'
- 636. 35.5.5 Symmetry with Map The following two methods mainly exist so that Sets and Maps have similar interfaces. Each Set element is handled as if it were a Map entry whose key and value are both the element. Set.prototype.entries(): Iterable<[T,T]> [ES6] Set.prototype.keys(): Iterable<T> [ES6] .entries() enables you to convert a Set to a Map: const set = new Set(['a', 'b', 'c']); const map = new Map(set.entries()); assert.deepEqual( [...map.entries()], [['a','a'], ['b','b'], ['c','c']]);
- 637. 35.6 FAQ: Sets 35.6.1 Why do Sets have a .size, while Arrays have a .length? The answer to this question is given in §33.6.4 “Why do Maps have a .size, while Arrays have a .length?”. Quiz See quiz app.
- 638. 36 WeakSets (WeakSet) 36.1 Example: Marking objects as safe to use with a method 36.2 WeakSet API WeakSets are similar to Sets, with the following differences: They can hold objects without preventing those objects from being garbage-collected. They are black boxes: we only get any data out of a WeakSet if we have both the WeakSet and a value. The only methods that are supported are .add(), .delete(), .has(). Consult the section on WeakMaps as black boxes for an explanation of why WeakSets don’t allow iteration, looping, and clearing. Given that we can’t iterate over their elements, there are not that many use cases for WeakSets. They do enable us to mark objects.
- 639. 36.1 Example: Marking objects as safe to use with a method Domenic Denicola shows how a class Foo can ensure that its methods are only applied to instances that were created by it: const foos = new WeakSet(); class Foo { constructor() { foos.add(this); } method() { if (!foos.has(this)) { throw new TypeError('Incompatible object!'); } } } const foo = new Foo(); foo.method(); // works assert.throws( () => { const obj = {}; Foo.prototype.method.call(obj); // throws an exception }, TypeError );
- 640. 36.2 WeakSet API The constructor and the three methods of WeakSet work the same as their Set equivalents: new WeakSet<T>(values?: Iterable<T>) [ES6] .add(value: T): this [ES6] .delete(value: T): boolean [ES6] .has(value: T): boolean [ES6]
- 641. 37 Destructuring 37.1 A first taste of destructuring 37.2 Constructing vs. extracting 37.3 Where can we destructure? 37.4 Object-destructuring 37.4.1 Property value shorthands 37.4.2 Rest properties 37.4.3 Syntax pitfall: assigning via object destructuring 37.5 Array-destructuring 37.5.1 Array-destructuring works with any iterable 37.5.2 Rest elements 37.6 Examples of destructuring 37.6.1 Array-destructuring: swapping variable values 37.6.2 Array-destructuring: operations that return Arrays 37.6.3 Object-destructuring: multiple return values 37.7 What happens if a pattern part does not match anything? 37.7.1 Object-destructuring and missing properties 37.7.2 Array-destructuring and missing elements 37.8 What values can’t be destructured? 37.8.1 You can’t object-destructure undefined and null 37.8.2 You can’t Array-destructure non-iterable values 37.9 (Advanced) 37.10 Default values 37.10.1 Default values in Array-destructuring 37.10.2 Default values in object-destructuring
- 642. 37.11 Parameter definitions are similar to destructuring 37.12 Nested destructuring
- 643. 37.1 A first taste of destructuring With normal assignment, you extract one piece of data at a time – for example: With destructuring, you can extract multiple pieces of data at the same time via patterns in locations that receive data. The left-hand side of = in the previous code is one such location. In the following code, the square brackets in line A are a destructuring pattern: This code does the same as the previous code. Note that the pattern is “smaller” than the data: we are only extracting what we need. const arr = ['a', 'b', 'c']; const x = arr[0]; // extract const y = arr[1]; // extract const arr = ['a', 'b', 'c']; const [x, y] = arr; // (A) assert.equal(x, 'a'); assert.equal(y, 'b');
- 644. 37.2 Constructing vs. extracting In order to understand what destructuring is, consider that JavaScript has two kinds of operations that are opposites: You can construct compound data, for example, by setting properties and via object literals. You can extract data out of compound data, for example, by getting properties. Constructing data looks as follows: Extracting data looks as follows: // Constructing: one property at a time const jane1 = {}; jane1.first = 'Jane'; jane1.last = 'Doe'; // Constructing: multiple properties const jane2 = { first: 'Jane', last: 'Doe', }; assert.deepEqual(jane1, jane2); const jane = { first: 'Jane', last: 'Doe', }; // Extracting: one property at a time const f1 = jane.first; const l1 = jane.last;
- 645. The operation in line A is new: we declare two variables f2 and l2 and initialize them via destructuring (multivalue extraction). The following part of line A is a destructuring pattern: Destructuring patterns are syntactically similar to the literals that are used for multivalue construction. But they appear where data is received (e.g., at the left-hand side of assignments), not where data is created (e.g., at the right-hand side of assignments). assert.equal(f1, 'Jane'); assert.equal(l1, 'Doe'); // Extracting: multiple properties (NEW!) const {first: f2, last: l2} = jane; // (A) assert.equal(f2, 'Jane'); assert.equal(l2, 'Doe'); {first: f2, last: l2}
- 646. 37.3 Where can we destructure? Destructuring patterns can be used at “data sink locations” such as: Variable declarations: Assignments: Parameter definitions: Note that variable declarations include const and let declarations in for-of loops: In the next two sections, we’ll look deeper into the two kinds of destructuring: object-destructuring and Array-destructuring. const [a] = ['x']; assert.equal(a, 'x'); let [b] = ['y']; assert.equal(b, 'y'); let b; [b] = ['z']; assert.equal(b, 'z'); const f = ([x]) => x; assert.equal(f(['a']), 'a'); const arr = ['a', 'b']; for (const [index, element] of arr.entries()) { console.log(index, element); } // Output: // 0, 'a' // 1, 'b'
- 647. 37.4 Object-destructuring Object-destructuring lets you batch-extract values of properties via patterns that look like object literals: You can think of the pattern as a transparent sheet that you place over the data: the pattern key 'street' has a match in the data. Therefore, the data value 'Evergreen Terrace' is assigned to the pattern variable s. You can also object-destructure primitive values: And you can object-destructure Arrays: Why does that work? Array indices are also properties. const address = { street: 'Evergreen Terrace', number: '742', city: 'Springfield', state: 'NT', zip: '49007', }; const { street: s, city: c } = address; assert.equal(s, 'Evergreen Terrace'); assert.equal(c, 'Springfield'); const {length: len} = 'abc'; assert.equal(len, 3); const {0:x, 2:y} = ['a', 'b', 'c']; assert.equal(x, 'a'); assert.equal(y, 'c');
- 648. 37.4.1 Property value shorthands Object literals support property value shorthands and so do object patterns: Exercise: Object-destructuring exercises/destructuring/object_destructuring_exrc.mjs 37.4.2 Rest properties In object literals, you can have spread properties. In object patterns, you can have rest properties (which must come last): A rest property variable, such as remaining (line A), is assigned an object with all data properties whose keys are not mentioned in the pattern. remaining can also be viewed as the result of non-destructively removing property a from obj. 37.4.3 Syntax pitfall: assigning via object destructuring const { street, city } = address; assert.equal(street, 'Evergreen Terrace'); assert.equal(city, 'Springfield'); const obj = { a: 1, b: 2, c: 3 }; const { a: propValue, ...remaining } = obj; // (A) assert.equal(propValue, 1); assert.deepEqual(remaining, {b:2, c:3});
- 649. If we object-destructure in an assignment, we are facing a pitfall caused by syntactic ambiguity – you can’t start a statement with a curly brace because then JavaScript thinks you are starting a block: Why eval()? eval() delays parsing (and therefore the SyntaxError) until the callback of assert.throws() is executed. If we didn’t use it, we’d already get an error when this code is parsed and assert.throws() wouldn’t even be executed. The workaround is to put the whole assignment in parentheses: let prop; assert.throws( () => eval("{prop} = { prop: 'hello' };"), { name: 'SyntaxError', message: 'Unexpected token =', }); let prop; ({prop} = { prop: 'hello' }); assert.equal(prop, 'hello');
- 650. 37.5 Array-destructuring Array-destructuring lets you batch-extract values of Array elements via patterns that look like Array literals: You can skip elements by mentioning holes inside Array patterns: The first element of the Array pattern in line A is a hole, which is why the Array element at index 0 is ignored. 37.5.1 Array-destructuring works with any iterable Array-destructuring can be applied to any value that is iterable, not just to Arrays: const [x, y] = ['a', 'b']; assert.equal(x, 'a'); assert.equal(y, 'b'); const [, x, y] = ['a', 'b', 'c']; // (A) assert.equal(x, 'b'); assert.equal(y, 'c'); // Sets are iterable const mySet = new Set().add('a').add('b').add('c'); const [first, second] = mySet; assert.equal(first, 'a'); assert.equal(second, 'b'); // Strings are iterable const [a, b] = 'xyz'; assert.equal(a, 'x'); assert.equal(b, 'y');
- 651. 37.5.2 Rest elements In Array literals, you can have spread elements. In Array patterns, you can have rest elements (which must come last): A rest element variable, such as remaining (line A), is assigned an Array with all elements of the destructured value that were not mentioned yet. const [x, y, ...remaining] = ['a', 'b', 'c', 'd']; // (A) assert.equal(x, 'a'); assert.equal(y, 'b'); assert.deepEqual(remaining, ['c', 'd']);
- 652. 37.6 Examples of destructuring 37.6.1 Array-destructuring: swapping variable values You can use Array-destructuring to swap the values of two variables without needing a temporary variable: 37.6.2 Array-destructuring: operations that return Arrays Array-destructuring is useful when operations return Arrays, as does, for example, the regular expression method .exec(): let x = 'a'; let y = 'b'; [x,y] = [y,x]; // swap assert.equal(x, 'b'); assert.equal(y, 'a'); // Skip the element at index 0 (the whole match): const [, year, month, day] = /^([0-9]{4})-([0-9]{2})-([0-9]{2})$/ .exec('2999-12-31'); assert.equal(year, '2999'); assert.equal(month, '12'); assert.equal(day, '31');
- 653. 37.6.3 Object-destructuring: multiple return values Destructuring is very useful if a function returns multiple values – either packaged as an Array or packaged as an object. Consider a function findElement() that finds elements in an Array: Its second parameter is a function that receives the value and index of an element and returns a boolean indicating if this is the element the caller is looking for. We are now faced with a dilemma: Should findElement() return the value of the element it found or the index? One solution would be to create two separate functions, but that would result in duplicated code because both functions would be very similar. The following implementation avoids duplication by returning an object that contains both index and value of the element that is found: findElement(array, (value, index) => «boolean expression») function findElement(arr, predicate) { for (let index=0; index < arr.length; index++) { const value = arr[index]; if (predicate(value)) { // We found something: return { value, index }; } } // We didn’t find anything: return { value: undefined, index: -1 }; }
- 654. Destructuring helps us with processing the result of findElement(): As we are working with property keys, the order in which we mention value and index doesn’t matter: The kicker is that destructuring also serves us well if we are only interested in one of the two results: All of these conveniences combined make this way of handling multiple return values quite versatile. const arr = [7, 8, 6]; const {value, index} = findElement(arr, x => x % 2 === 0); assert.equal(value, 8); assert.equal(index, 1); const {index, value} = findElement(arr, x => x % 2 === 0); const arr = [7, 8, 6]; const {value} = findElement(arr, x => x % 2 === 0); assert.equal(value, 8); const {index} = findElement(arr, x => x % 2 === 0); assert.equal(index, 1);
- 655. 37.7 What happens if a pattern part does not match anything? What happens if there is no match for part of a pattern? The same thing that happens if you use non-batch operators: you get undefined. 37.7.1 Object-destructuring and missing properties If a property in an object pattern has no match on the right-hand side, you get undefined: 37.7.2 Array-destructuring and missing elements If an element in an Array pattern has no match on the right-hand side, you get undefined: const {prop: p} = {}; assert.equal(p, undefined); const [x] = []; assert.equal(x, undefined);
- 656. 37.8 What values can’t be destructured? 37.8.1 You can’t object-destructure undefined and null Object-destructuring only fails if the value to be destructured is either undefined or null. That is, it fails whenever accessing a property via the dot operator would fail too. 37.8.2 You can’t Array-destructure non- iterable values Array-destructuring demands that the destructured value be iterable. Therefore, you can’t Array-destructure undefined and null. But you assert.throws( () => { const {prop} = undefined; }, { name: 'TypeError', message: "Cannot destructure property `prop` of " + "'undefined' or 'null'.", } ); assert.throws( () => { const {prop} = null; }, { name: 'TypeError', message: "Cannot destructure property `prop` of " + "'undefined' or 'null'.", } );
- 657. can’t Array-destructure non-iterable objects either: Quiz: basic See quiz app. assert.throws( () => { const [x] = {}; }, { name: 'TypeError', message: '{} is not iterable', } );
- 658. 37.9 (Advanced) All of the remaining sections are advanced.
- 659. 37.10 Default values Normally, if a pattern has no match, the corresponding variable is set to undefined: If you want a different value to be used, you need to specify a default value (via =): In line A, we specify the default value for p to be 123. That default is used because the data that we are destructuring has no property named prop. 37.10.1 Default values in Array- destructuring Here, we have two default values that are assigned to the variables x and y because the corresponding elements don’t exist in the Array that is destructured. The default value for the first element of the Array pattern is 1; the default value for the second element is 2. const {prop: p} = {}; assert.equal(p, undefined); const {prop: p = 123} = {}; // (A) assert.equal(p, 123); const [x=1, y=2] = []; assert.equal(x, 1); assert.equal(y, 2);
- 660. 37.10.2 Default values in object- destructuring You can also specify default values for object-destructuring: Neither property key first nor property key last exist in the object that is destructured. Therefore, the default values are used. With property value shorthands, this code becomes simpler: const {first: f='', last: l=''} = {}; assert.equal(f, ''); assert.equal(l, ''); const {first='', last=''} = {}; assert.equal(first, ''); assert.equal(last, '');
- 661. 37.11 Parameter definitions are similar to destructuring Considering what we have learned in this chapter, parameter definitions have much in common with an Array pattern (rest elements, default values, etc.). In fact, the following two function declarations are equivalent: function f1(«pattern1», «pattern2») { // ··· } function f2(...args) { const [«pattern1», «pattern2»] = args; // ··· }
- 662. 37.12 Nested destructuring Until now, we have only used variables as assignment targets (data sinks) inside destructuring patterns. But you can also use patterns as assignment targets, which enables you to nest patterns to arbitrary depths: Inside the Array pattern in line A, there is a nested object pattern at index 1. Nested patterns can become difficult to understand, so they are best used in moderation. Quiz: advanced See quiz app. const arr = [ { first: 'Jane', last: 'Bond' }, { first: 'Lars', last: 'Croft' }, ]; const [, {first}] = arr; assert.equal(first, 'Lars');
- 663. 38 Synchronous generators (advanced) 38.1 What are synchronous generators? 38.1.1 Generator functions return iterables and fill them via yield 38.1.2 yield pauses a generator function 38.1.3 Why does yield pause execution? 38.1.4 Example: Mapping over iterables 38.2 Calling generators from generators (advanced) 38.2.1 Calling generators via yield* 38.2.2 Example: Iterating over a tree 38.3 Background: external iteration vs. internal iteration 38.4 Use case for generators: reusing traversals 38.4.1 The traversal to reuse 38.4.2 Internal iteration (push) 38.4.3 External iteration (pull) 38.5 Advanced features of generators
- 664. 38.1 What are synchronous generators? Synchronous generators are special versions of function definitions and method definitions that always return synchronous iterables: Asterisks (*) mark functions and methods as generators: Functions: The pseudo-keyword function* is a combination of the keyword function and an asterisk. Methods: The * is a modifier (similar to static and get). // Generator function declaration function* genFunc1() { /*···*/ } // Generator function expression const genFunc2 = function* () { /*···*/ }; // Generator method definition in an object literal const obj = { * generatorMethod() { // ··· } }; // Generator method definition in a class definition // (class declaration or class expression) class MyClass { * generatorMethod() { // ··· } }
- 665. 38.1.1 Generator functions return iterables and fill them via yield If you call a generator function, it returns an iterable (actually, an iterator that is also iterable). The generator fills that iterable via the yield operator: 38.1.2 yield pauses a generator function Using a generator function involves the following steps: Function-calling it returns an iterator iter (that is also an iterable). Iterating over iter repeatedly invokes iter.next(). Each time, we jump into the body of the generator function until there is a yield that returns a value. function* genFunc1() { yield 'a'; yield 'b'; } const iterable = genFunc1(); // Convert the iterable to an Array, to check what’s inside: assert.deepEqual([...iterable], ['a', 'b']); // You can also use a for-of loop for (const x of genFunc1()) { console.log(x); } // Output: // 'a' // 'b'
- 666. Therefore, yield does more than just add values to iterables – it also pauses and exits the generator function: Like return, a yield exits the body of the function and returns a value (via .next()). Unlike return, if you repeat the invocation (of .next()), execution resumes directly after the yield. Let’s examine what that means via the following generator function. In order to use genFunc2(), we must first create the iterator/iterable iter. genFunc2() is now paused “before” its body. iter implements the iteration protocol. Therefore, we control the execution of genFunc2() via iter.next(). Calling that method resumes the paused genFunc2() and executes it until there is a yield. Then execution pauses and .next() returns the operand of the yield: let location = 0; function* genFunc2() { location = 1; yield 'a'; location = 2; yield 'b'; location = 3; } const iter = genFunc2(); // genFunc2() is now paused “before” its body: assert.equal(location, 0); assert.deepEqual( iter.next(), {value: 'a', done: false}); // genFunc2() is now paused directly after the first `yield`: assert.equal(location, 1);
- 667. Note that the yielded value 'a' is wrapped in an object, which is how iterators always deliver their values. We call iter.next() again and execution continues where we previously paused. Once we encounter the second yield, genFunc2() is paused and .next() returns the yielded value 'b'. We call iter.next() one more time and execution continues until it leaves the body of genFunc2(): This time, property .done of the result of .next() is true, which means that the iterator is finished. 38.1.3 Why does yield pause execution? What are the benefits of yield pausing execution? Why doesn’t it simply work like the Array method .push() and fill the iterable with values without pausing? Due to pausing, generators provide many of the features of coroutines (think processes that are multitasked cooperatively). For example, when you ask for the next value of an iterable, that value is assert.deepEqual( iter.next(), {value: 'b', done: false}); // genFunc2() is now paused directly after the second `yield`: assert.equal(location, 2); assert.deepEqual( iter.next(), {value: undefined, done: true}); // We have reached the end of genFunc2(): assert.equal(location, 3);
- 668. computed lazily (on demand). The following two generator functions demonstrate what that means. Note that the yield in numberLines() appears inside a for-of loop. yield can be used inside loops, but not inside callbacks (more on that later). Let’s combine both generators to produce the iterable numberedLines: The key benefit of using generators here is that everything works incrementally: via numberedLines.next(), we ask numberLines() for /** * Returns an iterable over lines */ function* genLines() { yield 'A line'; yield 'Another line'; yield 'Last line'; } /** * Input: iterable over lines * Output: iterable over numbered lines */ function* numberLines(lineIterable) { let lineNumber = 1; for (const line of lineIterable) { // input yield lineNumber + ': ' + line; // output lineNumber++; } } const numberedLines = numberLines(genLines()); assert.deepEqual( numberedLines.next(), {value: '1: A line', done: false}); assert.deepEqual( numberedLines.next(), {value: '2: Another line', done: false})
- 669. only a single numbered line. In turn, it asks genLines() for only a single unnumbered line. This incrementalism continues to work if, for example, genLines() reads its lines from a large text file: If we ask numberLines() for a numbered line, we get one as soon as genLines() has read its first line from the text file. Without generators, genLines() would first read all lines and return them. Then numberLines() would number all lines and return them. We therefore have to wait much longer until we get the first numbered line. Exercise: Turning a normal function into a generator exercises/sync-generators/fib_seq_test.mjs 38.1.4 Example: Mapping over iterables The following function mapIter() is similar to the Array method .map(), but it returns an iterable, not an Array, and produces its results on demand. function* mapIter(iterable, func) { let index = 0; for (const x of iterable) { yield func(x, index); index++; } } const iterable = mapIter(['a', 'b'], x => x + x); assert.deepEqual([...iterable], ['aa', 'bb']);
- 671. 38.2 Calling generators from generators (advanced) 38.2.1 Calling generators via yield* yield only works directly inside generators – so far we haven’t seen a way of delegating yielding to another function or method. Let’s first examine what does not work: in the following example, we’d like foo() to call bar(), so that the latter yields two values for the former. Alas, a naive approach fails: Why doesn’t this work? The function call bar() returns an iterable, which we ignore. What we want is for foo() to yield everything that is yielded by bar(). That’s what the yield* operator does: function* bar() { yield 'a'; yield 'b'; } function* foo() { // Nothing happens if we call `bar()`: bar(); } assert.deepEqual( [...foo()], []); function* bar() { yield 'a'; yield 'b'; }
- 672. In other words, the previous foo() is roughly equivalent to: Note that yield* works with any iterable: 38.2.2 Example: Iterating over a tree yield* lets us make recursive calls in generators, which is useful when iterating over recursive data structures such as trees. Take, for example, the following data structure for binary trees. function* foo() { yield* bar(); } assert.deepEqual( [...foo()], ['a', 'b']); function* foo() { for (const x of bar()) { yield x; } } function* gen() { yield* [1, 2]; } assert.deepEqual( [...gen()], [1, 2]); class BinaryTree { constructor(value, left=null, right=null) { this.value = value; this.left = left; this.right = right; } /** Prefix iteration: parent before children */ * [Symbol.iterator]() { yield this.value;
- 673. Method [Symbol.iterator]() adds support for the iteration protocol, which means that we can use a for-of loop to iterate over an instance of BinaryTree: Exercise: Iterating over a nested Array exercises/sync-generators/iter_nested_arrays_test.mjs if (this.left) { // Same as yield* this.left[Symbol.iterator]() yield* this.left; } if (this.right) { yield* this.right; } } } const tree = new BinaryTree('a', new BinaryTree('b', new BinaryTree('c'), new BinaryTree('d')), new BinaryTree('e')); for (const x of tree) { console.log(x); } // Output: // 'a' // 'b' // 'c' // 'd' // 'e'
- 674. 38.3 Background: external iteration vs. internal iteration In preparation for the next section, we need to learn about two different styles of iterating over the values “inside” an object: External iteration (pull): Your code asks the object for the values via an iteration protocol. For example, the for-of loop is based on JavaScript’s iteration protocol: Internal iteration (push): You pass a callback function to a method of the object and the method feeds the values to the callback. For example, Arrays have the method .forEach(): The next section has examples for both styles of iteration. for (const x of ['a', 'b']) { console.log(x); } // Output: // 'a' // 'b' ['a', 'b'].forEach((x) => { console.log(x); }); // Output: // 'a' // 'b'
- 675. 38.4 Use case for generators: reusing traversals One important use case for generators is extracting and reusing traversals. 38.4.1 The traversal to reuse As an example, consider the following function that traverses a tree of files and logs their paths (it uses the Node.js API for doing so): Consider the following directory: mydir/ a.txt b.txt subdir/ c.txt Let’s log the paths inside mydir/: function logPaths(dir) { for (const fileName of fs.readdirSync(dir)) { const filePath = path.resolve(dir, fileName); console.log(filePath); const stats = fs.statSync(filePath); if (stats.isDirectory()) { logPaths(filePath); // recursive call } } } logPaths('mydir');
- 676. How can we reuse this traversal and do something other than logging the paths? 38.4.2 Internal iteration (push) One way of reusing traversal code is via internal iteration: Each traversed value is passsed to a callback (line A). 38.4.3 External iteration (pull) // Output: // 'mydir/a.txt' // 'mydir/b.txt' // 'mydir/subdir' // 'mydir/subdir/c.txt' function visitPaths(dir, callback) { for (const fileName of fs.readdirSync(dir)) { const filePath = path.resolve(dir, fileName); callback(filePath); // (A) const stats = fs.statSync(filePath); if (stats.isDirectory()) { visitPaths(filePath, callback); } } } const paths = []; visitPaths('mydir', p => paths.push(p)); assert.deepEqual( paths, [ 'mydir/a.txt', 'mydir/b.txt', 'mydir/subdir', 'mydir/subdir/c.txt', ]);
- 677. Another way of reusing traversal code is via external iteration: We can write a generator that yields all traversed values (line A). function* iterPaths(dir) { for (const fileName of fs.readdirSync(dir)) { const filePath = path.resolve(dir, fileName); yield filePath; // (A) const stats = fs.statSync(filePath); if (stats.isDirectory()) { yield* iterPaths(filePath); } } } const paths = [...iterPaths('mydir')];
- 678. 38.5 Advanced features of generators The chapter on generators in Exploring ES6 covers two features that are beyond the scope of this book: yield can also receive data, via an argument of .next(). Generators can also return values (not just yield them). Such values do not become iteration values, but can be retrieved via yield*.
- 679. 39 Asynchronous programming in JavaScript 39.1 A roadmap for asynchronous programming in JavaScript 39.1.1 Synchronous functions 39.1.2 JavaScript executes tasks sequentially in a single process 39.1.3 Callback-based asynchronous functions 39.1.4 Promise-based asynchronous functions 39.1.5 Async functions 39.1.6 Next steps 39.2 The call stack 39.3 The event loop 39.4 How to avoid blocking the JavaScript process 39.4.1 The user interface of the browser can be blocked 39.4.2 How can we avoid blocking the browser? 39.4.3 Taking breaks 39.4.4 Run-to-completion semantics 39.5 Patterns for delivering asynchronous results 39.5.1 Delivering asynchronous results via events 39.5.2 Delivering asynchronous results via callbacks 39.6 Asynchronous code: the downsides 39.7 Resources
- 680. This chapter explains the foundations of asynchronous programming in JavaScript.
- 681. 39.1 A roadmap for asynchronous programming in JavaScript This section provides a roadmap for the content on asynchronous programming in JavaScript. Don’t worry about the details! Don’t worry if you don’t understand everything yet. This is just a quick peek at what’s coming up. 39.1.1 Synchronous functions Normal functions are synchronous: the caller waits until the callee is finished with its computation. divideSync() in line A is a synchronous function call: 39.1.2 JavaScript executes tasks sequentially in a single process function main() { try { const result = divideSync(12, 3); // (A) assert.equal(result, 4); } catch (err) { assert.fail(err); } }
- 682. By default, JavaScript tasks are functions that are executed sequentially in a single process. That looks like this: This loop is also called the event loop because events, such as clicking a mouse, add tasks to the queue. Due to this style of cooperative multitasking, we don’t want a task to block other tasks from being executed while, for example, it waits for results coming from a server. The next subsection explores how to handle this case. 39.1.3 Callback-based asynchronous functions What if divide() needs a server to compute its result? Then the result should be delivered in a different manner: The caller shouldn’t have to wait (synchronously) until the result is ready; it should be notified (asynchronously) when it is. One way of delivering the result asynchronously is by giving divide() a callback function that it uses to notify the caller. while (true) { const task = taskQueue.dequeue(); task(); // run task } function main() { divideCallback(12, 3, (err, result) => { if (err) { assert.fail(err); } else { assert.equal(result, 4); }
- 683. When there is an asynchronous function call: Then the following steps happen: divideCallback() sends a request to a server. Then the current task main() is finished and other tasks can be executed. When a response from the server arrives, it is either: An error err: Then the following task is added to the queue. A result r: Then the following task is added to the queue. 39.1.4 Promise-based asynchronous functions Promises are two things: A standard pattern that makes working with callbacks easier. The mechanism on which async functions (the topic of the next subsection) are built. Invoking a Promise-based function looks as follows. }); } divideCallback(x, y, callback) taskQueue.enqueue(() => callback(err)); taskQueue.enqueue(() => callback(null, r));
- 684. 39.1.5 Async functions One way of looking at async functions is as better syntax for Promise- based code: The dividePromise() we are calling in line A is the same Promise- based function as in the previous section. But we now have synchronous-looking syntax for handling the call. await can only be used inside a special kind of function, an async function (note the keyword async in front of the keyword function). await pauses the current async function and returns from it. Once the awaited result is ready, the execution of the function continues where it left off. 39.1.6 Next steps In this chapter, we’ll see how synchronous function calls work. We’ll also explore JavaScript’s way of executing code in a single process, via its event loop. function main() { dividePromise(12, 3) .then(result => assert.equal(result, 4)) .catch(err => assert.fail(err)); } async function main() { try { const result = await dividePromise(12, 3); // (A) assert.equal(result, 4); } catch (err) { assert.fail(err); } }
- 685. Asynchronicity via callbacks is also described in this chapter. The following chapters cover Promises and async functions. This series of chapters on asynchronous programming concludes with the chapter on asynchronous iteration, which is similar to synchronous iteration, but iterated values are delivered asynchronously.
- 686. 39.2 The call stack Whenever a function calls another function, we need to remember where to return to after the latter function is finished. That is typically done via a stack – the call stack: the caller pushes onto it the location to return to, and the callee jumps to that location after it is done. This is an example where several calls happen: Initially, before running this piece of code, the call stack is empty. After the function call f(3) in line 11, the stack has one entry: Line 12 (location in top-level scope) After the function call g(x + 1) in line 9, the stack has two entries: Line 10 (location in f()) Line 12 (location in top-level scope) unction h(z) { const error = new Error(); console.log(error.stack); unction g(y) { h(y + 1); unction f(x) { g(x + 1); (3); / done
- 687. After the function call h(y + 1) in line 6, the stack has three entries: Line 7 (location in g()) Line 10 (location in f()) Line 12 (location in top-level scope) Logging error in line 3, produces the following output: Error: at h (demos/async-js/stack_trace.mjs:2:17) at g (demos/async-js/stack_trace.mjs:6:3) at f (demos/async-js/stack_trace.mjs:9:3) at demos/async-js/stack_trace.mjs:11:1 This is a so-called stack trace of where the Error object was created. Note that it records where calls were made, not return locations. Creating the exception in line 2 is yet another call. That’s why the stack trace includes a location inside h(). After line 3, each of the functions terminates and each time, the top entry is removed from the call stack. After function f is done, we are back in top-level scope and the stack is empty. When the code fragment ends then that is like an implicit return. If we consider the code fragment to be a task that is executed, then returning with an empty call stack ends the task.
- 688. 39.3 The event loop By default, JavaScript runs in a single process – in both web browsers and Node.js. The so-called event loop sequentially executes tasks (pieces of code) inside that process. The event loop is depicted in fig. 30. onDone onClick onClick Task sources: • DOM manipulation • User interaction • Networking • History traversal • … Event loop ↺ Task queue Call stack func1 onTimeout func2 func3 running Figure 30: Task sources add code to run to the task queue, which is emptied by the event loop. Two parties access the task queue: Task sources add tasks to the queue. Some of those sources run concurrently to the JavaScript process. For example, one task source takes care of user interface events: if a user clicks somewhere and a click listener was registered, then an invocation of that listener is added to the task queue.
- 689. The event loop runs continuously inside the JavaScript process. During each loop iteration, it takes one task out of the queue (if the queue is empty, it waits until it isn’t) and executes it. That task is finished when the call stack is empty and there is a return. Control goes back to the event loop, which then retrieves the next task from the queue and executes it. And so on. The following JavaScript code is an approximation of the event loop: while (true) { const task = taskQueue.dequeue(); task(); // run task }
- 690. 39.4 How to avoid blocking the JavaScript process 39.4.1 The user interface of the browser can be blocked Many of the user interface mechanisms of browsers also run in the JavaScript process (as tasks). Therefore, long-running JavaScript code can block the user interface. Let’s look at a web page that demonstrates that. There are two ways in which you can try out that page: You can run it online. You can open the following file inside the repository with the exercises: demos/async-js/blocking.html The following HTML is the page’s user interface: The idea is that you click “Block” and a long-running loop is executed via JavaScript. During that loop, you can’t click the button because the browser/JavaScript process is blocked. A simplified version of the JavaScript code looks like this: <a id="block" href="">Block</a> <div id="statusMessage"></div> <button>Click me!</button> document.getElementById('block') .addEventListener('click', doBlock); // (A)
- 691. These are the key parts of the code: Line A: We tell the browser to call doBlock() whenever the HTML element is clicked whose ID is block. doBlock() displays status information and then calls sleep() to block the JavaScript process for 5000 milliseconds (line B). sleep() blocks the JavaScript process by looping until enough time has passed. displayStatus() displays status messages inside the <div> whose ID is statusMessage. 39.4.2 How can we avoid blocking the browser? There are several ways in which you can prevent a long-running operation from blocking the browser: function doBlock(event) { // ··· displayStatus('Blocking...'); // ··· sleep(5000); // (B) displayStatus('Done'); } function sleep(milliseconds) { const start = Date.now(); while ((Date.now() - start) < milliseconds); } function displayStatus(status) { document.getElementById('statusMessage') .textContent = status; }
- 692. The operation can deliver its result asynchronously: Some operations, such as downloads, can be performed concurrently to the JavaScript process. The JavaScript code triggering such an operation registers a callback, which is invoked with the result once the operation is finished. The invocation is handled via the task queue. This style of delivering a result is called asynchronous because the caller doesn’t wait until the results are ready. Normal function calls deliver their results synchronously. Perform long computations in separate processes: This can be done via so-called Web Workers. Web Workers are heavyweight processes that run concurrently to the main process. Each one of them has its own runtime environment (global variables, etc.). They are completely isolated and must be communicated with via message passing. Consult MDN web docs for more information. Take breaks during long computations. The next subsection explains how. 39.4.3 Taking breaks The following global function executes its parameter callback after a delay of ms milliseconds (the type signature is simplified – setTimeout() has more features): function setTimeout(callback: () => void, ms: number): any
- 693. The function returns a handle (an ID) that can be used to clear the timeout (cancel the execution of the callback) via the following global function: setTimeout() is available on both browsers and Node.js. The next subsection shows it in action. setTimeout() lets tasks take breaks Another way of looking at setTimeout() is that the current task takes a break and continues later via the callback. 39.4.4 Run-to-completion semantics JavaScript makes a guarantee for tasks: Each task is always finished (“run to completion”) before the next task is executed. As a consequence, tasks don’t have to worry about their data being changed while they are working on it (concurrent modification). That simplifies programming in JavaScript. The following example demonstrates this guarantee: function clearTimeout(handle?: any): void console.log('start'); setTimeout(() => { console.log('callback'); }, 0); console.log('end');
- 694. setTimeout() puts its parameter into the task queue. The parameter is therefore executed sometime after the current piece of code (task) is completely finished. The parameter ms only specifies when the task is put into the queue, not when exactly it runs. It may even never run – for example, if there is a task before it in the queue that never terminates. That explains why the previous code logs 'end' before 'callback', even though the parameter ms is 0. // Output: // 'start' // 'end' // 'callback'
- 695. 39.5 Patterns for delivering asynchronous results In order to avoid blocking the main process while waiting for a long- running operation to finish, results are often delivered asynchronously in JavaScript. These are three popular patterns for doing so: Events Callbacks Promises The first two patterns are explained in the next two subsections. Promises are explained in the next chapter. 39.5.1 Delivering asynchronous results via events Events as a pattern work as follows: They are used to deliver values asynchronously. They do so zero or more times. There are three roles in this pattern: The event (an object) carries the data to be delivered. The event listener is a function that receives events via a parameter. The event source sends events and lets you register event listeners.
- 696. Multiple variations of this pattern exist in the world of JavaScript. We’ll look at three examples next. 39.5.1.1 Events: IndexedDB IndexedDB is a database that is built into web browsers. This is an example of using it: indexedDB has an unusual way of invoking operations: Each operation has an associated method for creating request objects. For example, in line A, the operation is “open”, the method is .open(), and the request object is openRequest. The parameters for the operation are provided via the request object, not via parameters of the method. For example, the event listeners (functions) are stored in the properties .onsuccess and .onerror. The invocation of the operation is added to the task queue via the method (in line A). That is, we configure the operation after its invocation has already been added to the queue. Only run-to- const openRequest = indexedDB.open('MyDatabase', 1); // (A) openRequest.onsuccess = (event) => { const db = event.target.result; // ··· }; openRequest.onerror = (error) => { console.error(error); };
- 697. completion semantics saves us from race conditions here and ensures that the operation runs after the current code fragment is finished. 39.5.1.2 Events: XMLHttpRequest The XMLHttpRequest API lets us make downloads from within a web browser. This is how we download the file http://example.com/textfile.txt: With this API, we first create a request object (line A), then configure it, then activate it (line E). The configuration consists of: Specifying which HTTP request method to use (line B): GET, POST, PUT, etc. Registering a listener (line C) that is notified if something could be downloaded. Inside the listener, we still need to determine if const xhr = new XMLHttpRequest(); // (A) xhr.open('GET', 'http://example.com/textfile.txt'); // (B) xhr.onload = () => { // (C) if (xhr.status == 200) { processData(xhr.responseText); } else { assert.fail(new Error(xhr.statusText)); } }; xhr.onerror = () => { // (D) assert.fail(new Error('Network error')); }; xhr.send(); // (E) function processData(str) { assert.equal(str, 'Content of textfile.txtn'); }
- 698. the download contains what we requested or informs us of an error. Note that some of the result data is delivered via the request object xhr. (I’m not a fan of this kind of mixing of input and output data.) Registering a listener (line D) that is notified if there was a network error. 39.5.1.3 Events: DOM We have already seen DOM events in action in §39.4.1 “The user interface of the browser can be blocked”. The following code also handles click events: We first ask the browser to retrieve the HTML element whose ID is 'my-link' (line A). Then we add a listener for all click events (line B). In the listener, we first tell the browser not to perform its default action (line C) – going to the target of the link. Then we log to the console if the shift key is currently pressed (line D). 39.5.2 Delivering asynchronous results via callbacks const element = document.getElementById('my-link'); // (A) element.addEventListener('click', clickListener); // (B) function clickListener(event) { event.preventDefault(); // (C) console.log(event.shiftKey); // (D) }
- 699. Callbacks are another pattern for handling asynchronous results. They are only used for one-off results and have the advantage of being less verbose than events. As an example, consider a function readFile() that reads a text file and returns its contents asynchronously. This is how you call readFile() if it uses Node.js-style callbacks: There is a single callback that handles both success and failure. If the first parameter is not null then an error happened. Otherwise, the result can be found in the second parameter. Exercises: Callback-based code The following exercises use tests for asynchronous code, which are different from tests for synchronous code. Consult §11.3.2 “Asynchronous tests in AVA” for more information. From synchronous to callback-based code: exercises/async- js/read_file_cb_exrc.mjs Implementing a callback-based version of .map(): exercises/async-js/map_cb_test.mjs readFile('some-file.txt', {encoding: 'utf8'}, (error, data) => { if (error) { assert.fail(error); return; } assert.equal(data, 'The content of some-file.txtn'); });
- 700. 39.6 Asynchronous code: the downsides In many situations, on either browsers or Node.js, you have no choice, you must use asynchronous code. In this chapter, we have seen several patterns that such code can use. All of them have two disadvantages: Asynchronous code is more verbose than synchronous code. If you call asynchronous code, your code must become asynchronous too. That’s because you can’t wait synchronously for an asynchronous result. Asynchronous code has an infectious quality. The first disadvantage becomes less severe with Promises (covered in the next chapter) and mostly disappears with async functions (covered in the chapter after next). Alas, the infectiousness of async code does not go away. But it is mitigated by the fact that switching between sync and async is easy with async functions.
- 701. 39.7 Resources “Help, I’m stuck in an event-loop” by Philip Roberts (video). “Event loops”, section in HTML5 spec.
- 702. 40 Promises for asynchronous programming 40.1 The basics of using Promises 40.1.1 Using a Promise-based function 40.1.2 What is a Promise? 40.1.3 Implementing a Promise-based function 40.1.4 States of Promises 40.1.5 Promise.resolve(): create a Promise fulfilled with a given value 40.1.6 Promise.reject(): create a Promise rejected with a given value 40.1.7 Returning and throwing in .then() callbacks 40.1.8 .catch() and its callback 40.1.9 Chaining method calls 40.1.10 Advantages of promises 40.2 Examples 40.2.1 Node.js: Reading a file asynchronously 40.2.2 Browsers: Promisifying XMLHttpRequest 40.2.3 Node.js: util.promisify() 40.2.4 Browsers: Fetch API 40.3 Error handling: don’t mix rejections and exceptions 40.4 Promise-based functions start synchronously, settle asynchronously 40.5 Promise.all(): concurrency and Arrays of Promises 40.5.1 Sequential execution vs. concurrent execution
- 703. 40.5.2 Concurrency tip: focus on when operations start 40.5.3 Promise.all() is fork-join 40.5.4 Asynchronous .map() via Promise.all() 40.6 Tips for chaining Promises 40.6.1 Chaining mistake: losing the tail 40.6.2 Chaining mistake: nesting 40.6.3 Chaining mistake: more nesting than necessary 40.6.4 Not all nesting is bad 40.6.5 Chaining mistake: creating Promises instead of chaining 40.7 Advanced topics In this chapter, we explore Promises, yet another pattern for delivering asynchronous results. Recommended reading This chapter builds on the previous chapter with background on asynchronous programming in JavaScript.
- 704. 40.1 The basics of using Promises Promises are a pattern for delivering results asynchronously. 40.1.1 Using a Promise-based function The following code is an example of using the Promise-based function addAsync() (whose implementation is shown soon): Promises are similar to the event pattern: There is an object (a Promise), where we register callbacks: Method .then() registers callbacks that handle results. Method .catch() registers callbacks that handle errors. A Promise-based function returns a Promise and sends it a result or an error (if and when it is done). The Promise passes it on to the relevant callbacks. In contrast to the event pattern, Promises are optimized for one-off results: A result (or an error) is cached so that it doesn’t matter if we register a callback before or after the result (or error) was sent. addAsync(3, 4) .then(result => { // success assert.equal(result, 7); }) .catch(error => { // failure assert.fail(error); });
- 705. We can chain the Promise methods .then() and .catch() because they both return Promises. That helps with sequentially invoking multiple asynchronous functions. More on that later. 40.1.2 What is a Promise? What is a Promise? There are two ways of looking at it: On one hand, it is a placeholder or container for the final result that will eventually be delivered. On the other hand, it is an object with which we can register listeners. 40.1.3 Implementing a Promise-based function This is an implementation of a Promise-based function that adds two numbers x and y: addAsync() immediately invokes the Promise constructor. The actual implementation of that function resides in the callback that is passed function addAsync(x, y) { return new Promise( (resolve, reject) => { // (A) if (x === undefined || y === undefined) { reject(new Error('Must provide two parameters')); } else { resolve(x + y); } }); }
- 706. to that constructor (line A). That callback is provided with two functions: resolve is used for delivering a result (in case of success). reject is used for delivering an error (in case of failure). 40.1.4 States of Promises Pending Fulfilled Rejected Settled Figure 31: A Promise can be in either one of three states: pending, fulfilled, or rejected. If a Promise is in a final (non-pending) state, it is called settled. Fig. 31 depicts the three states a Promise can be in. Promises specialize in one-off results and protect us against race conditions (registering too early or too late): If we register a .then() callback or a .catch() callback too early, it is notified once a Promise is settled. Once a Promise is settled, the settlement value (result or error) is cached. Thus, if .then() or .catch() are called after the settlement, they receive the cached value. Additionally, once a Promise is settled, its state and settlement value can’t change anymore. That helps make code predictable and enforces the one-off nature of Promises.
- 707. Some Promises are never settled It is possible that a Promise is never settled. For example: 40.1.5 Promise.resolve(): create a Promise fulfilled with a given value Promise.resolve(x) creates a Promise that is fulfilled with the value x: If the parameter is already a Promise, it is returned unchanged: Therefore, given an arbitrary value x, we can use Promise.resolve(x) to ensure we have a Promise. Note that the name is resolve, not fulfill, because .resolve() returns a rejected Promise if its Parameter is a rejected Promise. 40.1.6 Promise.reject(): create a Promise rejected with a given value new Promise(() => {}) Promise.resolve(123) .then(x => { assert.equal(x, 123); }); const abcPromise = Promise.resolve('abc'); assert.equal( Promise.resolve(abcPromise), abcPromise);
- 708. Promise.reject(err) creates a Promise that is rejected with the value err: 40.1.7 Returning and throwing in .then() callbacks .then() handles Promise fulfillments. It also returns a fresh Promise. How that Promise is settled depends on what happens inside the callback. Let’s look at three common cases. 40.1.7.1 Returning a non-Promise value First, the callback can return a non-Promise value (line A). Consequently, the Promise returned by .then() is fulfilled with that value (as checked in line B): 40.1.7.2 Returning a Promise const myError = new Error('My error!'); Promise.reject(myError) .catch(err => { assert.equal(err, myError); }); Promise.resolve('abc') .then(str => { return str + str; // (A) }) .then(str2 => { assert.equal(str2, 'abcabc'); // (B) });
- 709. Second, the callback can return a Promise p (line A). Consequently, p “becomes” what .then() returns. In other words: the Promise that .then() has already returned is effectively replaced by p. Why is that useful? We can return the result of a Promise-based operation and process its fulfillment value via a “flat” (non-nested) .then(). Compare: 40.1.7.3 Throwing an exception Promise.resolve('abc') .then(str => { return Promise.resolve(123); // (A) }) .then(num => { assert.equal(num, 123); }); // Flat asyncFunc1() .then(result1 => { /*···*/ return asyncFunc2(); }) .then(result2 => { /*···*/ }); // Nested asyncFunc1() .then(result1 => { /*···*/ asyncFunc2() .then(result2 => { /*···*/ }); });
- 710. Third, the callback can throw an exception. Consequently, the Promise returned by .then() is rejected with that exception. That is, a synchronous error is converted into an asynchronous error. 40.1.8 .catch() and its callback The only difference between .then() and .catch() is that the latter is triggered by rejections, not fulfillments. However, both methods turn the actions of their callbacks into Promises in the same manner. For example, in the following code, the value returned by the .catch() callback in line A becomes a fulfillment value: 40.1.9 Chaining method calls const myError = new Error('My error!'); Promise.resolve('abc') .then(str => { throw myError; }) .catch(err => { assert.equal(err, myError); }); const err = new Error(); Promise.reject(err) .catch(e => { assert.equal(e, err); // Something went wrong, use a default value return 'default value'; // (A) }) .then(str => { assert.equal(str, 'default value'); });
- 711. .then() and .catch() always return Promises. That enables us to create arbitrary long chains of method calls: Due to chaining, the return in line A returns the result of the last .then(). In a way, .then() is the asynchronous version of the synchronous semicolon: .then() executes two asynchronous operations sequentially. The semicolon executes two synchronous operations sequentially. We can also add .catch() into the mix and let it handle multiple error sources at the same time: function myAsyncFunc() { return asyncFunc1() // (A) .then(result1 => { // ··· return asyncFunc2(); // a Promise }) .then(result2 => { // ··· return result2 || '(Empty)'; // not a Promise }) .then(result3 => { // ··· return asyncFunc4(); // a Promise }); } asyncFunc1() .then(result1 => { // ··· return asyncFunction2(); })
- 712. 40.1.10 Advantages of promises These are some of the advantages of Promises over plain callbacks when it comes to handling one-off results: The type signatures of Promise-based functions and methods are cleaner: if a function is callback-based, some parameters are about input, while the one or two callbacks at the end are about output. With Promises, everything output-related is handled via the returned value. Chaining asynchronous processing steps is more convenient. Promises handle both asynchronous errors (via rejections) and synchronous errors: Inside the callbacks for new Promise(), .then(), and .catch(), exceptions are converted to rejections. In contrast, if we use callbacks for asynchronicity, exceptions are normally not handled for us; we have to do it ourselves. Promises are a single standard that is slowly replacing several, mutually incompatible alternatives. For example, in Node.js, many functions are now available in Promise-based versions. And new asynchronous browser APIs are usually Promise- based. .then(result2 => { // ··· }) .catch(error => { // Failure: handle errors of asyncFunc1(), asyncFunc2() // and any (sync) exceptions thrown in previous callbacks });
- 713. One of the biggest advantages of Promises involves not working with them directly: they are the foundation of async functions, a synchronous-looking syntax for performing asynchronous computations. Asynchronous functions are covered in the next chapter.
- 714. 40.2 Examples Seeing Promises in action helps with understanding them. Let’s look at examples. 40.2.1 Node.js: Reading a file asynchronously Consider the following text file person.json with JSON data in it: Let’s look at two versions of code that reads this file and parses it into an object. First, a callback-based version. Second, a Promise- based version. 40.2.1.1 The callback-based version The following code reads the contents of this file and converts it to a JavaScript object. It is based on Node.js-style callbacks: { "first": "Jane", "last": "Doe" } import * as fs from 'fs'; fs.readFile('person.json', (error, text) => { if (error) { // (A) // Failure assert.fail(error); } else { // Success try { // (B)
- 715. fs is a built-in Node.js module for file system operations. We use the callback-based function fs.readFile() to read a file whose name is person.json. If we succeed, the content is delivered via the parameter text as a string. In line C, we convert that string from the text-based data format JSON into a JavaScript object. JSON is an object with methods for consuming and producing JSON. It is part of JavaScript’s standard library and documented later in this book. Note that there are two error-handling mechanisms: the if in line A takes care of asynchronous errors reported by fs.readFile(), while the try in line B takes care of synchronous errors reported by JSON.parse(). 40.2.1.2 The Promise-based version The following code uses readFileAsync(), a Promise-based version of fs.readFile() (created via util.promisify(), which is explained later): const obj = JSON.parse(text); // (C) assert.deepEqual(obj, { first: 'Jane', last: 'Doe', }); } catch (e) { // Invalid JSON assert.fail(e); } } }); readFileAsync('person.json') .then(text => { // (A) // Success const obj = JSON.parse(text);
- 716. Function readFileAsync() returns a Promise. In line A, we specify a success callback via method .then() of that Promise. The remaining code in then’s callback is synchronous. .then() returns a Promise, which enables the invocation of the Promise method .catch() in line B. We use it to specify a failure callback. Note that .catch() lets us handle both the asynchronous errors of readFileAsync() and the synchronous errors of JSON.parse() because exceptions inside a .then() callback become rejections. 40.2.2 Browsers: Promisifying XMLHttpRequest We have previously seen the event-based XMLHttpRequest API for downloading data in web browsers. The following function promisifies that API: assert.deepEqual(obj, { first: 'Jane', last: 'Doe', }); }) .catch(err => { // (B) // Failure: file I/O error or JSON syntax error assert.fail(err); }); function httpGet(url) { return new Promise( (resolve, reject) => { const xhr = new XMLHttpRequest(); xhr.onload = () => { if (xhr.status === 200) {
- 717. Note how the results and errors of XMLHttpRequest are handled via resolve() and reject(): A successful outcome leads to the returned Promise being fullfilled with it (line A). An error leads to the Promise being rejected (lines B and C). This is how to use httpGet(): Exercise: Timing out a Promise exercises/promises/promise_timeout_test.mjs 40.2.3 Node.js: util.promisify() resolve(xhr.responseText); // (A) } else { // Something went wrong (404, etc.) reject(new Error(xhr.statusText)); // (B) } } xhr.onerror = () => { reject(new Error('Network error')); // (C) }; xhr.open('GET', url); xhr.send(); }); } httpGet('http://example.com/textfile.txt') .then(content => { assert.equal(content, 'Content of textfile.txtn'); }) .catch(error => { assert.fail(error); });
- 718. util.promisify() is a utility function that converts a callback-based function f into a Promise-based one. That is, we are going from this type signature: f(arg_1, ···, arg_n, (err: Error, result: T) => void) : void To this type signature: f(arg_1, ···, arg_n) : Promise<T> The following code promisifies the callback-based fs.readFile() (line A) and uses it: Exercises: util.promisify() Using util.promisify(): exercises/promises/read_file_async_exrc.mjs Implementing util.promisify() yourself: exercises/promises/my_promisify_test.mjs 40.2.4 Browsers: Fetch API import * as fs from 'fs'; import {promisify} from 'util'; const readFileAsync = promisify(fs.readFile); // (A) readFileAsync('some-file.txt', {encoding: 'utf8'}) .then(text => { assert.equal(text, 'The content of some-file.txtn'); }) .catch(err => { assert.fail(err); });
- 719. All modern browsers support Fetch, a new Promise-based API for downloading data. Think of it as a Promise-based version of XMLHttpRequest. The following is an excerpt of the API: That means we can use fetch() as follows: Exercise: Using the fetch API exercises/promises/fetch_json_test.mjs interface Body { text() : Promise<string>; ··· } interface Response extends Body { ··· } declare function fetch(str) : Promise<Response>; fetch('http://example.com/textfile.txt') .then(response => response.text()) .then(text => { assert.equal(text, 'Content of textfile.txtn'); });
- 720. 40.3 Error handling: don’t mix rejections and exceptions Rule for implementing functions and methods: Don’t mix (asynchronous) rejections and (synchronous) exceptions. This makes our synchronous and asynchronous code more predictable and simpler because we can always focus on a single error-handling mechanism. For Promise-based functions and methods, the rule means that they should never throw exceptions. Alas, it is easy to accidentally get this wrong – for example: The problem is that if an exception is thrown in line A, then asyncFunc() will throw an exception. Callers of that function only expect rejections and are not prepared for an exception. There are three ways in which we can fix this issue. We can wrap the whole body of the function in a try-catch statement and return a rejected Promise if an exception is thrown: // Don’t do this function asyncFunc() { doSomethingSync(); // (A) return doSomethingAsync() .then(result => { // ··· }); }
- 721. Given that .then() converts exceptions to rejections, we can execute doSomethingSync() inside a .then() callback. To do so, we start a Promise chain via Promise.resolve(). We ignore the fulfillment value undefined of that initial Promise. Lastly, new Promise() also converts exceptions to rejections. Using this constructor is therefore similar to the previous solution: // Solution 1 function asyncFunc() { try { doSomethingSync(); return doSomethingAsync() .then(result => { // ··· }); } catch (err) { return Promise.reject(err); } } // Solution 2 function asyncFunc() { return Promise.resolve() .then(() => { doSomethingSync(); return doSomethingAsync(); }) .then(result => { // ··· }); } // Solution 3 function asyncFunc() { return new Promise((resolve, reject) => { doSomethingSync(); resolve(doSomethingAsync()); }) .then(result => {
- 722. // ··· }); }
- 723. 40.4 Promise-based functions start synchronously, settle asynchronously Most Promise-based functions are executed as follows: Their execution starts right away, synchronously (in the current task). But the Promise they return is guaranteed to be settled asynchronously (in a later task) – if ever. The following code demonstrates that: function asyncFunc() { console.log('asyncFunc'); return new Promise( (resolve, _reject) => { console.log('new Promise()'); resolve(); }); } console.log('START'); asyncFunc() .then(() => { console.log('.then()'); // (A) }); console.log('END'); // Output: // 'START' // 'asyncFunc' // 'new Promise()' // 'END' // '.then()'
- 724. We can see that the callback of new Promise() is executed before the end of the code, while the result is delivered later (line A). Benefits of this approach: Starting synchronously helps avoid race conditions because we can rely on the order in which Promise-based functions begin. There is an example in the next chapter, where text is written to a file and race conditions are avoided. Chaining Promises won’t starve other tasks of processing time because before a Promise is settled, there will always be a break, during which the event loop can run. Promise-based functions always return results asynchronously; we can be sure that there is never a synchronous return. This kind of predictability makes code easier to work with. More information on this approach “Designing APIs for Asynchrony” by Isaac Z. Schlueter
- 725. 40.5 Promise.all(): concurrency and Arrays of Promises 40.5.1 Sequential execution vs. concurrent execution Consider the following code: Using .then() in this manner executes Promise-based functions sequentially: only after the result of asyncFunc1() is settled will asyncFunc2() be executed. The static method Promise.all() helps execute Promise-based functions more concurrently: Its type signature is: const asyncFunc1 = () => Promise.resolve('one'); const asyncFunc2 = () => Promise.resolve('two'); asyncFunc1() .then(result1 => { assert.equal(result1, 'one'); return asyncFunc2(); }) .then(result2 => { assert.equal(result2, 'two'); }); Promise.all([asyncFunc1(), asyncFunc2()]) .then(arr => { assert.deepEqual(arr, ['one', 'two']); });
- 726. The parameter promises is an iterable of Promises. The result is a single Promise that is settled as follows: If and when all input Promises are fulfilled, the output Promise is fulfilled with an Array of the fulfillment values. As soon as at least one input Promise is rejected, the output Promise is rejected with the rejection value of that input Promise. In other words: We go from an iterable of Promises to a Promise for an Array. 40.5.2 Concurrency tip: focus on when operations start Tip for determining how “concurrent” asynchronous code is: Focus on when asynchronous operations start, not on how their Promises are handled. For example, each of the following functions executes asyncFunc1() and asyncFunc2() concurrently because they are started at nearly the same time. Promise.all<T>(promises: Iterable<Promise<T>>): Promise<T[]> function concurrentAll() { return Promise.all([asyncFunc1(), asyncFunc2()]); } function concurrentThen() { const p1 = asyncFunc1(); const p2 = asyncFunc2();
- 727. On the other hand, both of the following functions execute asyncFunc1() and asyncFunc2() sequentially: asyncFunc2() is only invoked after the Promise of asyncFunc1() is fulfilled. 40.5.3 Promise.all() is fork-join Promise.all() is loosely related to the concurrency pattern “fork join” – for example: httpGet() is the promisified version of XMLHttpRequest that we implemented earlier. return p1.then(r1 => p2.then(r2 => [r1, r2])); } function sequentialThen() { return asyncFunc1() .then(r1 => asyncFunc2() .then(r2 => [r1, r2])); } function sequentialAll() { const p1 = asyncFunc1(); const p2 = p1.then(() => asyncFunc2()); return Promise.all([p1, p2]); } Promise.all([ // Fork async computations httpGet('http://example.com/file1.txt'), httpGet('http://example.com/file2.txt'), ]) // Join async computations .then(([text1, text2]) => { assert.equal(text1, 'Content of file1.txtn'); assert.equal(text2, 'Content of file2.txtn'); });
- 728. 40.5.4 Asynchronous .map() via Promise.all() Array transformation methods such as .map(), .filter(), etc., are made for synchronous computations – for example: What happens if the callback of .map() is a Promise-based function (a function that maps normal values to Promises)? Then the result of .map() is an Array of Promises. Alas, that is not data that normal code can work with. Thankfully, we can fix that via Promise.all(): It converts an Array of Promises into a Promise that is fulfilled with an Array of normal values. 40.5.4.1 A more realistic example The following code is a more realistic example: in the section on fork-join, there was an example where we downloaded two resources identified by two fixed URLs. Let’s turn that code fragment into a function timesTwoSync(x) { return 2 * x; } const arr = [1, 2, 3]; const result = arr.map(timesTwoSync); assert.deepEqual(result, [2, 4, 6]); function timesTwoAsync(x) { return new Promise(resolve => resolve(x * 2)); } const arr = [1, 2, 3]; const promiseArr = arr.map(timesTwoAsync); Promise.all(promiseArr) .then(result => { assert.deepEqual(result, [2, 4, 6]); });
- 729. function that accepts an Array of URLs and downloads the corresponding resources: Exercise: Promise.all() and listing files exercises/promises/list_files_async_test.mjs function downloadTexts(urls) { const promisedTexts = urls.map(httpGet); return Promise.all(promisedTexts); } downloadTexts([ 'http://example.com/file1.txt', 'http://example.com/file2.txt', ]) .then(texts => { assert.deepEqual( texts, [ 'Content of file1.txtn', 'Content of file2.txtn', ]); });
- 730. 40.6 Tips for chaining Promises This section gives tips for chaining Promises. 40.6.1 Chaining mistake: losing the tail Problem: Computation starts with the Promise returned by asyncFunc(). But afterward, computation continues and another Promise is created via .then(). foo() returns the former Promise, but should return the latter. This is how to fix it: 40.6.2 Chaining mistake: nesting Problem: // Don’t do this function foo() { const promise = asyncFunc(); promise.then(result => { // ··· }); return promise; } function foo() { const promise = asyncFunc(); return promise.then(result => { // ··· }); }
- 731. The .then() in line A is nested. A flat structure would be better: 40.6.3 Chaining mistake: more nesting than necessary This is another example of avoidable nesting: We can once again get a flat structure: // Don’t do this asyncFunc1() .then(result1 => { return asyncFunc2() .then(result2 => { // (A) // ··· }); }); asyncFunc1() .then(result1 => { return asyncFunc2(); }) .then(result2 => { // ··· }); // Don’t do this asyncFunc1() .then(result1 => { if (result1 < 0) { return asyncFuncA() .then(resultA => 'Result: ' + resultA); } else { return asyncFuncB() .then(resultB => 'Result: ' + resultB); } });
- 732. 40.6.4 Not all nesting is bad In the following code, we actually benefit from nesting: We are receiving an asynchronous result in line A. In line B, we are nesting so that we have access to variable connection inside the callback and in line C. 40.6.5 Chaining mistake: creating Promises instead of chaining Problem: asyncFunc1() .then(result1 => { return result1 < 0 ? asyncFuncA() : asyncFuncB(); }) .then(resultAB => { return 'Result: ' + resultAB; }); db.open() .then(connection => { // (A) return connection.select({ name: 'Jane' }) .then(result => { // (B) // Process result // Use `connection` to make more queries }) // ··· .finally(() => { connection.close(); // (C) }); }) // Don’t do this class Model { insertInto(db) {
- 733. In line A, we are creating a Promise to deliver the result of db.insert(). That is unnecessarily verbose and can be simplified: The key idea is that we don’t need to create a Promise; we can return the result of the .then() call. An additional benefit is that we don’t need to catch and re-reject the failure of db.insert(). We simply pass its rejection on to the caller of .insertInto(). return new Promise((resolve, reject) => { // (A) db.insert(this.fields) .then(resultCode => { this.notifyObservers({event: 'created', model: this}); resolve(resultCode); }).catch(err => { reject(err); }) }); } // ··· } class Model { insertInto(db) { return db.insert(this.fields) .then(resultCode => { this.notifyObservers({event: 'created', model: this}); return resultCode; }); } // ··· }
- 734. 40.7 Advanced topics In addition to Promise.all(), there is also Promise.race(), which is not used often and described in Exploring ES6. Exploring ES6 has a section that shows a very simple implementation of Promises. That may be helpful if you want a deeper understanding of how Promises work.
- 735. 41 Async functions 41.1 Async functions: the basics 41.1.1 Async constructs 41.2 Returning from async functions 41.2.1 Async functions always return Promises 41.2.2 Returned Promises are not wrapped 41.2.3 Executing async functions: synchronous start, asynchronous settlement (advanced) 41.3 await: working with Promises 41.3.1 await and fulfilled Promises 41.3.2 await and rejected Promises 41.3.3 await is shallow (we can’t use it in callbacks) 41.4 (Advanced) 41.5 Immediately invoked async arrow functions 41.6 Concurrency and await 41.6.1 await: running asynchronous functions sequentially 41.6.2 await: running asynchronous functions concurrently 41.7 Tips for using async functions 41.7.1 We don’t need await if we “fire and forget” 41.7.2 It can make sense to await and ignore the result Roughly, async functions provide better syntax for code that uses Promises. In order to use async functions, we should therefore understand Promises. They are explained in the previous chapter.
- 736. 41.1 Async functions: the basics Consider the following async function: The previous, rather synchronous-looking code is equivalent to the following code that uses Promises directly: A few observations about the async function fetchJsonAsync(): Async functions are marked with the keyword async. Inside the body of an async function, we write Promise-based code as if it were synchronous. We only need to apply the await operator whenever a value is a Promise. That operator pauses the async function and resumes it once the Promise is settled: async function fetchJsonAsync(url) { try { const request = await fetch(url); // async const text = await request.text(); // async return JSON.parse(text); // sync } catch (error) { assert.fail(error); } } function fetchJsonViaPromises(url) { return fetch(url) // async .then(request => request.text()) // async .then(text => JSON.parse(text)) // sync .catch(error => { assert.fail(error); }); }
- 737. If the Promise is fulfilled, await returns the fulfillment value. If the Promise is rejected, await throws the rejection value. The result of an async function is always a Promise: Any value that is returned (explicitly or implicitly) is used to fulfill the Promise. Any exception that is thrown is used to reject the Promise. Both fetchJsonAsync() and fetchJsonViaPromises() are called in exactly the same way, like this: Async functions are as Promise-based as functions that use Promises directly From the outside, it is virtually impossible to tell the difference between an async function and a function that returns a Promise. 41.1.1 Async constructs JavaScript has the following async versions of synchronous callable entities. Their roles are always either real function or method. fetchJsonAsync('http://example.com/person.json') .then(obj => { assert.deepEqual(obj, { first: 'Jane', last: 'Doe', }); }); // Async function declaration async function func1() {}
- 738. Asynchronous functions vs. async functions The difference between the terms asynchronous function and async function is subtle, but important: An asynchronous function is any function that delivers its result asynchronously – for example, a callback-based function or a Promise-based function. An async function is defined via special syntax, involving the keywords async and await. It is also called async/await due to these two keywords. Async functions are based on Promises and therefore also asynchronous functions (which is somewhat confusing). // Async function expression const func2 = async function () {}; // Async arrow function const func3 = async () => {}; // Async method definition in an object literal const obj = { async m() {} }; // Async method definition in a class definition class MyClass { async m() {} }
- 739. 41.2 Returning from async functions 41.2.1 Async functions always return Promises Each async function always returns a Promise. Inside the async function, we fulfill the result Promise via return (line A): As usual, if we don’t explicitly return anything, undefined is returned for us: We reject the result Promise via throw (line A): async function asyncFunc() { return 123; // (A) } asyncFunc() .then(result => { assert.equal(result, 123); }); async function asyncFunc() { } asyncFunc() .then(result => { assert.equal(result, undefined); });
- 740. 41.2.2 Returned Promises are not wrapped If we return a Promise p from an async function, then p becomes the result of the function (or rather, the result “locks in” on p and behaves exactly like it). That is, the Promise is not wrapped in yet another Promise. Recall that any Promise q is treated similarly in the following situations: resolve(q) inside new Promise((resolve, reject) => { ··· }) return q inside .then(result => { ··· }) return q inside .catch(err => { ··· }) 41.2.3 Executing async functions: synchronous start, asynchronous async function asyncFunc() { throw new Error('Problem!'); // (A) } asyncFunc() .catch(err => { assert.deepEqual(err, new Error('Problem!')); }); async function asyncFunc() { return Promise.resolve('abc'); } asyncFunc() .then(result => assert.equal(result, 'abc'));
- 741. settlement (advanced) Async functions are executed as follows: The Promise p for the result is created when the async function is started. Then the body is executed. There are two ways in which execution can leave the body: Execution can leave permanently while settling p: A return fulfills p. A throw rejects p. Execution can also leave temporarily when awaiting the settlement of another Promise q via await. The async function is paused and execution leaves it. It is resumed once q is settled. Promise p is returned after execution has left the body for the first time (permanently or temporarily). Note that the notification of the settlement of the result p happens asynchronously, as is always the case with Promises. The following code demonstrates that an async function is started synchronously (line A), then the current task finishes (line C), then the result Promise is settled – asynchronously (line B). async function asyncFunc() { console.log('asyncFunc() starts'); // (A) return 'abc'; } asyncFunc(). then(x => { // (B) console.log(`Resolved: ${x}`);
- 742. }); console.log('Task ends'); // (C) // Output: // 'asyncFunc() starts' // 'Task ends' // 'Resolved: abc'
- 743. 41.3 await: working with Promises The await operator can only be used inside async functions and async generators (which are explained in §42.2 “Asynchronous generators”). Its operand is usually a Promise and leads to the following steps being performed: The current async function is paused and returned from. This step is similar to how yield works in sync generators. Eventually, the current task is finished and processing of the task queue continues. When and if the Promise is settled, the async function is resumed in a new task: If the Promise is fulfilled, await returns the fulfillment value. If the Promise is rejected, await throws the rejection value. Read on to find out more about how await handles Promises in various states. 41.3.1 await and fulfilled Promises If its operand ends up being a fulfilled Promise, await returns its fulfillment value: Non-Promise values are allowed, too, and simply passed on (synchronously, without pausing the async function): assert.equal(await Promise.resolve('yes!'), 'yes!');
- 744. 41.3.2 await and rejected Promises If its operand is a rejected Promise, then await throws the rejection value: Instances of Error (including instances of its subclasses) are treated specially and also thrown: Exercise: Fetch API via async functions exercises/async-functions/fetch_json2_test.mjs 41.3.3 await is shallow (we can’t use it in callbacks) If we are inside an async function and want to pause it via await, we must do so directly within that function; we can’t use it inside a nested function, such as a callback. That is, pausing is shallow. assert.equal(await 'yes!', 'yes!'); try { await Promise.reject(new Error()); assert.fail(); // we never get here } catch (e) { assert.equal(e instanceof Error, true); } try { await new Error(); assert.fail(); // we never get here } catch (e) { assert.equal(e instanceof Error, true); }
- 745. For example, the following code can’t be executed: The reason is that normal arrow functions don’t allow await inside their bodies. OK, let’s try an async arrow function then: Alas, this doesn’t work either: Now .map() (and therefore downloadContent()) returns an Array with Promises, not an Array with (unwrapped) values. One possible solution is to use Promise.all() to unwrap all Promises: Can this code be improved? Yes it can: in line A, we are unwrapping a Promise via await, only to re-wrap it immediately via return. If we omit await, we don’t even need an async arrow function: async function downloadContent(urls) { return urls.map((url) => { return await httpGet(url); // SyntaxError! }); } async function downloadContent(urls) { return urls.map(async (url) => { return await httpGet(url); }); } async function downloadContent(urls) { const promiseArray = urls.map(async (url) => { return await httpGet(url); // (A) }); return await Promise.all(promiseArray); }
- 746. For the same reason, we can also omit await in line B. Exercise: Mapping and filtering asynchronously exercises/async-functions/map_async_test.mjs async function downloadContent(urls) { const promiseArray = urls.map( url => httpGet(url)); return await Promise.all(promiseArray); // (B) }
- 747. 41.4 (Advanced) All remaining sections are advanced.
- 748. 41.5 Immediately invoked async arrow functions If we need an await outside an async function (e.g., at the top level of a module), then we can immediately invoke an async arrow function: The result of an immediately invoked async arrow function is a Promise: (async () => { // start const promise = Promise.resolve('abc'); const value = await promise; assert.equal(value, 'abc'); })(); // end const promise = (async () => 123)(); promise.then(x => assert.equal(x, 123));
- 749. 41.6 Concurrency and await In the next two subsections, we’ll use the helper function paused(): 41.6.1 await: running asynchronous functions sequentially If we prefix the invocations of multiple asynchronous functions with await, then those functions are executed sequentially: /** * Resolves after `ms` milliseconds */ function delay(ms) { return new Promise((resolve, _reject) => { setTimeout(resolve, ms); }); } async function paused(id) { console.log('START ' + id); await delay(10); // pause console.log('END ' + id); return id; } async function sequentialAwait() { const result1 = await paused('first'); assert.equal(result1, 'first'); const result2 = await paused('second'); assert.equal(result2, 'second'); } // Output: // 'START first' // 'END first'
- 750. That is, paused('second') is only started after paused('first') is completely finished. 41.6.2 await: running asynchronous functions concurrently If we want to run multiple functions concurrently, we can use the tool method Promise.all(): Here, both asynchronous functions are started at the same time. Once both are settled, await gives us either an Array of fulfillment values or – if at least one Promise is rejected – an exception. Recall from §40.5.2 “Concurrency tip: focus on when operations start” that what counts is when we start a Promise-based computation; not how we process its result. Therefore, the following code is as “concurrent” as the previous one: // 'START second' // 'END second' async function concurrentPromiseAll() { const result = await Promise.all([ paused('first'), paused('second') ]); assert.deepEqual(result, ['first', 'second']); } // Output: // 'START first' // 'START second' // 'END first' // 'END second'
- 751. async function concurrentAwait() { const resultPromise1 = paused('first'); const resultPromise2 = paused('second'); assert.equal(await resultPromise1, 'first'); assert.equal(await resultPromise2, 'second'); } // Output: // 'START first' // 'START second' // 'END first' // 'END second'
- 752. 41.7 Tips for using async functions 41.7.1 We don’t need await if we “fire and forget” await is not required when working with a Promise-based function; we only need it if we want to pause and wait until the returned Promise is settled. If we only want to start an asynchronous operation, then we don’t need it: In this code, we don’t await .write() because we don’t care when it is finished. We do, however, want to wait until .close() is done. Note: Each invocation of .write() starts synchronously. That prevents race conditions. 41.7.2 It can make sense to await and ignore the result It can occasionally make sense to use await, even if we ignore its result – for example: async function asyncFunc() { const writer = openFile('someFile.txt'); writer.write('hello'); // don’t wait writer.write('world'); // don’t wait await writer.close(); // wait for file to close } await longRunningAsyncOperation(); console.log('Done!');
- 753. Here, we are using await to join a long-running asynchronous operation. That ensures that the logging really happens after that operation is done.
- 754. 42 Asynchronous iteration 42.1 Basic asynchronous iteration 42.1.1 Protocol: async iteration 42.1.2 Using async iteration directly 42.1.3 Using async iteration via for-await-of 42.2 Asynchronous generators 42.2.1 Example: creating an async iterable via an async generator 42.2.2 Example: converting a sync iterable to an async iterable 42.2.3 Example: converting an async iterable to an Array 42.2.4 Example: transforming an async iterable 42.2.5 Example: mapping over asynchronous iterables 42.3 Async iteration over Node.js streams 42.3.1 Node.js streams: async via callbacks (push) 42.3.2 Node.js streams: async via async iteration (pull) 42.3.3 Example: from chunks to lines Required knowledge For this chapter, you should be familiar with: Promises Async functions
- 755. 42.1 Basic asynchronous iteration 42.1.1 Protocol: async iteration To understand how asynchronous iteration works, let’s first revisit synchronous iteration. It comprises the following interfaces: An Iterable is a data structure whose contents can be accessed via iteration. It is a factory for iterators. An Iterator is a factory for iteration results that we retrieve by calling the method .next(). Each IterationResult contains the iterated .value and a boolean .done that is true after the last element and false before. For the protocol for asynchronous iteration, we only want to change one thing: the values produced by .next() should be delivered asynchronously. There are two conceivable options: The .value could contain a Promise<T>. .next() could return Promise<IteratorResult<T>>. interface Iterable<T> { [Symbol.iterator]() : Iterator<T>; } interface Iterator<T> { next() : IteratorResult<T>; } interface IteratorResult<T> { value: T; done: boolean; }
- 756. In other words, the question is whether to wrap just values or whole iterator results in Promises. It has to be the latter because when .next() returns a result, it starts an asynchronous computation. Whether or not that computation produces a value or signals the end of the iteration can only be determined after it is finished. Therefore, both .done and .value need to be wrapped in a Promise. The interfaces for async iteration look as follows. The only difference to the synchronous interfaces is the return type of .next() (line A). 42.1.2 Using async iteration directly The following code uses the asynchronous iteration protocol directly: interface AsyncIterable<T> { [Symbol.asyncIterator]() : AsyncIterator<T>; } interface AsyncIterator<T> { next() : Promise<IteratorResult<T>>; // (A) } interface IteratorResult<T> { value: T; done: boolean; } const asyncIterable = syncToAsyncIterable(['a', 'b']); // (A) const asyncIterator = asyncIterable[Symbol.asyncIterator](); // Call .next() until .done is true: asyncIterator.next() // (B) .then(iteratorResult => {
- 757. In line A, we create an asynchronous iterable over the value 'a' and 'b'. We’ll see an implementation of syncToAsyncIterable() later. We call .next() in line B, line C and line D. Each time, we use .next() to unwrap the Promise and assert.deepEqual() to check the unwrapped value. We can simplify this code if we use an async function. Now we unwrap Promises via await and the code looks almost like we are doing synchronous iteration: assert.deepEqual( iteratorResult, { value: 'a', done: false }); return asyncIterator.next(); // (C) }) .then(iteratorResult => { assert.deepEqual( iteratorResult, { value: 'b', done: false }); return asyncIterator.next(); // (D) }) .then(iteratorResult => { assert.deepEqual( iteratorResult, { value: undefined, done: true }); }) ; async function f() { const asyncIterable = syncToAsyncIterable(['a', 'b']); const asyncIterator = asyncIterable[Symbol.asyncIterator](); // Call .next() until .done is true: assert.deepEqual( await asyncIterator.next(), { value: 'a', done: false }); assert.deepEqual( await asyncIterator.next(),
- 758. 42.1.3 Using async iteration via for-await- of The asynchronous iteration protocol is not meant to be used directly. One of the language constructs that supports it is the for-await-of loop, which is an asynchronous version of the for-of loop. It can be used in async functions and async generators (which are introduced later in this chapter). This is an example of for-await-of in use: for-await-of is relatively flexible. In addition to asynchronous iterables, it also supports synchronous iterables: And it supports synchronous iterables over values that are wrapped in Promises: { value: 'b', done: false }); assert.deepEqual( await asyncIterator.next(), { value: undefined, done: true }); } for await (const x of syncToAsyncIterable(['a', 'b'])) { console.log(x); } // Output: // 'a' // 'b' for await (const x of ['a', 'b']) { console.log(x); } // Output: // 'a' // 'b'
- 759. Exercise: Convert an async iterable to an Array Warning: We’ll soon see the solution for this exercise in this chapter. exercises/async- iteration/async_iterable_to_array_test.mjs const arr = [Promise.resolve('a'), Promise.resolve('b')]; for await (const x of arr) { console.log(x); } // Output: // 'a' // 'b'
- 760. 42.2 Asynchronous generators An asynchronous generator is two things at the same time: An async function (input): We can use await and for-await-of to retrieve data. A generator that returns an asynchronous iterable (output): We can use yield and yield* to produce data. Asynchronous generators are very similar to synchronous generators Due to async generators and sync generators being so similar, I don’t explain how exactly yield and yield* work. Please consult §38 “Synchronous generators” if you have doubts. Therefore, an asynchronous generator has: Input that can be: synchronous (single values, sync iterables) or asynchronous (Promises, async iterables). Output that is an asynchronous iterable. This looks as follows: async function* asyncGen() { // Input: Promises, async iterables const x = await somePromise; for await (const y of someAsyncIterable) { // ··· }
- 761. 42.2.1 Example: creating an async iterable via an async generator Let’s look at an example. The following code creates an async iterable with three numbers: Does the result of yield123() conform to the async iteration protocol? // Output yield someValue; yield* otherAsyncGen(); } async function* yield123() { for (let i=1; i<=3; i++) { yield i; } } (async () => { const asyncIterable = yield123(); const asyncIterator = asyncIterable[Symbol.asyncIterator](); assert.deepEqual( await asyncIterator.next(), { value: 1, done: false }); assert.deepEqual( await asyncIterator.next(), { value: 2, done: false }); assert.deepEqual( await asyncIterator.next(), { value: 3, done: false }); assert.deepEqual( await asyncIterator.next(), { value: undefined, done: true }); })();
- 762. We wrapped the code in an immediately invoked async arrow function. 42.2.2 Example: converting a sync iterable to an async iterable The following asynchronous generator converts a synchronous iterable to an asynchronous iterable. It implements the function syncToAsyncIterable() that we have used previously. Note: The input is synchronous in this case (no await is needed). 42.2.3 Example: converting an async iterable to an Array The following function is a solution to a previous exercise. It converts an async iterable to an Array (think spreading, but for async iterables instead of sync iterables). async function* syncToAsyncIterable(syncIterable) { for (const elem of syncIterable) { yield elem; } } async function asyncIterableToArray(asyncIterable) { const result = []; for await (const value of asyncIterable) { result.push(value); } return result; }
- 763. Note that we can’t use an async generator in this case: We get our input via for-await-of and return an Array wrapped in a Promise. The latter requirement rules out async generators. This is a test for asyncIterableToArray(): Note the await in line A, which is needed to unwrap the Promise returned by asyncIterableToArray(). In order for await to work, this code fragment must be run inside an async function. 42.2.4 Example: transforming an async iterable Let’s implement an async generator that produces a new async iterable by transforming an existing async iterable. To test this function, we use asyncIterableToArray() from the previous section. async function* createAsyncIterable() { yield 'a'; yield 'b'; } const asyncIterable = createAsyncIterable(); assert.deepEqual( await asyncIterableToArray(asyncIterable), // (A) ['a', 'b'] ); async function* timesTwo(asyncNumbers) { for await (const x of asyncNumbers) { yield x * 2; } }
- 764. Exercise: Async generators Warning: We’ll soon see the solution for this exercise in this chapter. exercises/async-iteration/number_lines_test.mjs 42.2.5 Example: mapping over asynchronous iterables As a reminder, this is how to map over synchronous iterables: The asynchronous version looks as follows: async function* createAsyncIterable() { for (let i=1; i<=3; i++) { yield i; } } assert.deepEqual( await asyncIterableToArray(timesTwo(createAsyncIterable())), [2, 4, 6] ); function* mapSync(iterable, func) { let index = 0; for (const x of iterable) { yield func(x, index); index++; } } const syncIterable = mapSync(['a', 'b', 'c'], s => s.repeat(3)); assert.deepEqual( [...syncIterable], ['aaa', 'bbb', 'ccc']);
- 765. Note how similar the sync implementation and the async implementation are. The only two differences are the async in line A and the await in line B. That is comparable to going from a synchronous function to an asynchronous function – we only need to add the keyword async and the occasional await. To test mapAsync(), we use the helper function asyncIterableToArray() (shown earlier in this chapter): Once again, we await to unwrap a Promise (line A) and this code fragment must run inside an async function. Exercise: filterAsyncIter() exercises/async-iteration/filter_async_iter_test.mjs async function* mapAsync(asyncIterable, func) { // (A) let index = 0; for await (const x of asyncIterable) { // (B) yield func(x, index); index++; } } async function* createAsyncIterable() { yield 'a'; yield 'b'; } const mapped = mapAsync( createAsyncIterable(), s => s.repeat(3)); assert.deepEqual( await asyncIterableToArray(mapped), // (A) ['aaa', 'bbb']);
- 766. 42.3 Async iteration over Node.js streams 42.3.1 Node.js streams: async via callbacks (push) Traditionally, reading asynchronously from Node.js streams is done via callbacks: That is, the stream is in control and pushes data to the reader. 42.3.2 Node.js streams: async via async iteration (pull) Starting with Node.js 10, we can also use asynchronous iteration to read from streams: function main(inputFilePath) { const readStream = fs.createReadStream(inputFilePath, { encoding: 'utf8', highWaterMark: 1024 }); readStream.on('data', (chunk) => { console.log('>>> '+chunk); }); readStream.on('end', () => { console.log('### DONE ###'); }); } async function main(inputFilePath) { const readStream = fs.createReadStream(inputFilePath, { encoding: 'utf8', highWaterMark: 1024 });
- 767. This time, the reader is in control and pulls data from the stream. 42.3.3 Example: from chunks to lines Node.js streams iterate over chunks (arbitrarily long pieces) of data. The following asynchronous generator converts an async iterable over chunks to an async iterable over lines: Let’s apply chunksToLines() to an async iterable over chunks (as produced by chunkIterable()): for await (const chunk of readStream) { console.log('>>> '+chunk); } console.log('### DONE ###'); } /** * Parameter: async iterable of chunks (strings) * Result: async iterable of lines (incl. newlines) */ async function* chunksToLines(chunksAsync) { let previous = ''; for await (const chunk of chunksAsync) { // input previous += chunk; let eolIndex; while ((eolIndex = previous.indexOf('n')) >= 0) { // line includes the EOL (Windows 'rn' or Unix 'n') const line = previous.slice(0, eolIndex+1); yield line; // output previous = previous.slice(eolIndex+1); } } if (previous.length > 0) { yield previous; } }
- 768. Now that we have an asynchronous iterable over lines, we can use the solution of a previous exercise, numberLines(), to number those lines: async function* chunkIterable() { yield 'FirstnSec'; yield 'ondnThirdnF'; yield 'ourth'; } const linesIterable = chunksToLines(chunkIterable()); assert.deepEqual( await asyncIterableToArray(linesIterable), [ 'Firstn', 'Secondn', 'Thirdn', 'Fourth', ]); async function* numberLines(linesAsync) { let lineNumber = 1; for await (const line of linesAsync) { yield lineNumber + ': ' + line; lineNumber++; } } const numberedLines = numberLines(chunksToLines(chunkIterable()) assert.deepEqual( await asyncIterableToArray(numberedLines), [ '1: Firstn', '2: Secondn', '3: Thirdn', '4: Fourth', ]);
- 769. 43 Regular expressions (RegExp) 43.1 Creating regular expressions 43.1.1 Literal vs. constructor 43.1.2 Cloning and non-destructively modifying regular expressions 43.2 Syntax 43.2.1 Syntax characters 43.2.2 Basic atoms 43.2.3 Unicode property escapes [ES2018] 43.2.4 Character classes 43.2.5 Groups 43.2.6 Quantifiers 43.2.7 Assertions 43.2.8 Disjunction (|) 43.3 Flags 43.3.1 Flag: Unicode mode via /u 43.4 Properties of regular expression objects 43.4.1 Flags as properties 43.4.2 Other properties 43.5 Methods for working with regular expressions 43.5.1 In general, regular expressions match anywhere in a string 43.5.2 regExp.test(str): is there a match? [ES3]
- 770. 43.5.3 str.search(regExp): at what index is the match? [ES3] 43.5.4 regExp.exec(str): capturing groups [ES3] 43.5.5 str.match(regExp): return all matching substrings [ES3] 43.5.6 str.replace(searchValue, replacementValue) [ES3] 43.5.7 Other methods for working with regular expressions 43.6 Flag /g and its pitfalls 43.6.1 Pitfall: You can’t inline a regular expression with flag /g 43.6.2 Pitfall: Removing /g can break code 43.6.3 Pitfall: Adding /g can break code 43.6.4 Pitfall: Code can break if .lastIndex isn’t zero 43.6.5 Dealing with /g and .lastIndex 43.7 Techniques for working with regular expressions 43.7.1 Escaping arbitrary text for regular expressions 43.7.2 Matching everything or nothing Availability of features Unless stated otherwise, each regular expression feature has been available since ES3.
- 771. 43.1 Creating regular expressions 43.1.1 Literal vs. constructor The two main ways of creating regular expressions are: Literal: compiled statically (at load time). Constructor: compiled dynamically (at runtime). Both regular expressions have the same two parts: The body abc – the actual regular expression. The flags u and i. Flags configure how the pattern is interpreted. For example, i enables case-insensitive matching. A list of available flags is given later in this chapter. 43.1.2 Cloning and non-destructively modifying regular expressions There are two variants of the constructor RegExp(): new RegExp(pattern : string, flags = '') [ES3] A new regular expression is created as specified via pattern. If flags is missing, the empty string '' is used. /abc/ui new RegExp('abc', 'ui')
- 772. new RegExp(regExp : RegExp, flags = regExp.flags) [ES6] regExp is cloned. If flags is provided, then it determines the flags of the clone. The second variant is useful for cloning regular expressions, optionally while modifying them. Flags are immutable and this is the only way of changing them – for example: function copyAndAddFlags(regExp, flagsToAdd='') { // The constructor doesn’t allow duplicate flags; // make sure there aren’t any: const newFlags = [...new Set(regExp.flags + flagsToAdd)].join( return new RegExp(regExp, newFlags); } assert.equal(/abc/i.flags, 'i'); assert.equal(copyAndAddFlags(/abc/i, 'g').flags, 'gi');
- 773. 43.2 Syntax 43.2.1 Syntax characters At the top level of a regular expression, the following syntax characters are special. They are escaped by prefixing a backslash (). ^ $ . * + ? ( ) [ ] { } | In regular expression literals, you must escape slashs: In the argument of new RegExp(), you don’t have to escape slashes: 43.2.2 Basic atoms Atoms are the basic building blocks of regular expressions. Pattern characters are all characters except syntax characters (^, $, etc.). Pattern characters match themselves. Examples: A b % . matches any character. You can use the flag /s (dotall) to control if the dot matches line terminators or not (more below). Character escapes (each escape matches a single fixed character): Control escapes (for a few control characters): > ///.test('/') true > new RegExp('/').test('/') true
- 774. f: form feed (FF) n: line feed (LF) r: carriage return (CR) t: character tabulation v: line tabulation Arbitrary control characters: cA (Ctrl-A), …, cZ (Ctrl-Z) Unicode code units: u00E4 Unicode code points (require flag /u): u{1F44D} Character class escapes (each escape matches one out of a set of characters): d: digits (same as [0-9]) D: non-digits w: “word” characters (same as [A-Za-z0-9_], related to identifiers in programming languages) W: non-word characters s: whitespace (space, tab, line terminators, etc.) S: non-whitespace Unicode property escapes [ES2018]: p{White_Space}, P{White_Space}, etc. Require flag /u. Described in the next subsection. 43.2.3 Unicode property escapes [ES2018] 43.2.3.1 Unicode character properties
- 775. In the Unicode standard, each character has properties – metadata describing it. Properties play an important role in defining the nature of a character. Quoting the Unicode Standard, Sect. 3.3, D3: The semantics of a character are determined by its identity, normative properties, and behavior. These are a few examples of properties: Name: a unique name, composed of uppercase letters, digits, hyphens, and spaces – for example: A: Name = LATIN CAPITAL LETTER A 🙂: Name = SLIGHTLY SMILING FACE General_Category: categorizes characters – for example: x: General_Category = Lowercase_Letter $: General_Category = Currency_Symbol White_Space: used for marking invisible spacing characters, such as spaces, tabs and newlines – for example: t: White_Space = True π: White_Space = False Age: version of the Unicode Standard in which a character was introduced – for example: The Euro sign € was added in version 2.1 of the Unicode standard. €: Age = 2.1 Block: a contiguous range of code points. Blocks don’t overlap and their names are unique. For example: S: Block = Basic_Latin (range U+0000..U+007F) 🙂: Block = Emoticons (range U+1F600..U+1F64F)
- 776. Script: is a collection of characters used by one or more writing systems. Some scripts support several writing systems. For example, the Latin script supports the writing systems English, French, German, Latin, etc. Some languages can be written in multiple alternate writing systems that are supported by multiple scripts. For example, Turkish used the Arabic script before it transitioned to the Latin script in the early 20th century. Examples: α: Script = Greek Д: Script = Cyrillic 43.2.3.2 Unicode property escapes Unicode property escapes look like this: 1. p{prop=value}: matches all characters whose property prop has the value value. 2. P{prop=value}: matches all characters that do not have a property prop whose value is value. 3. p{bin_prop}: matches all characters whose binary property bin_prop is True. 4. P{bin_prop}: matches all characters whose binary property bin_prop is False. Comments: You can only use Unicode property escapes if the flag /u is set. Without /u, p is the same as p.
- 777. Forms (3) and (4) can be used as abbreviations if the property is General_Category. For example, the following two escapes are equivalent: p{Lowercase_Letter} p{General_Category=Lowercase_Letter} Examples: Checking for whitespace: Checking for Greek letters: Deleting any letters: Deleting lowercase letters: Further reading: Lists of Unicode properties and their values: “Unicode Standard Annex #44: Unicode Character Database” (Editors: Mark Davis, Laurențiu Iancu, Ken Whistler) 43.2.4 Character classes > /^p{White_Space}+$/u.test('t nr') true > /^p{Script=Greek}+$/u.test('μετά') true > '1π2ü3é4'.replace(/p{Letter}/ug, '') '1234' > 'AbCdEf'.replace(/p{Lowercase_Letter}/ug, '') 'ACE'
- 778. A character class wraps class ranges in square brackets. The class ranges specify a set of characters: [«class ranges»] matches any character in the set. [^«class ranges»] matches any character not in the set. Rules for class ranges: Non-syntax characters stand for themselves: [abc] Only the following four characters are special and must be escaped via slashes: ^ - ] ^ only has to be escaped if it comes first. - need not be escaped if it comes first or last. Character escapes (n, u{1F44D}, etc.) have the usual meaning. Watch out: b stands for backspace. Elsewhere in a regular expression, it matches word boundaries. Character class escapes (d, p{White_Space}, etc.) have the usual meaning. Ranges of characters are specified via dashes: [a-z] 43.2.5 Groups Positional capture group: (#+) Backreference: 1, 2, etc. Named capture group [ES2018]: (?<hashes>#+)
- 779. Backreference: k<hashes> Noncapturing group: (?:#+) 43.2.6 Quantifiers By default, all of the following quantifiers are greedy (they match as many characters as possible): ?: match never or once *: match zero or more times +: match one or more times {n}: match n times {n,}: match n or more times {n,m}: match at least n times, at most m times. To make them reluctant (so that they match as few characters as possible), put question marks (?) after them: 43.2.7 Assertions ^ matches only at the beginning of the input $ matches only at the end of the input b matches only at a word boundary B matches only when not at a word boundary 43.2.7.1 Lookahead > /".*"/.exec('"abc"def"')[0] // greedy '"abc"def"' > /".*?"/.exec('"abc"def"')[0] // reluctant '"abc"'
- 780. Positive lookahead: (?=«pattern») matches if pattern matches what comes next. Example: sequences of lowercase letters that are followed by an X. Note that the X itself is not part of the matched substring. Negative lookahead: (?!«pattern») matches if pattern does not match what comes next. Example: sequences of lowercase letters that are not followed by an X. 43.2.7.2 Lookbehind [ES2018] Positive lookbehind: (?<=«pattern») matches if pattern matches what came before. Example: sequences of lowercase letters that are preceded by an X. Negative lookbehind: (?<!«pattern») matches if pattern does not match what came before. Example: sequences of lowercase letters that are not preceded by an X. > 'abcX def'.match(/[a-z]+(?=X)/g) [ 'abc' ] > 'abcX def'.match(/[a-z]+(?!X)/g) [ 'ab', 'def' ] > 'Xabc def'.match(/(?<=X)[a-z]+/g) [ 'abc' ]
- 781. Example: replace “.js” with “.html”, but not in “Node.js”. 43.2.8 Disjunction (|) Caveat: this operator has low precedence. Use groups if necessary: ^aa|zz$ matches all strings that start with aa and/or end with zz. Note that | has a lower precedence than ^ and $. ^(aa|zz)$ matches the two strings 'aa' and 'zz'. ^a(a|z)z$ matches the two strings 'aaz' and 'azz'. > 'Xabc def'.match(/(?<!X)[a-z]+/g) [ 'bc', 'def' ] > 'Node.js: index.js and main.js'.replace(/(?<!Node).js/g, '.ht 'Node.js: index.html and main.html'
- 782. 43.3 Flags Table 20: These are the regular expression flags supported by JavaScript. Literal flag Property name ES Description g global ES3 Match multiple times i ignoreCase ES3 Match case-insensitively m multiline ES3 ^ and $ match per line s dotall ES2018 Dot matches line terminators u unicode ES6 Unicode mode (recommended) y sticky ES6 No characters between matches The following regular expression flags are available in JavaScript (tbl. 20 provides a compact overview): /g (.global): fundamentally changes how the following methods work. RegExp.prototype.test() RegExp.prototype.exec() String.prototype.match() How, is explained in §43.6 “Flag /g and its pitfalls”. In a nutshell, without /g, the methods only consider the first match
- 783. for a regular expression in an input string. With /g, they consider all matches. /i (.ignoreCase): switches on case-insensitive matching: /m (.multiline): If this flag is on, ^ matches the beginning of each line and $ matches the end of each line. If it is off, ^ matches the beginning of the whole input string and $ matches the end of the whole input string. /u (.unicode): This flag switches on the Unicode mode for a regular expression. That mode is explained in the next subsection. /y (.sticky): This flag mainly makes sense in conjunction with /g. When both are switched on, any match must directly follow the previous one (that is, it must start at index .lastIndex of the regular expression object). Therefore, the first match must be at index 0. > /a/.test('A') false > /a/i.test('A') true > 'a1na2na3'.match(/^a./gm) [ 'a1', 'a2', 'a3' ] > 'a1na2na3'.match(/^a./g) [ 'a1' ] > 'a1a2 a3'.match(/a./gy) [ 'a1', 'a2' ] > '_a1a2 a3'.match(/a./gy) // first match must be at index 0 null
- 784. The main use case for /y is tokenization (during parsing). /s (.dotall): By default, the dot does not match line terminators. With this flag, it does: Workaround if /s isn’t supported: Use [^] instead of a dot. 43.3.1 Flag: Unicode mode via /u The flag /u switches on a special Unicode mode for regular expressions. That mode enables several features: In patterns, you can use Unicode code point escapes such as u{1F42A} to specify characters. Code unit escapes such as u03B1 only have a range of four hexadecimal digits (which corresponds to the basic multilingual plane). In patterns, you can use Unicode property escapes such as p{White_Space}. Many escapes are now forbidden. For example: a - : > 'a1a2 a3'.match(/a./g) [ 'a1', 'a2', 'a3' ] > '_a1a2 a3'.match(/a./g) [ 'a1', 'a2', 'a3' ] > /./.test('n') false > /./s.test('n') true > /[^]/.test('n') true
- 785. Pattern characters always match themselves: Without /u, there are some pattern characters that still match themselves if you escape them with backslashes: With /u: p starts a Unicode property escape. The remaining “self-matching” escapes are forbidden. As a consequence, they can now be used for new features in the future. The atomic units for matching are Unicode characters (code points), not JavaScript characters (code units). The following subsections explain the last item in more detail. They use the following Unicode character to explain when the atomic units are Unicode characters and when they are JavaScript characters: I’m only switching between 🙂 and uD83DuDE42, to illustrate how JavaScript sees things. Both are equivalent and can be used interchangeably in strings and regular expressions. > /pa-:/.test('pa-:') true > /pa-:/.test('pa-:') true const codePoint = '🙂'; const codeUnits = 'uD83DuDE42'; // UTF-16 assert.equal(codePoint, codeUnits); // same string!
- 786. 43.3.1.1 Consequence: you can put Unicode characters in character classes With /u, the two code units of 🙂 are treated as a single character: Without /u, 🙂 is treated as two characters: Note that ^ and $ demand that the input string have a single character. That’s why the first result is false. 43.3.1.2 Consequence: the dot operator (.) matches Unicode characters, not JavaScript characters With /u, the dot operator matches Unicode characters: .match() plus /g returns an Array with all the matches of a regular expression. Without /u, the dot operator matches JavaScript characters: > /^[🙂]$/u.test('🙂') true > /^[uD83DuDE42]$/.test('uD83DuDE42') false > /^[uD83DuDE42]$/.test('uDE42') true > '🙂'.match(/./gu).length 1 > 'uD83DuDE80'.match(/./g).length 2
- 787. 43.3.1.3 Consequence: quantifiers apply to Unicode characters, not JavaScript characters With /u, a quantifier applies to the whole preceding Unicode character: Without /u, a quantifier only applies to the preceding JavaScript character: > /^🙂{3}$/u.test('🙂🙂🙂') true > /^uD83DuDE80{3}$/.test('uD83DuDE80uDE80uDE80') true
- 788. 43.4 Properties of regular expression objects Noteworthy: Strictly speaking, only .lastIndex is a real instance property. All other properties are implemented via getters. Accordingly, .lastIndex is the only mutable property. All other properties are read-only. If you want to change them, you need to copy the regular expression (consult §43.1.2 “Cloning and non-destructively modifying regular expressions” for details). 43.4.1 Flags as properties Each regular expression flag exists as a property with a longer, more descriptive name: This is the complete list of flag properties: .dotall (/s) .global (/g) .ignoreCase (/i) .multiline (/m) .sticky (/y) .unicode (/u) > /a/i.ignoreCase true > /a/.ignoreCase false
- 789. 43.4.2 Other properties Each regular expression also has the following properties: .source [ES3]: The regular expression pattern .flags [ES6]: The flags of the regular expression .lastIndex [ES3]: Used when flag /g is switched on. Consult §43.6 “Flag /g and its pitfalls” for details. > /abc/ig.source 'abc' > /abc/ig.flags 'gi'
- 790. 43.5 Methods for working with regular expressions 43.5.1 In general, regular expressions match anywhere in a string Note that, in general, regular expressions match anywhere in a string: You can change that by using assertions such as ^ or by using the flag /y: 43.5.2 regExp.test(str): is there a match? [ES3] The regular expression method .test() returns true if regExp matches str: > /a/.test('__a__') true > /^a/.test('__a__') false > /^a/.test('a__') true > /bc/.test('ABCD') false > /bc/i.test('ABCD') true > /.mjs$/.test('main.mjs') true
- 791. With .test() you should normally avoid the /g flag. If you use it, you generally don’t get the same result every time you call the method: The results are due to /a/ having two matches in the string. After all of those were found, .test() returns false. 43.5.3 str.search(regExp): at what index is the match? [ES3] The string method .search() returns the first index of str at which there is a match for regExp: 43.5.4 regExp.exec(str): capturing groups [ES3] 43.5.4.1 Getting a match object for the first match Without the flag /g, .exec() returns the captures of the first match for regExp in str: > const r = /a/g; > r.test('aab') true > r.test('aab') true > r.test('aab') false > '_abc_'.search(/abc/) 1 > 'main.mjs'.search(/.mjs$/) 4
- 792. The result is a match object with the following properties: [0]: the complete substring matched by the regular expression [1]: capture of positional group 1 (etc.) .index: where did the match occur? .input: the string that was matched against .groups: captures of named groups 43.5.4.2 Named capture groups [ES2018] The previous example contained a single positional group. The following example demonstrates named groups: In the result of .exec(), you can see that a named group is also a positional group – its capture exists twice: assert.deepEqual( /(a+)b/.exec('ab aab'), { 0: 'ab', 1: 'a', index: 0, input: 'ab aab', groups: undefined, } ); assert.deepEqual( /(?<as>a+)b/.exec('ab aab'), { 0: 'ab', 1: 'a', index: 0, input: 'ab aab', groups: { as: 'a' }, } );
- 793. Once as a positional capture (property '1'). Once as a named capture (property groups.as). 43.5.4.3 Looping over multiple matches If you want to retrieve all matches of a regular expression (not just the first one), you need to switch on the flag /g. Then you can call .exec() multiple times and get one match each time. After the last match, .exec() returns null. Therefore, you can loop over all matches as follows: Sharing regular expressions with /g has a few pitfalls, which are explained later. > const regExp = /(a+)b/g; > regExp.exec('ab aab') { 0: 'ab', 1: 'a', index: 0, input: 'ab aab', groups: undefined > regExp.exec('ab aab') { 0: 'aab', 1: 'aa', index: 3, input: 'ab aab', groups: undefine > regExp.exec('ab aab') null const regExp = /(a+)b/g; const str = 'ab aab'; let match; // Check for null via truthiness // Alternative: while ((match = regExp.exec(str)) !== null) while (match = regExp.exec(str)) { console.log(match[1]); } // Output: // 'a' // 'aa'
- 794. Exercise: Extract quoted text via .exec() exercises/regexps/extract_quoted_test.mjs 43.5.5 str.match(regExp): return all matching substrings [ES3] Without /g, .match() works like .exec() – it returns a single match object. With /g, .match() returns all substrings of str that match regExp: If there is no match, .match() returns null: You can use the Or operator to protect yourself against null: 43.5.6 str.replace(searchValue, replacementValue) [ES3] .replace() is overloaded – it works differently, depending on the types of its parameters: If searchValue is: > 'ab aab'.match(/(a+)b/g) [ 'ab', 'aab' ] > 'xyz'.match(/(a+)b/g) null const numberOfMatches = (str.match(regExp) || []).length;
- 795. Regular expression without /g: Replace first match of this regular expression. Regular expression with /g: Replace all matches of this regular expression. String: Replace first occurrence of this string (the string is interpreted verbatim, not as a regular expression). Alas, there is no way to replace every occurrence of a string. Later in this chapter, we’ll see a tool function that converts a string into a regular expression that matches this string (e.g., '*' becomes /*/). If replacementValue is: String: Replace matches with this string. The character $ has special meaning and lets you insert captures of groups and more (read on for details). Function: Compute strings that replace matches via this function. The next two subsubsections assume that a regular expression with /g is being used. 43.5.6.1 replacementValue is a string If the replacement value is a string, the dollar sign has special meaning – it inserts text matched by the regular expression: Text Result $$ single $ $& complete match
- 796. Text Result $` text before match $' text after match $n capture of positional group n (n > 0) $<name> capture of named group name [ES2018] Example: Inserting the text before, inside, and after the matched substring. Example: Inserting the captures of positional groups. Example: Inserting the captures of named groups. 43.5.6.2 replacementValue is a function If the replacement value is a function, you can compute each replacement. In the following example, we multiply each non- negative integer that we find by two. > 'a1 a2'.replace(/a/g, "($`|$&|$')") '(|a|1 a2)1 (a1 |a|2)2' > const regExp = /^([A-Za-z]+): (.*)$/ug; > 'first: Jane'.replace(regExp, 'KEY: $1, VALUE: $2') 'KEY: first, VALUE: Jane' > const regExp = /^(?<key>[A-Za-z]+): (?<value>.*)$/ug; > 'first: Jane'.replace(regExp, 'KEY: $<key>, VALUE: $<value>') 'KEY: first, VALUE: Jane' assert.equal( '3 cats and 4 dogs'.replace(/[0-9]+/g, (all) => 2 * Number(all '6 cats and 8 dogs' );
- 797. The replacement function gets the following parameters. Note how similar they are to match objects. These parameters are all positional, but I’ve included how one might name them: all: complete match g1: capture of positional group 1 Etc. index: where did the match occur? input: the string in which we are replacing groups [ES2018]: captures of named groups (an object) Exercise: Change quotes via .replace() and a named group exercises/regexps/change_quotes_test.mjs 43.5.7 Other methods for working with regular expressions String.prototype.split() is described in the chapter on strings. Its first parameter of String.prototype.split() is either a string or a regular expression. If it is the latter, then captures of groups appear in the result: > 'a:b : c'.split(':') [ 'a', 'b ', ' c' ] > 'a:b : c'.split(/ *: */) [ 'a', 'b', 'c' ] > 'a:b : c'.split(/( *):( *)/) [ 'a', '', '', 'b', ' ', ' ', 'c' ]
- 798. 43.6 Flag /g and its pitfalls The following two regular expression methods work differently if /g is switched on: RegExp.prototype.exec() RegExp.prototype.test() Then they can be called repeatedly and deliver all matches inside a string. Property .lastIndex of the regular expression is used to track the current position inside the string – for example: The next subsections explain the pitfalls of using /g. They are followed by a subsection that explains how to work around those pitfalls. 43.6.1 Pitfall: You can’t inline a regular expression with flag /g const r = /a/g; assert.equal(r.lastIndex, 0); assert.equal(r.test('aa'), true); // 1st match? assert.equal(r.lastIndex, 1); // after 1st match assert.equal(r.test('aa'), true); // 2nd match? assert.equal(r.lastIndex, 2); // after 2nd match assert.equal(r.test('aa'), false); // 3rd match? assert.equal(r.lastIndex, 0); // start over
- 799. A regular expression with /g can’t be inlined. For example, in the following while loop, the regular expression is created fresh, every time the condition is checked. Therefore, its .lastIndex is always zero and the loop never terminates. 43.6.2 Pitfall: Removing /g can break code If code expects a regular expression with /g and has a loop over the results of .exec() or .test(), then a regular expression without /g can cause an infinite loop: Why? Because .exec() always returns the first result, a match object, and never null. 43.6.3 Pitfall: Adding /g can break code let count = 0; // Infinite loop while (/a/g.test('babaa')) { count++; } function countMatches(regExp) { let count = 0; // Infinite loop while (regExp.exec('babaa')) { count++; } return count; } countMatches(/a/); // Missing: flag /g
- 800. With .test(), there is another caveat: if you want to check exactly once if a regular expression matches a string, then the regular expression must not have /g. Otherwise, you generally get a different result every time you call .test(): Normally, you won’t add /g if you intend to use .test() in this manner. But it can happen if, for example, you use the same regular expression for testing and for replacing. 43.6.4 Pitfall: Code can break if .lastIndex isn’t zero If you match a regular expression multiple times via .exec() or .test(), the current position inside the input string is stored in the regular expression property .lastIndex. Therefore, code that matches multiple times may break if .lastIndex is not zero: function isMatching(regExp) { return regExp.test('Xa'); } const myRegExp = /^X/g; assert.equal(isMatching(myRegExp), true); assert.equal(isMatching(myRegExp), false); function countMatches(regExp) { let count = 0; while (regExp.exec('babaa')) { count++; } return count; } const myRegExp = /a/g; myRegExp.lastIndex = 4; assert.equal(countMatches(myRegExp), 1); // should be 3
- 801. Note that .lastIndex is always zero in newly created regular expressions, but it may not be if the same regular expression is used multiple times. 43.6.5 Dealing with /g and .lastIndex As an example of dealing with /g and .lastIndex, we will implement the following function: It counts how often regExp has a match inside str. How do we prevent a wrong regExp from breaking our code? Let’s look at three approaches. First, we can throw an exception if /g isn’t set or .lastIndex isn’t zero: Second, we can clone the parameter. That has the added benefit that regExp won’t be changed. countMatches(regExp, str) function countMatches(regExp, str) { if (!regExp.global) { throw new Error('Flag /g of regExp must be set'); } if (regExp.lastIndex !== 0) { throw new Error('regExp.lastIndex must be zero'); } let count = 0; while (regExp.test(str)) { count++; } return count; }
- 802. Third, we can use .match() to count occurrences, which doesn’t change or depend on .lastIndex. function countMatches(regExp, str) { const cloneFlags = regExp.flags + (regExp.global ? '' : 'g'); const clone = new RegExp(regExp, cloneFlags); let count = 0; while (clone.test(str)) { count++; } return count; } function countMatches(regExp, str) { if (!regExp.global) { throw new Error('Flag /g of regExp must be set'); } return (str.match(regExp) || []).length; }
- 803. 43.7 Techniques for working with regular expressions 43.7.1 Escaping arbitrary text for regular expressions The following function escapes an arbitrary text so that it is matched verbatim if you put it inside a regular expression: In line A, we escape all syntax characters. We have to be selective because the regular expression flag /u forbids many escapes – for example: a : - The regular expression method .replace() only lets you replace plain text once. With escapeForRegExp(), we can work around that limitation and replace plain text multiple times: 43.7.2 Matching everything or nothing function escapeForRegExp(str) { return str.replace(/[^$.*+?()[]{}|]/g, '$&'); // (A) } assert.equal(escapeForRegExp('[yes?]'), String.raw`[yes?]`); assert.equal(escapeForRegExp('_g_'), String.raw`_g_`); const plainText = ':-)'; const regExp = new RegExp(escapeForRegExp(plainText), 'ug'); assert.equal( ':-) :-) :-)'.replace(regExp, '🙂'), '🙂 🙂 🙂');
- 804. Sometimes, you may need a regular expression that matches everything or nothing – for example, as a default value. Match everything: /(?:)/ The empty group () matches everything. We make it non- capturing (via ?:), to avoid unnecessary work. Match nothing: /.^/ ^ only matches at the beginning of a string. The dot moves matching beyond the first character and now ^ doesn’t match anymore. > /(?:)/.test('') true > /(?:)/.test('abc') true > /.^/.test('') false > /.^/.test('abc') false
- 805. 44 Dates (Date) 44.1 Best practice: avoid the built-in Date 44.1.1 Things to look for in a date library 44.2 Time standards 44.2.1 Background: UTC vs. Z vs. GMT 44.2.2 Dates do not support time zones 44.3 Background: date time formats (ISO) 44.3.1 Tip: append a Z to make date parsing deterministic 44.4 Time values 44.4.1 Creating time values 44.4.2 Getting and setting time values 44.5 Creating Dates 44.5.1 Creating dates via numbers 44.5.2 Parsing dates from strings 44.5.3 Other ways of creating dates 44.6 Getters and setters 44.6.1 Time unit getters and setters 44.7 Converting Dates to strings 44.7.1 Strings with times 44.7.2 Strings with dates 44.7.3 Strings with dates and times 44.7.4 Other methods This chapter describes JavaScript’s API for working with dates – the class Date.
- 806. 44.1 Best practice: avoid the built- in Date The JavaScript Date API is cumbersome to use. Hence, it’s best to rely on a library for anything related to dates. Popular libraries include: Moment.js Day.js Luxon js-joda date-fns Consult the blog post “Why you shouldn’t use Moment.js…” for the pros and cons of these libraries. Additionally, TC39 is working on a new date API for JavaScript: temporal. 44.1.1 Things to look for in a date library Two things are important to keep in mind: Tree-shaking can considerably reduce the size of a library. It is a technique of only deploying those exports of a library to a web server that are imported somewhere. Functions are much more amenable to tree-shaking than classes.
- 807. Support for time zones: As explained later, Date does not support time zones, which introduces a number of pitfalls and is a key weakness. Make sure that your date library supports them.
- 808. 44.2 Time standards 44.2.1 Background: UTC vs. Z vs. GMT UTC, Z, and GMT are ways of specifying time that are similar, but subtly different: UTC (Coordinated Universal Time) is the time standard that all times zones are based on. They are specified relative to it. That is, no country or territory has UTC as its local time zone. Z (Zulu Time Zone) is a military time zone that is often used in aviation and the military as another name for UTC+0. GMT (Greenwich Mean Time) is a time zone used in some European and African countries. It is UTC plus zero hours and therefore has the same time as UTC. Sources: “The Difference Between GMT and UTC” at TimeAndDate.com “Z – Zulu Time Zone (Military Time)” at TimeAndDate.com 44.2.2 Dates do not support time zones Dates support the following time standards: The local time zone (which depends on the current location) UTC
- 809. Time offsets (relative to UTC) Depending on the operation, only some of those options are available. For example, when converting dates to strings or extracting time units such as the day of the month, you can only choose between the local time zone and UTC. Internally, Dates are stored as UTC. When converting from or to the local time zone, the necessary offsets are determined via the date. In the following example, the local time zone is Europe/Paris: Whenever you create or convert dates, you need to be mindful of the time standard being used – for example: new Date() uses the local time zone while .toISOString() uses UTC. Dates interpret 0 as January. The day of the month is 27 in the local time zone, but 26 in UTC. Documenting the time standards supported by each operation In the remainder of this chapter, the supported time standards are noted for each operation. // CEST (Central European Summer Time) assert.equal( new Date('2122-06-29').getTimezoneOffset(), -120); // CET (Central European Time) assert.equal( new Date('2122-12-29').getTimezoneOffset(), -60); > new Date(2077, 0, 27).toISOString() '2077-01-26T23:00:00.000Z'
- 810. 44.2.2.1 The downsides of not being able to specify time zones Not being able to specify time zones has two downsides: It makes it impossible to support multiple time zones. It can lead to location-specific bugs. For example, the previous example produces different results depending on where it is executed. To be safe: Use UTC-based operations whenever possible Use Z or a time offset when parsing strings (see the next section for more information).
- 811. 44.3 Background: date time formats (ISO) Date time formats describe: The strings accepted by: Date.parse() new Date() The strings returned by (always longest format): Date.prototype.toISOString() The following is an example of a date time string returned by .toISOString(): Date time formats have the following structures: Date formats: Y=year; M=month; D=day YYYY-MM-DD YYYY-MM YYYY Time formats: T=separator (the string 'T'); H=hour; m=minute; s=second and millisecond; Z=Zulu Time Zone (the string 'Z') THH:mm:ss.sss THH:mm:ss.sssZ THH:mm:ss THH:mm:ssZ '2033-05-28T15:59:59.123Z'
- 812. THH:mm THH:mmZ Date time formats: are date formats followed by time formats. For example (longest): YYYY-MM-DDTHH:mm:ss.sssZ Instead of Z (which is UTC+0), we can also specify time offsets relative to UTC: THH:mm+HH:mm (etc.) THH:mm-HH:mm (etc.) 44.3.1 Tip: append a Z to make date parsing deterministic If you add a Z to the end of a string, date parsing doesn’t produce different results at different locations: Without Z: Input is January 27 (in the Europe/Paris time zone), output is January 26 (in UTC). With Z: Input is January 27, output is January 27. > new Date('2077-01-27T00:00').toISOString() '2077-01-26T23:00:00.000Z' > new Date('2077-01-27T00:00Z').toISOString() '2077-01-27T00:00:00.000Z'
- 813. 44.4 Time values A time value represents a date via the number of milliseconds since 1 January 1970 00:00:00 UTC. Time values can be used to create Dates: Coercing a Date to a number returns its time value: Ordering operators coerce their operands to numbers. Therefore, you can use these operators to compare Dates: 44.4.1 Creating time values The following methods create time values: Date.now(): number (UTC) Returns the current time as a time value. const timeValue = 0; assert.equal( new Date(timeValue).toISOString(), '1970-01-01T00:00:00.000Z'); > Number(new Date(123)) 123 assert.equal( new Date('1972-05-03') < new Date('2001-12-23'), true); // Internally: assert.equal(73699200000 < 1009065600000, true);
- 814. Date.parse(dateTimeStr: string): number (local time zone, UTC, time offset) Parses dateTimeStr and returns the corresponding time value. Date.UTC(year, month, date?, hours?, minutes?, seconds?, milliseconds?): number (UTC) Returns the time value for the specified UTC date time. 44.4.2 Getting and setting time values Date.prototype.getTime(): number (UTC) Returns the time value corresponding to the Date. Date.prototype.setTime(timeValue) (UTC) Sets this to the date encoded by timeValue.
- 815. 44.5 Creating Dates 44.5.1 Creating dates via numbers new Date(year: number, month: number, date?: number, hours?: number, minutes?: number, seconds?: number, milliseconds?: number) (local time zone) Two of the parameters have pitfalls: For month, 0 is January, 1 is February, etc. If 0 ≤ year ≤ 99, then 1900 is added: That’s why, elsewhere in this chapter, we avoid the time unit year and always use fullYear. But in this case, we have no choice. Example: Note that the input hours (21) are different from the output hours (20). The former refer to the local time zone, the latter to UTC. 44.5.2 Parsing dates from strings new Date(dateTimeStr: string) (local time zone, UTC, time offset) > new Date(12, 1, 22, 19, 11).getFullYear() 1912 > new Date(2077,0,27, 21,49).toISOString() // CET (UTC+1) '2077-01-27T20:49:00.000Z'
- 816. If there is a Z at the end, UTC is used: If there is not Z or time offset at the end, the local time zone is used: If a string only contains a date, it is interpreted as UTC: 44.5.3 Other ways of creating dates new Date(timeValue: number) (UTC) new Date() (UTC) The same as new Date(Date.now()). > new Date('2077-01-27T00:00Z').toISOString() '2077-01-27T00:00:00.000Z' > new Date('2077-01-27T00:00').toISOString() // CET (UTC+1) '2077-01-26T23:00:00.000Z' > new Date('2077-01-27').toISOString() '2077-01-27T00:00:00.000Z' > new Date(0).toISOString() '1970-01-01T00:00:00.000Z'
- 817. 44.6 Getters and setters 44.6.1 Time unit getters and setters Dates have getters and setters for time units – for example: Date.prototype.getFullYear() Date.prototype.setFullYear(num) These getters and setters conform to the following patterns: Local time zone: Date.prototype.get«Unit»() Date.prototype.set«Unit»(num) UTC: Date.prototype.getUTC«Unit»() Date.prototype.setUTC«Unit»(num) These are the time units that are supported: Date FullYear Month: month (0–11). Pitfall: 0 is January, etc. Date: day of the month (1–31) Day (getter only): day of the week (0–6, 0 is Sunday) Time Hours: hour (0–23) Minutes: minutes (0–59) Seconds: seconds (0–59)
- 818. Milliseconds: milliseconds (0–999) There is one more getter that doesn’t conform to the previously mentioned patterns: Date.prototype.getTimezoneOffset() Returns the time difference between local time zone and UTC in minutes. For example, for Europe/Paris, it returns -120 (CEST, Central European Summer Time) or -60 (CET, Central European Time): > new Date('2122-06-29').getTimezoneOffset() -120 > new Date('2122-12-29').getTimezoneOffset() -60
- 819. 44.7 Converting Dates to strings Example Date: 44.7.1 Strings with times Date.prototype.toTimeString() (local time zone) 44.7.2 Strings with dates Date.prototype.toDateString() (local time zone) 44.7.3 Strings with dates and times Date.prototype.toString() (local time zone) Date.prototype.toUTCString() (UTC) const d = new Date(0); > d.toTimeString() '01:00:00 GMT+0100 (Central European Standard Time)' > d.toDateString() 'Thu Jan 01 1970' > d.toString() 'Thu Jan 01 1970 01:00:00 GMT+0100 (Central European Standar > d.toUTCString() 'Thu, 01 Jan 1970 00:00:00 GMT'
- 820. Date.prototype.toISOString() (UTC) 44.7.4 Other methods The following three methods are not really part of ECMAScript, but rather of the ECMAScript internationalization API. That API has much functionality for formatting dates (including support for time zones), but not for parsing them. Date.prototype.toLocaleTimeString() Date.prototype.toLocaleDateString() Date.prototype.toLocaleString() Exercise: Creating a date string exercises/dates/create_date_string_test.mjs > d.toISOString() '1970-01-01T00:00:00.000Z'
- 821. 45 Creating and parsing JSON (JSON) 45.1 The discovery and standardization of JSON 45.1.1 JSON’s grammar is frozen 45.2 JSON syntax 45.3 Using the JSON API 45.3.1 JSON.stringify(data, replacer?, space?) 45.3.2 JSON.parse(text, reviver?) 45.3.3 Example: converting to and from JSON 45.4 Customizing stringification and parsing (advanced) 45.4.1 .stringfy(): specifying which properties of objects to stringify 45.4.2 .stringify() and .parse(): value visitors 45.4.3 Example: visiting values 45.4.4 Example: stringifying unsupported values 45.4.5 Example: parsing unsupported values 45.5 FAQ 45.5.1 Why doesn’t JSON support comments? JSON (“JavaScript Object Notation”) is a storage format that uses text to encode data. Its syntax is a subset of JavaScript expressions. As an example, consider the following text, stored in a file jane.json: { "first": "Jane",
- 822. JavaScript has the global namespace object JSON that provides methods for creating and parsing JSON. "last": "Porter", "married": true, "born": 1890, "friends": [ "Tarzan", "Cheeta" ] }
- 823. 45.1 The discovery and standardization of JSON A specification for JSON was published by Douglas Crockford in 2001, at json.org. He explains: I discovered JSON. I do not claim to have invented JSON because it already existed in nature. What I did was I found it, I named it, I described how it was useful. I don’t claim to be the first person to have discovered it; I know that there are other people who discovered it at least a year before I did. The earliest occurrence I’ve found was, there was someone at Netscape who was using JavaScript array literals for doing data communication as early as 1996, which was at least five years before I stumbled onto the idea. Later, JSON was standardized as ECMA-404: 1st edition: October 2013 2nd edition: December 2017 45.1.1 JSON’s grammar is frozen Quoting the ECMA-404 standard: Because it is so simple, it is not expected that the JSON grammar will ever change. This gives JSON, as a foundational notation, tremendous stability.
- 824. Therefore, JSON will never get improvements such as optional trailing commas, comments, or unquoted keys – independently of whether or not they are considered desirable. However, that still leaves room for creating supersets of JSON that compile to plain JSON.
- 825. 45.2 JSON syntax JSON consists of the following parts of JavaScript: Compound: Object literals: Property keys are double-quoted strings. Property values are JSON values. No trailing commas are allowed. Array literals: Elements are JSON values. No holes or trailing commas are allowed. Atomic: null (but not undefined) Booleans Numbers (excluding NaN, +Infinity, -Infinity) Strings (must be double-quoted) As a consequence, you can’t (directly) represent cyclic structures in JSON.
- 826. 45.3 Using the JSON API The global namespace object JSON contains methods for working with JSON data. 45.3.1 JSON.stringify(data, replacer?, space?) .stringify() converts JavaScript data to a JSON string. In this section, we are ignoring the parameter replacer; it is explained in §45.4 “Customizing stringification and parsing”. 45.3.1.1 Result: a single line of text If you only provide the first argument, .stringify() returns a single line of text: 45.3.1.2 Result: a tree of indented lines If you provide a non-negative integer for space, then .stringify() returns one or more lines and indents by space spaces per level of nesting: assert.equal( JSON.stringify({foo: ['a', 'b']}), '{"foo":["a","b"]}' ); assert.equal( JSON.stringify({foo: ['a', 'b']}, null, 2), `{ "foo": [
- 827. 45.3.1.3 Details on how JavaScript data is stringified Primitive values: Supported primitive values are stringified as expected: Unsupported numbers: 'null' Other unsupported primitive values are not stringified; they produce the result undefined: Objects: If an object has a method .toJSON(), then the result of that method is stringified: "a", "b" ] }`); > JSON.stringify('abc') '"abc"' > JSON.stringify(123) '123' > JSON.stringify(null) 'null' > JSON.stringify(NaN) 'null' > JSON.stringify(Infinity) 'null' > JSON.stringify(undefined) undefined > JSON.stringify(Symbol()) undefined
- 828. Dates have a method .toJSON() that returns a string: Wrapped primitive values are unwrapped and stringified: Arrays are stringified as Array literals. Unsupported Array elements are stringified as if they were null: All other objects – except for functions – are stringified as object literals. Properties with unsupported values are omitted: Functions are not stringified: 45.3.2 JSON.parse(text, reviver?) .parse() converts a JSON text to a JavaScript value. In this section, we are ignoring the parameter reviver; it is explained §45.4 > JSON.stringify({toJSON() {return true}}) 'true' > JSON.stringify(new Date(2999, 11, 31)) '"2999-12-30T23:00:00.000Z"' > JSON.stringify(new Boolean(true)) 'true' > JSON.stringify(new Number(123)) '123' > JSON.stringify([undefined, 123, Symbol()]) '[null,123,null]' > JSON.stringify({a: Symbol(), b: true}) '{"b":true}' > JSON.stringify(() => {}) undefined
- 829. “Customizing stringification and parsing”. This is an example of using .parse(): 45.3.3 Example: converting to and from JSON The following class implements conversions from (line A) and to (line B) JSON. Converting JSON to a point: We use the static method Point.fromJson() to parse JSON and create an instance of Point. > JSON.parse('{"foo":["a","b"]}') { foo: [ 'a', 'b' ] } class Point { static fromJson(jsonObj) { // (A) return new Point(jsonObj.x, jsonObj.y); } constructor(x, y) { this.x = x; this.y = y; } toJSON() { // (B) return {x: this.x, y: this.y}; } } assert.deepEqual( Point.fromJson(JSON.parse('{"x":3,"y":5}')), new Point(3, 5) );
- 830. Converting a point to JSON: JSON.stringify() internally calls the previously mentioned method .toJSON(). Exercise: Converting an object to and from JSON exercises/json/to_from_json_test.mjs assert.equal( JSON.stringify(new Point(3, 5)), '{"x":3,"y":5}' );
- 831. 45.4 Customizing stringification and parsing (advanced) Stringification and parsing can be customized as follows: JSON.stringify(data, replacer?, space?) The optional parameter replacer contains either: An Array with names of properties. If a value in data is stringified as an object literal, then only the mentioned properties are considered. All other properties are ignored. A value visitor, a function that can transform JavaScript data before it is stringified. JSON.parse(text, reviver?) The optional parameter reviver contains a value visitor that can transform the parsed JSON data before it is returned. 45.4.1 .stringfy(): specifying which properties of objects to stringify If the second parameter of .stringify() is an Array, then only object properties, whose names are mentioned there, are included in the result: const obj = { a: 1, b: {
- 832. 45.4.2 .stringify() and .parse(): value visitors What I call a value visitor is a function that transforms JavaScript data: JSON.stringify() lets the value visitor in its parameter replacer transform JavaScript data before it is stringified. JSON.parse() lets the value visitor in its parameter reviver transform parsed JavaScript data before it is returned. In this section, JavaScript data is considered to be a tree of values. If the data is atomic, it is a tree that only has a root. All values in the tree are fed to the value visitor, one at a time. Depending on what the visitor returns, the current value is omitted, changed, or preserved. A value visitor has the following type signature: The parameters are: value: The current value. this: Parent of current value. The parent of the root value r is {'': r}. c: 2, d: 3, } }; assert.equal( JSON.stringify(obj, ['b', 'c']), '{"b":{"c":2}}'); type ValueVisitor = (key: string, value: any) => any;
- 833. Note: this is an implicit parameter and only available if the value visitor is an ordinary function. key: Key or index of the current value inside its parent. The key of the root value is ''. The value visitor can return: value: means there won’t be any change. A different value x: leads to value being replaced with x in the output tree. undefined: leads to value being omitted in the output tree. 45.4.3 Example: visiting values The following code shows in which order a value visitor sees values: const log = []; function valueVisitor(key, value) { log.push({this: this, key, value}); return value; // no change } const root = { a: 1, b: { c: 2, d: 3, } }; JSON.stringify(root, valueVisitor); assert.deepEqual(log, [ { this: { '': root }, key: '', value: root }, { this: root , key: 'a', value: 1 }, { this: root , key: 'b', value: root.b }, { this: root.b , key: 'c', value: 2 },
- 834. As we can see, the replacer of JSON.stringify() visits values top- down (root first, leaves last). The rationale for going in that direction is that we are converting JavaScript values to JSON values. And a single JavaScript object may be expanded into a tree of JSON- compatible values. In contrast, the reviver of JSON.parse() visits values bottom-up (leaves first, root last). The rationale for going in that direction is that we are assembling JSON values into JavaScript values. Therefore, we need to convert the parts before we can convert the whole. 45.4.4 Example: stringifying unsupported values JSON.stringify() has no special support for regular expression objects – it stringifies them as if they were plain objects: We can fix that via a replacer: { this: root.b , key: 'd', value: 3 }, ]); const obj = { name: 'abc', regex: /abc/ui, }; assert.equal( JSON.stringify(obj), '{"name":"abc","regex":{}}'); function replacer(key, value) { if (value instanceof RegExp) { return {
- 835. 45.4.5 Example: parsing unsupported values To JSON.parse() the result from the previous section, we need a reviver: __type__: 'RegExp', source: value.source, flags: value.flags, }; } else { return value; // no change } } assert.equal( JSON.stringify(obj, replacer, 2), `{ "name": "abc", "regex": { "__type__": "RegExp", "source": "abc", "flags": "iu" } }`); function reviver(key, value) { // Very simple check if (value && value.__type__ === 'RegExp') { return new RegExp(value.source, value.flags); } else { return value; } } const str = `{ "name": "abc", "regex": { "__type__": "RegExp", "source": "abc", "flags": "iu" }
- 836. }`; assert.deepEqual( JSON.parse(str, reviver), { name: 'abc', regex: /abc/ui, });
- 837. 45.5 FAQ 45.5.1 Why doesn’t JSON support comments? Douglas Crockford explains why in a Google+ post from 1 May 2012: I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn’t. Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin [a minifier for JavaScript] before handing it to your JSON parser.
- 838. 46 Next steps: overview of web development (bonus) 46.1 Tips against feeling overwhelmed 46.2 Things worth learning for web development 46.2.1 Keep an eye on WebAssembly (Wasm)! 46.3 Example: tool-based JavaScript workflow 46.4 An overview of JavaScript tools 46.4.1 Building: getting from the JavaScript you write to the JavaScript you deploy 46.4.2 Static checking 46.4.3 Testing 46.4.4 Package managers 46.4.5 Libraries 46.5 Tools not related to JavaScript You now know most of the JavaScript language. This chapter gives an overview of web development and describes next steps. It answers questions such as: What should I learn next for web development? What JavaScript-related tools should I know about?
- 839. 46.1 Tips against feeling overwhelmed Web development has become a vast field: Between JavaScript, web browsers, server-side JavaScript, JavaScript libraries, and JavaScript tools, there is a lot to know. Additionally, everything is always changing: some things go out of style, new things are invented, etc. How can you avoid feeling overwhelmed when faced with this constantly changing vastness of knowledge? Focus on the web technologies that you work with most often and learn them well. If you do frontend development, that may be JavaScript, CSS, SVG, or something else. For JavaScript: Know the language, but also try out one tool in each of the following categories (which are covered in more detail later). Compilers: compile future JavaScript or supersets of JavaScript to normal JavaScript. Bundlers: combine all modules used by a web app into a single file (a script or a module). That makes loading faster and enables dead code elimination. Static checkers. For example: Linters: check for anti-patterns, style violations, and more. Type checkers: type JavaScript statically and report errors. Test libraries and tools
- 840. Version control (usually git) Trust in your ability to learn on demand It is commendable to learn something out of pure curiosity. But I’m wary of trying to learn everything and spreading yourself too thin. That also induces an anxiety of not knowing enough (because you never will). Instead, trust in your ability to learn things on demand!
- 841. 46.2 Things worth learning for web development These are a few things worth learning for web development: Browser APIs such as the Document Object Model (DOM), the browsers’ representation of HTML in memory. They are the foundations of any kind of frontend development. JavaScript-adjacent technologies such as HTML and CSS. Frontend frameworks: When you get started with web development, it can be instructive to write user interfaces without any libraries. Once you feel more confident, frontend frameworks make many things easier, especially for larger apps. Popular frameworks include React, Angular, Vue, Ember, Svelte. Node.js is the most popular platform for server-side JavaScript. But it also lets you run JavaScript in the command line. Most JavaScript-related tools (even compilers!) are implemented in Node.js-based JavaScript and installed via npm. A good way to get started with Node.js, is to use it for shell scripting. JavaScript tooling: Modern web development involves many tools. Later in this chapter, there is an overview of the current tooling ecosystem. Progressive web apps: The driving idea behind progressive web apps is to give web apps features that, traditionally, only native
- 842. apps had – for example: native installation on mobile and desktop operating systems; offline operation; showing notifications to users. Google has published a checklist detailing what makes a web app progressive. The minimum requirements are: The app must be served over HTTPS (not the unsecure HTTP). The app must have a Web App Manifest file, specifying metadata such as app name and icon (often in multiple resolutions). The file(s) of the icon must also be present. The app must have a service worker: a base layer of the app that runs in the background, in a separate process (independently of web pages). One of its responsibilities is to keep the app functioning when there is no internet connection. Among others, two mechanisms help it do that: It is a local proxy that supervises all of the web resource requests of the app. And it has access to a browser’s cache. Therefore, it can use the cache to fulfill requests when the app is offline – after initially caching all critical resources. Other capabilities of service workers include synchronizing data in the background; receiving server-sent push messages; and the aforementioned showing notifications to users. One good resource for learning web development – including and beyond JavaScript – is MDN web docs.
- 843. 46.2.1 Keep an eye on WebAssembly (Wasm)! WebAssembly is a universal virtual machine that is built into most JavaScript engines. You get the following distribution of work: JavaScript is for dynamic, higher-level code. WebAssembly is for static, lower-level code. For static code, WebAssembly is quite fast: C/C++ code, compiled to WebAssembly, is about 50% as fast as the same code, compiled to native (source). Use cases include support for new video formats, machine learning, gaming, etc. WebAssembly works well as a compilation target for various languages. Does this mean JavaScript will be compiled to WebAssembly or replaced by another language? 46.2.1.1 Will JavaScript be compiled to WebAssembly? JavaScript engines perform many optimizations for JavaScript’s highly dynamic features. If you wanted to compile JavaScript to WebAssembly, you’d have to implement these optimizations on top of WebAssembly. The result would be slower than current engines and have a similar code base. Therefore, you wouldn’t gain anything. 46.2.1.2 Will JavaScript be replaced by another language? Does WebAssembly mean that JavaScript is about to be replaced by another language? WebAssembly does make it easier to support
- 844. languages other than JavaScript in web browsers. But those languages face several challenges on that platform: All browser APIs are based on JavaScript. The runtimes (standard library, etc.) of other languages incur an additional memory overhead, whereas JavaScript’s runtime is already built into web browsers. JavaScript is well-known, has many libraries and tools, etc. Additionally, many parts of the WebAssembly ecosystem (e.g., debugging) are works in progress. For dynamic code, JavaScript is comparatively fast. Therefore, for the foreseeable future, it will probably remain the most popular choice for high-level code. For low-level code, compiling more static languages (such as Rust) to WebAssembly is an intriguing option. Given that it is just a virtual machine, there are not that many practically relevant things to learn about WebAssembly. But it is worth keeping an eye on its evolving role in web development. It is also becoming popular as a stand-alone virtual machine; e.g., supported by the WebAssembly System Interface.
- 845. 46.3 Example: tool-based JavaScript workflow <script src="code.js"> <script src="library.js"> loads loads Figure 32: A classic, very simple web app: An HTML file refers to a JavaScript file code.js, which imbues the former with interactivity. code.js uses the library library.js, which must also be loaded by the HTML file. Fig. 32 depicts a classic web app – when web development was less sophisticated (for better and for worse): index.html contains the HTML file that is opened in web browsers. code.js contains the JavaScript code loaded and used by index.html. That code depends on the library library.js, a file that was downloaded manually and put next to code.js. It is accessed via a global variable. Note that the HTML file needs to load the dependency library.js for code.js. code.js can’t do that itself.
- 846. Since then, JavaScript workflows have become more complex. Fig. 33 shows such a workflow – one that is based on the JavaScript bundler webpack. Entry Output imports imports <script src="bundle.js"> compiled to compiled to loads a d d e d t o added to added to Figure 33: This is the workflow when developing a web app with the bundler webpack. Our web app consists of multiple modules. We tell webpack, in which one execution starts (the so-called entry point). It then analyzes the imports of the entry point, the imports of the imports, etc., to determine what code is needed to run the app. All of that code is put into a single script file. Let’s examine the pieces (data, tools, technologies) involved in this workflow: The app itself consists of multiple modules, written in TypeScript – a language that is a statically typed superset of JavaScript. Each file is an ECMAScript module, plus static type annotations.
- 847. The library used by the app is now downloaded and installed via the npm package manager. It also transparently handles transitive dependencies – if this package depends on other packages, etc. All TypeScript files are compiled to plain JS via a loader, a plugin for webpack. The tool webpack combines all plain JavaScript files into a single JavaScript script file. This process is called bundling. Bundling is done for two reasons: Downloading a single file is usually faster in web browsers. During bundling, you can perform various optimizations, such as leaving out code that isn’t used. The basic structure is still the same: the HTML file loads a JavaScript script file via a <script> element. However: The code is now modular without the HTML file having to know the modules. bundle.js only includes the code that is needed to run the app (vs. all of library.js). We used a package manager to install the libraries that our code depends on. The libraries aren’t accessed via global variables but via ES module specifiers. In modern browsers, you can also deliver the bundle as a module (vs. as a script file).
- 848. 46.4 An overview of JavaScript tools Now that we have seen one workflow, let’s look at various categories of tools that are popular in the world of JavaScript. You’ll see categories of tools and lots of names of specific tools. The former are much more important. The names change, as tools come into and out of style, but I wanted you to see at least some of them. 46.4.1 Building: getting from the JavaScript you write to the JavaScript you deploy Building JavaScript means getting from the JavaScript you write to the JavaScript you deploy. The following tools are often involved in this process: Transpilers: A transpiler is a compiler that compiles source code to source code. Two transpilers that are popular in the JavaScript community are: Babel compiles upcoming and modern JavaScript features to older versions of the language. That means you can use new features in your code and still run it on older browsers. TypeScript is a superset of JavaScript. Roughly, it is the latest version of JavaScript plus static typing.
- 849. Minifiers: A minifier compiles JavaScript to equivalent, smaller (as in fewer characters) JavaScript. It does so by renaming variables, removing comments, removing whitespace, etc. For example, given the following input: A minifier might produce: Popular minifiers include: UglifyJS, babel-minify, Terser, and Closure Compiler. Bundlers: compile and optimize the code of a JavaScript app. The input of a bundler is many files – all of the app’s code plus the libraries it uses. A bundler combines these input files to produce fewer output files (which tends to improve performance). A bundler minimizes the size of its output via techniques such as tree-shaking. Tree-shaking is a form of dead code elimination: only those module exports are put in the output that are imported somewhere (across all code, while considering transitive imports). It is also common to perform compilation steps such as transpiling and minification while bundling. In these cases, a let numberOfOccurrences = 5; if (Math.random()) { // Math.random() is not zero numberOfOccurrences++ } let a=5;Math.random()&&a++;
- 850. bundler relies on the previously mentioned tools, packaged as libraries. Popular bundlers include webpack, browserify, Rollup, and Parcel. All of these tools and build steps are usually coordinated via so-called task runners (think “make” in Unix). There are: Dedicated task runners: grunt, gulp, broccoli, etc. Tools that can be used as simple task runners: npm (via its “scripts”) and webpack (via plugins). 46.4.2 Static checking Static checking means analyzing source code statically (without running it). It can be used to detect a variety of problems. Tools include: Linters: check the source code for problematic patterns, unused variables, etc. Linters are especially useful if you are still learning the language because they point out if you are doing something wrong. Popular linters include JSLint, JSHint, ESLint Code style checkers: check if code is formatted properly. They consider indentation, spaces after brackets, spaces after commas, etc. Example: JSCS (JavaScript Code Style checker) Code formatters: automatically format your code for you, according to rules that you can customize.
- 851. Example: Prettier Type checkers: add static type checking to JavaScript. Popular type checkers: TypeScript (which is also a transpiler), Flow. 46.4.3 Testing JavaScript has many testing frameworks – for example: Unit testing: Jasmine, Mocha, AVA, Jest, Karma, etc. Integration testing: Jenkins, Travis CI, etc. User interface testing: CasperJS, Protractor, Nightwatch.js, TestCafé, etc. 46.4.4 Package managers The most popular package manager for JavaScript is npm. It started as a package manager for Node.js but has since also become dominant for client-side web development and tools of any kind. There are alternatives to npm, but they are all based in one way or another on npm’s software registry: Yarn is a different take on npm; some of the features it pioneered are now also supported by npm. pnpm focuses on saving space when installing packages locally. 46.4.5 Libraries
- 852. Various helpers: lodash (which was originally based on the Underscore.js library) is one of the most popular general helper libraries for JavaScript. Data structures: The following libraries are two examples among many. Immutable.js provides immutable data structures for JavaScript. Immer is an interesting lightweight alternative to Immutable.js. It also doesn’t mutate the data it operates on, but it works with normal objects and Arrays. Date libraries: JavaScript’s built-in support for dates is limited and full of pitfalls. The chapter on dates lists libraries that you can use instead. Internationalization: In this area, ECMAScript’s standard library is complemented by the ECMAScript Internationalization API (ECMA-402). It is accessed via the global variable Intl and available in most modern browsers. Implementing and accessing services: The following are two popular options that are supported by a variety of libraries and tools. REST (Representative State Transfer) is one popular option for services and based on HTTP(S). GraphQL is more sophisticated (for example, it can combine multiple data sources) and supports a query language.
- 853. 46.5 Tools not related to JavaScript Given that JavaScript is just one of several kinds of artifacts involved in web development, more tools exist. These are but a few examples: CSS: Minifiers: reduce the size of CSS by removing comments, etc. Preprocessors: let you write compact CSS (sometimes augmented with control flow constructs, etc.) that is expanded into deployable, more verbose CSS. Frameworks: provide help with layout, decent-looking user interface components, etc. Images: Automatically optimizing the size of bitmap images, etc.
- 854. 47 Index Symbol !x ++x x++ +x , (comma operator) --x x-- -x x && y x + y x - y x / y x << y x === y x >>> y x >> y x & y x ** y x * y x ^ y x ¦ y x ¦¦ y x ٪ y
- 855. = c ? t : e __proto__ ~x A accessor (object literal) addition AMD module anonymous function expression argument argument vs. parameter Array Array hole Array index Array literal Array, dense Array, multidimensional Array, roles of an Array, sparse Array-destructuring Array-like object ArrayBuffer arrow function ASI (automatic semicolon insertion) assert (module) assertion assignment operator
- 856. async async function async function* async-await asynchronous generator asynchronous iterable asynchronous iteration asynchronous iterator asynchronous programming attribute of a property automatic semicolon insertion (ASI) await (async function) await (asynchronous generator) B big endian binary integer literal binding (variable) bitwise And bitwise Not bitwise Or bitwise Xor boolean Boolean() bound variable break bundler bundling
- 857. C call stack callback (asynchronous pattern) callback function camel case catch class class class declaration class definition class expression class, mixin classes, private data for closure code point code unit coercion comma operator CommonJS module comparing by identity comparing by value computed property key concatenating strings conditional operator console console.error() console.log() const
- 858. constant constructor function (role of an ordinary function) continue Converting to [type] Coordinated Universal Time (UTC) copy object deeply copy object shallowly currying D dash case DataView date date time format decimal floating point literal decimal integer literal decrementation operator (prefix) decrementation operator (suffix) deep copy of an object default export default value (destructuring) default value (parameter) delete deleting a property dense Array descriptor of a property destructive operation destructuring
- 859. destructuring an Array destructuring an object dictionary (role of an object) direct method call dispatched method call divided by operator division do-while dynamic this dynamic vs. static E early activation Ecma ECMA-262 ECMAScript ECMAScript module Eich, Brendan endianness (Typed Arrays) enumerability enumerable (property attribute) environment (variables) equality operator ES module escaping HTML eval() evaluating an expression event (asynchronous pattern)
- 860. event loop exception exercises, getting started with exponentiation export export default export, default export, named expression extends external iteration extracting a method F false falsiness falsy finally flags (regular expression) Float32Array Float64Array floating point literal for for-await-of for-in for-of free variable freezing an object
- 861. fulfilled (Promise state) function declaration function expression, anonymous function expression, named function, arrow function, ordinary function, roles of an ordinary function, specialized function* G garbage collection generator, asynchronous generator, synchronous getter (object literal) global global object global scope global variable globalThis GMT (Greenwich Mean Time) grapheme cluster Greenwich Mean Time (GMT) H heap hexadecimal integer literal hoisting
- 862. hole in an Array I identifier identity of an object if IIFE (immediately invoked function expression) immediately invoked function expression (IIFE) import import() import, named import, namespace in incrementation operator (prefix) incrementation operator (suffix) index of an Array Infinity inheritance, multiple inheritance, single instanceof instanceof Int16Array Int32Array Int8Array integer integer, safe internal iteration iterable (asynchronous)
- 863. iterable (synchronous) iteration, asynchronous iteration, external iteration, internal iteration, synchronous iterator (asynchronous) iterator (synchronous) J JSON (data format) JSON (namespace object) K kebab case keyword L label left shift operator let lexical this listing properties little endian logical And logical Not logical Or M
- 864. Map Map Map vs. object Math (namespace object) method method (object literal) method (role of an ordinary function) method call, direct method call, dispatched method, extracting a minification minifier minus operator (binary) minus operator (unary) mixin class module specifier module, AMD module, CommonJS multidimensional Array multiple inheritance multiple return values multiplication N named export named function expression named import named parameter
- 865. namespace import NaN node_modules npm npm package null number Number() O object object literal object vs. Map object vs. primitive value Object() object, copy deeply object, copy shallowly object, freezing an object, identity of an object, roles of an object-destructuring Object.is() octal integer literal ordinary function ordinary function, roles of an override a property P
- 866. package, npm package.json parameter parameter default value parameter vs. argument partial application passing by identity passing by value pattern (regular expression) pending (Promise state) plus operator (binary) plus operator (unary) polyfill polyfill, speculative ponyfill primitive value primitive value vs. object private data for classes progressive web app prollyfill Promise Promise, states of a properties, listing property (object) property attribute property descriptor property key property key, computed
- 867. property key, quoted property name property symbol property value shorthand property, deleting a prototype prototype chain publicly known symbol Q quizzes, getting started with quoted property key R real function (role of an ordinary function) receiver record (role of an object) RegExp regular expression regular expression literal rejected (Promise state) remainder operator REPL replica RequireJS reserved word rest element (Array-destructuring) rest parameter (function call)
- 868. rest property (object-destructuring) return values, multiple revealing module pattern roles of an Array roles of an object roles of an ordinary function run-to-completion semantics S safe integer scope of a variable script self sequence (role of an Array) Set Set setter (object literal) settled (Promise state) shadowing shallow copy of an object shim signed right shift operator single inheritance sloppy mode snake case sparse Array specialized function specifier, module
- 869. speculative polyfill spreading (...) into a function call spreading into an Array literal spreading into an object literal statement states of a Promise static static vs. dynamic strict mode string String() subclass subtraction switch symbol symbol, publicly known synchronous generator synchronous iterable synchronous iteration synchronous iterator syntax T tagged template task queue task runner TC39 TC39 process
- 870. TDZ (temporal dead zone) Technical Committee 39 template literal temporal dead zone ternary operator this this, dynamic this, lexical this, pitfalls of this, values of throw time value times operator to the power of operator transpilation transpiler tree-shaking true truthiness truthy try tuple (role of an Array) type type hierarchy type signature Typed Array typeof TypeScript
- 871. U Uint16Array Uint32Array Uint8Array Uint8ClampedArray undefined underscore case Unicode Unicode Transformation Format (UTF) unit test unsigned right shift operator UTC (Coordinated Universal Time) UTF (Unicode Transformation Format) UTF-16 UTF-32 UTF-8 V variable, bound variable, free variable, scope of a void operator W Wasm (WebAssembly) WeakMap WeakMap
- 872. WeakSet WeakSet Web Worker WebAssembly while window wrapper types (for primitive types) Y yield (asynchronous generator) yield (synchronous generator) yield* (asynchronous generator) yield* (synchronous generator) Z Z (Zulu Time Zone) Zulu Time Zone (Z)