Douglas Crockford - Programming Style and Your Brain

Programming Style
&

Your Brain
Douglas Crockford

Visual Processing.
An analogy.

Computer Programs.
The most complicated things
people make.

Programming uses
Head and Gut.
Tradeoffs.

JavaScript.
Good Parts.
Bad Parts.

JSLint.
JSLint deﬁnes a professional
subset of JavaScript.
http://www.JSLint.com/

WARNING!
JSLint will hurt your
feelings.

Left or Right?
block block {
{ ....
.... }
}

Left or Right?
block block {
{ ....
.... }
}

• Be consistent. • Everyone should do

Left or Right?
return return {
{ ok: true
ok: false };
};
• SILENT ERROR! • Works well in
JavaScript.

Prefer forms that are error
resistant.

switch statement.
The fallthrough hazard.

“That hardly ever happens”
is another way of saying
“It happens”.

A good style can help
produce better programs.
Style is should not be about
personal preference and self-
expression.

Medieval copyists
introduced lowercase, word

These innovations helped
reduce the error rate.

Good use of style can help
reduce the occurrence of

The Elements of Style

http://www.crockford.com/wrrrld/style.html

Programs must
communicate clearly to

Use elements of good
composition where
For example, use a space after
a comma, not before.

Use spaces to disambiguate
• No space between function name and (.
• One space between all other names and (.
• Wrong:
foo (bar);
return(a+b);
if(a=== 0) {…}
function foo (b) {…}
function(x) {…}

Immediately Invocable

function () {
...
}(); // Syntax error!


(function () {
...
})();


(function () {
... Dog balls
})();


(function () {
...
}()); // Neatness counts.

The Heartbreak of Automatic

x = y // <-- Missing semicolon
(function () {
...
}());

with statement.
with (o) {  o.foo = koda;
foo = koda;  o.foo = o.koda;
}  foo = koda;
 foo = o.koda;

with statement.
with (o) {  o.foo = koda;
foo = koda;  o.foo = o.koda;
}  foo = koda;
 foo = o.koda;

I am not saying that it isn’t useful.
I am saying that there is never a case where
it isn’t confusing.

Transitivity? What's That?
0 == '' // true
0 == '0' // true
'' == '0' // false
false == 'false' // false
false == '0' // true
" trn " == 0 // true
Always use ===, never ==.

If there is a feature of a
language that is sometimes
problematic, and if it can be
replaced with another feature
that is more reliable, then
always use the more reliable
feature.

Multiline string literals
var long_line_1 = "This is a
long line"; // ok

var long_line_2 = "This is a
long line"; // syntax error

Avoid forms that are
difficult to distinguish from
common errors.

if (a = b) {…}

a = b;
if (a) {…}

if (a === b) {…}

Make your programs look
like what they do.

Scope.
Block scope v function scope.

var statement.
• It gets split into two parts:
The declaration part gets hoisted to the top of the function, initializing with
undefined.
The initialization part turns into an ordinary assignment. So
function foo() {
...
var myVar = 0, myOtherVar;
• Expands into
function foo() {
var myVar = undefined,
myOtherVar = undefined;
...
myVar = 0;

Declare all variables at the
top of the function.

for (var i …) {…}
Variable i is not scoped to the loop.

Write in the language
you are writing in.

Global variables.
• Global variables are evil.
• Avoid global variables.
• When using global variables, be
explicit.
UPPER_CASE
• Global variables should be as rare as
hens teeth and stick out like a sore
thumb.

new preﬁx
Forgetting new causes a
constructor to clobber global
variables without warning.
Fixed in ES5/strict.

Constructor functions
should be named with
InitialCaps.
Nothing else should be named
with InitialCaps.

var a = b = 0;

var a = 0, b = 0;

b = 0;
var a = b;

Write in a way that clearly
communicates your intent.

For no cost, by adopting a
more rigorous style, many
classes of errors can be
automatically avoided.

if (a) b(); c();

if (a) {b(); c();}

if (a) {b();} c();

As our processes become
more agile, our coding
must be more resilient.

Bad stylists
• Under educated.
• Old school.
• Thrill seeker.
• Exhibitionist.

“That was intentional.”
“I know what I’m doing.”

Programming is the most
complicated thing that humans
do.
Computer programs must be perfect.
Humans are not good at perfect.

Designing a programming
style demands discipline.
It is not selecting features
because they are liked, or
pretty, or familiar.

The JSLint style was driven
by the need to automatically
detect defects.
Forms that can hide defects are
considered defective.

Language Subsetting.
Only a madman would use all of C++.

There will be bugs.
Do what you can to move the
odds to your favor.

Good style is good for your
gut.

The Analytical Language of John Wilkins
Jorge Luis Borges
These ambiguities, redundancies and deficiencies remind us of those which doctor
Franz Kuhn attributes to a certain Chinese encyclopedia entitled The Celestial
Emporium of Benevolent Knowledge. In its remote pages it is written that the animals
are divided into
a. belonging to the Emperor
b. embalmed
c. trained
d. piglets
e. sirens
f. fabulous
g. stray dogs
h. included in this classification
i. trembling like crazy
j. innumerables
k. drawn with a very fine camelhair brush
l. et cetera
m.just broke the vase
n. from a distance look like flies

Douglas Crockford - Programming Style and Your Brain

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