JavaScript For Loop
Loops can execute a block of code a number of times.
JavaScript Loops
Loops are handy, if you want to run the same code over and over again, each time with a different value.
Often this is the case when working with arrays:
Instead of writing:
text += cars[0] + "<br>";
text += cars[1] + "<br>";
text += cars[2] + "<br>";
text += cars[3] + "<br>";
text += cars[4] + "<br>";
text += cars[5] + "<br>";
You can write:
for (let i = 0; i < cars.length; i++) {
text += cars[i] + "<br>";
}
Try it Yourself »
Different Kinds of Loops
JavaScript supports different kinds of loops:
for- loops through a block of code a number of timesfor/in- loops through the properties of an objectfor/of- loops through the values of any iterablewhile- loops through a block of code while a specified condition is truedo/while- also loops through a block of code while a specified condition is true
The For Loop
The for statement creates a loop with 3 optional expressions:
for (expression 1; expression 2; expression 3) {
// code block to be executed
}
Expression 1 is executed (one time) before the execution of the code block.
Expression 2 defines the condition for executing the code block.
Expression 3 is executed (every time) after the code block has been executed.
From the example above, you can read:
Expression 1 sets a variable before the loop starts (let i = 0).
Expression 2 defines the condition for the loop to run (i must be less than 5).
Expression 3 increases a value (i++) each time the code block in the loop has been executed.
How to use Expression 1
Expression 1 is used to initialize the variable(s) used in the loop (let i = 0).
But, expression 1 is optional.
You can omit expression 1 when your values are set before the loop starts:
Example
let i = 2;
let len = cars.length;
let text = "";
for (; i < len; i++) {
text += cars[i] + "<br>";
}
Try it Yourself »
You can intiate many values in expression 1 (separated by comma):
Example
for (i = 0, len = cars.length, text = ""; i < len; i++) {
text += cars[i] + "<br>";
}
Try it Yourself »
How to use Expression 2
Expression 2 is used to evaluate the condition of the initial variable (i < len).
But, expression 2 is also optional.
If expression 2 returns true, the loop will start over again. If it returns false, the loop will end.
Note
If you omit expression 2, you must provide a break inside the loop. Otherwise the loop will never end. This will crash your browser. Read about breaks in a later chapter of this tutorial.
How to use Expression 3
Expression 3 increments the value of the initial variable (i++).
But, expression 3 is also optional.
Expression 3 can do anything like negative increment (i--), positive increment (i = i + 15), or anything else.
Expression 3 can also be omitted (like when you increment your values inside the loop):
Example
let i = 0;
let len = cars.length;
let text = "";
for (; i < len; ) {
text += cars[i] + "<br>";
i++;
}
Try it Yourself »
Loop Scope
Using var in a loop:
Using let in a loop:
In the first example, using var, the variable declared in
the loop redeclares the variable outside the loop.
In the second example, using let, the variable declared in
the loop does not redeclare the variable outside the loop.
When let is used to declare the i variable in a loop, the i
variable will only be visible within the loop.
