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 an iterable objectwhile
- 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.
Expression 1
Normally you will use expression 1 to initialize the variable used in the loop (let i = 0).
This is not always the case. JavaScript doesn't care. Expression 1 is optional.
You can initiate many values in expression 1 (separated by comma):
Example
for (let i = 0, len = cars.length, text = ""; i < len; i++) {
text += cars[i] + "<br>";
}
Try it Yourself »
And you can omit expression 1 (like 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 »
Expression 2
Often expression 2 is used to evaluate the condition of the initial variable.
This is not always the case. JavaScript doesn't care. Expression 2 is also optional.
If expression 2 returns true, the loop will start over again. If it returns false, the loop will end.
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.
Expression 3
Often expression 3 increments the value of the initial variable.
This is not always the case. JavaScript doesn't care. Expression 3 is 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.
For/Of and For/In Loops
The for/in
loop and the for/of
loop are explained in the next chapter.
While Loops
The while
loop and the do/while
are explained in the next chapters.