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:
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript For Loop</h2>
<p id="demo"></p>
<script>
const cars = ["BMW", "Volvo", "Saab", "Ford", "Fiat", "Audi"];
let text = "";
for (let i = 0; i < cars.length; i++) {
text += cars[i] + "<br>";
}
document.getElementById("demo").innerHTML = text;
</script>
</body>
</html>
Result:
JavaScript For Loop
BMW
Volvo
Saab
Ford
Fiat
Audi
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.
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript For Loop</h2>
<p id="demo"></p>
<script>
let text = "";
for (let i = 0; i < 5; i++) {
text += "The number is " + i + "<br>";
}
document.getElementById("demo").innerHTML = text;
</script>
</body>
</html>
Result:
JavaScript For Loop
The number is 0
The number is 1
The number is 2
The number is 3
The number is 4
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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript For Loop</h2>
<p id="demo"></p>
<script>
const cars = ["BMW", "Volvo", "Saab", "Ford"];
let i, len, text;
for (i = 0, len = cars.length, text = ""; i < len; i++) {
text += cars[i] + "<br>";
}
document.getElementById("demo").innerHTML = text;
</script>
</body>
</html>
Result:
JavaScript For Loop
BMW
Volvo
Saab
Ford
And you can omit expression 1 (like when your values are set before the loop starts):
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript For Loop</h2>
<p id="demo"></p>
<script>
const cars = ["BMW", "Volvo", "Saab", "Ford"];
let i = 2;
let len = cars.length;
let text = "";
for (; i < len; i++) {
text += cars[i] + "<br>";
}
document.getElementById("demo").innerHTML = text;
</script>
</body>
</html>
Result:
JavaScript For Loop
Saab
Ford
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, and 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
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript For Loop</h2>
<p id="demo"></p>
<script>
const cars = ["BMW", "Volvo", "Saab", "Ford"];
let i = 0;
let len = cars.length;
let text = "";
for (; i < len; ) {
text += cars[i] + "<br>";
i++;
}
document.getElementById("demo").innerHTML = text;
</script>
</body>
</html>
Result:
JavaScript For Loop
BMW
Volvo
Saab
Ford
Loop Scope
Using var in a loop:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript let</h2>
<p id="demo"></p>
<script>
var i = 5;
for (var i = 0; i < 10; i++) {
// some statements
}
document.getElementById("demo").innerHTML = i;
</script>
</body>
</html>
Result:
JavaScript let
10
Using let in a loop:
Example
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript let</h2>
<p id="demo"></p>
<script>
let i = 5;
for (let i = 0; i < 10; i++) {
// some statements
}
document.getElementById("demo").innerHTML = i;
</script>
</body>
</html>
Result:
JavaScript let
5
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.