JavaScript For Loop
For Loops can execute a block of code a number of times.
For Loops are fundamental for tasks like performing an action multiple times.
The For Loop
The for statement creates a loop with 3 optional expressions:
for (exp 1; exp 2; exp 3) {
// code block to be executed
}
exp 1 is executed (one time) before the execution of the code block.
exp 2 defines the condition for executing the code block.
exp 3 is executed (every time) after the code block has been executed.
Example
for (let i = 0; i < 5; i++) {
text += "The number is " + i + "<br>";
}
Output
JavaScript Loops
The for Loop
BMW
Volvo
Saab
Ford
From the example above, you can read:
exp 1 sets a variable before the loop starts (let i = 0).
exp 2 defines the condition for the loop to run (i must be less than 5).
exp 3 increases a value (i++) after the code block has been executed.
How to use exp 1
exp 1 is used to initialize the variable(s) used in the loop (let i = 0).
exp 1 is optional.
You can omit exp 1 if the value is set before the loop starts:
Example
const cars = ["BMW", "Volvo", "Saab", "Ford"];
let len = cars.length;
let i = 2;
let text = "";
for (; i < len; i++) {
text += cars[i] + "<br>";
}
How to use exp 2
exp 2 is used to evaluate the condition of the initial variable (i < len).
exp 2 is also optional.
If exp 2 returns false, the loop will end.
If you omit exp 2, you must provide a break inside the loop.
Otherwise the loop will never end.
This will crash your browser.
How to use exp 3
exp 3 increments the value of the initial variable (i++).
exp 3 is optional.
exp 3 can do anything like negative increment (i--), positive increment (i = i + 15), or anything else.
exp 3 can be omitted (if you increment the value inside the loop):
Example
const cars = ["BMW", "Volvo", "Saab", "Ford"];
let len = cars.length;
let i = 0;
let text = "";
for (; i < len; ) {
text += cars[i] + "<br>";
i++;
}
Output
JavaScript Loops
The for Loop
BMW
Volvo
Saab
Ford
Loop Scope
Using var in a loop:
Example
var i = 5;
for (var i = 0; i < 10; i++) {
// some code
}
// Here i is 10
Output
JavaScript Scope
Loop Scope
10
JavaScript While Loops
While Loops
While loops execute a block of code as long as a specified condition is true.
JavaScript have two types of while loops:
The while loop
The do while loop
The While Loop
The while loop loops through a block of code as long as a specified condition is true.
Syntax
while (condition) {
// code block to be executed
}
The Do While Loop
The do while loop is a variant of the while loop. This loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true.
Syntax
do {
// code block to be executed
}
while (condition);
The do while runs at least once, even if the condition is false from the start.
This is because the code block is executed before the condition is tested:
Example
do {
text += "The number is " + i;
i++;
}
while (i < 10);
Output
JavaScript Loops
The do...while Loop
The number is 0
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5
The number is 6
The number is 7
The number is 8
The number is 9