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JavaScript Loops - GeeksforGeeks

Last Updated : 26 Dec, 2024

Loops in JavaScript are used to reduce repetitive tasks by repeatedly executing a block of code as long as a specified condition is true. This makes code more concise and efficient.

Suppose we want to print 'Hello World' five times. Instead of manually writing the print statement repeatedly, we can use a loop to automate the task and execute it based on the given condition.

for (let i = 0; i < 5; i++) {
    console.log("Hello World!");
}

Hello World!
Hello World!
Hello World!
Hello World!
Hello World!

Let's now discuss the different types of loops available in JavaScript

The for loop repeats a block of code a specific number of times. It contains initialization, condition, and increment/decrement in one line.

Syntax

for (initialization; condition; increment/decrement) {
    // Code to execute
}

for (let i = 1; i <= 3; i++) {
    console.log("Count:", i);
}

Count: 1
Count: 2
Count: 3

In this example

• Initializes the counter variable (let i = 1).
• Tests the condition (i <= 3); runs while true.
• Executes the loop body and increments the counter (i++).

The while loop executes as long as the condition is true. It can be thought of as a repeating if statement. 

Syntax

while (condition) {
    // Code to execute
}

let i = 0;
while (i < 3) {
    console.log("Number:", i);
    i++;
}

Number: 0
Number: 1
Number: 2

In this example

• Initializes the variable (let i = 0).
• Runs the loop body while the condition (i < 3) is true.
• Increments the counter after each iteration (i++).

The do-while loop is similar to while loop except it executes the code block at least once before checking the condition.

Syntax

do {
    // Code to execute
} while (condition);

let i = 0;
do {
    console.log("Iteration:", i);
    i++;
} while (i < 3);

Iteration: 0
Iteration: 1
Iteration: 2

In this example:

• Executes the code block first.
• Checks the condition (i < 3) after each iteration.

The for...in loop is used to iterate over the properties of an object. It only iterate over keys of an object which have their enumerable property set to “true”.

Syntax

for (let key in object) {
    // Code to execute
}

const obj = { name: "Ashish", age: 25 };
for (let key in obj) {
    console.log(key, ":", obj[key]);
}

name : Ashish
age : 25

In this example:

• Iterates over the keys of the person object.
• Accesses both keys and values.

The for...of loop is used to iterate over iterable objects like arrays, strings, or sets. It directly iterate the value and has more concise syntax than for loop.

Syntax

for (let value of iterable) {
    // Code to execute
}

let a = [1, 2, 3, 4, 5];
for (let val of a) {
    console.log(val);
}

• Use for loop when the number of iterations is known.
• Use while loop when the condition depends on dynamic factors.
• Use do-while loop to ensure the block executes at least once.
• Use for...in loop to iterate over object properties.
• Use for...of loop for iterating through iterable objects.

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