JavaScript for loop

Introduction

Loops are an essential part of programming, allowing you to execute a block of code multiple times. In JavaScript, one of the most commonly used loops is the for loop.

In this guide, you’ll learn everything about the JavaScript for loop, including syntax, working, types, real-life examples, common mistakes, and interview questions.

What is a for Loop?

A for loop is a control structure that repeats a block of code as long as a specified condition is true.

It has 3 parts

1. Initialization
2. Condition
3. Increment/Decrement

Syntax of for Loop

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

Basic Example

for (let i = 1; i <= 5; i++) {
   console.log(i);
}

Output:

How for Loop Works

There are some steps in the Loop works

1. Initialization runs once
2. The condition is checked
3. Code block executes
4. Increment/decrement happens
5. Steps repeat until the condition is false

Flow Example

for (let i = 0; i < 3; i++) {
   console.log(i);
}

Execution order:

• i = 0 → print 0
• i = 1 → print 1
• i = 2 → print 2
• i = 3 → stop

Real-Life Examples: Multiplication Table

let num = 5;

for (let i = 1; i <= 10; i++) {
   console.log(num * i);
}

Output:

Types of for Loop

There are 3 types of for loops.

1. Standard for Loop

for (let i = 0; i < 5; i++) {}

2. for...in Loop

It is used for Objects and to iterate over object properties.

let user = { name: "John", age: 35 };

for (let key in user) {
   console.log(key, user[key]);
}

3. for...of Loop

It is used for arrays and to iterate over array properties.

let arr = [1, 2, 3];

for (let value of arr) {
   console.log(value);
}

Nested for Loop

When one loop runs inside another loop.

for (let i = 1; i <= 3; i++) {
   for (let j = 1; j <= 2; j++) {
       console.log(i, j);
   }
}

Output:

Break and Continue Statement in For Loop

You will see how to use break and continue statements in a for loop.

1. break Statement

It is used to stop the loop.

for (let i = 1; i <= 5; i++) {
   if (i === 3) break;
   console.log(i);
}

Output:

2. continue Statement

It is used to skip an iteration.

for (let i = 1; i <= 5; i++) {
   if (i === 3) continue;
   console.log(i);
}

Output:

Common Mistakes

Kindly check some common mistakes.

1. Infinite Loop

In the example below, the 3rd parameter should be i++ instead of i--. When this code runs, it will execute indefinitely.

for (let i = 1; i <= 5; i--) {} // never ends

2. Wrong Condition

In the example below, the code never runs.

for (let i = 0; i > 5; i++) {} 

Javascript for loop – Interview Questions

Javascript for loop – Objective Questions (MCQs)

Conclusion

The JavaScript for loop is one of the most powerful and commonly used tools for repeating tasks. It allows you to iterate over data efficiently and perform operations multiple times with minimal code.