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JavaScript Cheat Sheet 10.34: JavaScript For in

Mastering JavaScript’s for in Loop

JavaScript is a versatile programming language widely used for web development. One of the fundamental concepts in JavaScript is looping, which allows you to repeat a certain block of code. In this tutorial, we will delve into the ‘for…in’ loop in JavaScript, a powerful construct for iterating over the properties of an object.

Understanding the Basics of Loops

Before we dive into the ‘for…in’ loop, let’s briefly recap the basics of loops in JavaScript. Loops are used to execute a block of code repeatedly. The ‘for’ loop is a common choice, but it may not be the most suitable for iterating over object properties.

Introduction to ‘for…in’ Loop

The ‘for…in’ loop is specifically designed for iterating over the enumerable properties of an object. It simplifies the process of traversing through object properties, making your code more concise and readable.

Syntax of ‘for…in’ Loop

The syntax of the ‘for…in’ loop is as follows:

for (variable in object) {
  // code to be executed
  • variable: A variable that will be assigned the name of each property during each iteration.
  • object: The object whose enumerable properties will be iterated.

Using ‘for…in’ Loop with Arrays

While ‘for…in’ is designed for objects, it can also be used with arrays to iterate over their indices. Let’s look at an example:

let fruits = ['apple', 'orange', 'banana'];

for (let index in fruits) {
  console.log(index, fruits[index]);


  • The ‘index’ variable takes on the index of each element in the ‘fruits’ array during each iteration.
  • The output will be:
  0 apple
  1 orange
  2 banana

Iterating over Object Properties

The primary use of ‘for…in’ is to iterate over object properties. Consider the following example:

let person = {
  name: 'John',
  age: 30,
  occupation: 'developer'

for (let prop in person) {
  console.log(prop, person[prop]);


  • The ‘prop’ variable takes on the property name during each iteration.
  • The output will be:
  name John
  age 30
  occupation developer

Handling Prototype Properties

Be cautious when using ‘for…in’ with objects, as it iterates over all enumerable properties, including those in the object’s prototype chain. To avoid this, use the ‘hasOwnProperty’ method:

for (let prop in person) {
  if (person.hasOwnProperty(prop)) {
    console.log(prop, person[prop]);


Mastering the ‘for…in’ loop in JavaScript is a valuable skill for efficient object traversal. It simplifies the process of working with objects and arrays, making your code more elegant and concise.

Now that you have a solid understanding of the ‘for…in’ loop, practice incorporating it into your JavaScript projects to enhance your programming skills. Happy coding!

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