JavaScript For In Loop

Last Updated : 20 Nov, 2024

The JavaScript for…in loop iterates over the properties of an object. It allows you to access each key or property name of an object.

JavaScript

const car = {
    make: "Toyota",
    model: "Corolla",
    year: 2020
};

for (let key in car) {
    console.log(`${key}: ${car[key]}`);
}

Output

make: Toyota
model: Corolla
year: 2020

Syntax

for (key in object) { // Code}

The for…in loop can also works to iterate over the properties of an array, but it is not recommended. for..in is mainly suitable for objects.

For arrays, we should use below loops.

For of Loop if we need to put continue or break in the loop
forEach() if we need execute something for all elements without any condition

JavaScript

// Example of for in for arrays
// Not a recommended way to traverse
// an array
const a = [1, 2, 3, 4, 5];

for (const i in a) {
  	console.log(a[i]);
}

Output

Important Facts About for in Loop

The for…in loop is not recommended for use with arrays if maintaining index order is important.
The order of iteration in for…in loop is implementation-dependent, means the array elements may not be accessed in the expected sequence.
The order in which properties are iterated may not match the properties that are defined in the object.

JavaScript for...of Loop

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Important Facts About for in Loop

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