JavaScript For In Loop

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.

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 iterate over the properties of an array, but it is not recommended. for..in is mainly suitable for objects.

For arrays, we should use the following loops:

• for…of Loop: If we need to use continue or break inside the loop.
• forEach() Method: If we need to execute something for all elements without any condition.

Example: Using for…in for Arrays (Not Recommended)

const a = [1, 2, 3, 4, 5];
 
for (const i in a) {
    console.log(a[i]);
}

Output

1
2
3
4
5

Important Facts About for…in Loop

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