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]}`);
}

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

// 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]);
}

1
2
3
4
5

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.

for in

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