JavaScript For...in Loop: A Complete Guide

The for...in loop is used to iterate over the enumerable properties of an object or an array. It loops through the keys (or property names) of an object.

Syntax:

for (key in object) { 
  // Code to execute for each key 
}

key: A variable representing the current key in the iteration.
object: The object (or array) whose enumerable properties you want to loop through.

Iterating Over an Object

let user = {

name: "Alice",

age: 25,

city: "New York"

};

for (let key in user) {

console.log(`${key}: ${user[key]}`);

}

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Iterating Over an Array:

Although it's not the best practice to use for...in with arrays (use for or for...of instead), it is possible because arrays in JavaScript are also objects.

Example

let colors = ["Red", "Green", "Blue"];

for (let index in colors) {

console.log(`${index}: ${colors[index]}`);

}

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Iterating Over a String:

You can use for...in to loop through the indices of a string.

Example

let str = "hello";

for (let index in str) {

console.log(`${index}: ${str[index]}`);

}

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