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For In vs For Of

The for…in and for…of loops are used to iterate over the properties of objects or elements of an iterable, but they do it in different ways.

For In

The for…in loop iterates over all enumerable properties of an object, including inherited ones. It gives you the keys (property names) of the object.

Example with an Object

const obj = {a: 1, b: 2, c: 3};
for (const key in obj) {
  console.log(key, obj[key]); // Logs 'a 1', 'b 2', 'c 3'

Example with an Array

const arr = [10, 20, 30]; = 'bar';
for (const key in arr) {
  console.log(key, arr[key]); // Logs '0 10', '1 20', '2 30', 'foo bar'
Be cautious using for…in with arrays, as it will also include enumerable properties that are not indices.
const numbers = [4, 8, 15, 16, 23, 42]; = "I'm a property, not a number!";
for (const key in numbers) {
  console.log(`key = ${key}, numbers[${key}] = ${numbers[key]}`);
// Output will be:
// key = 0, numbers[0] = 4
// key = 1, numbers[1] = 8
// key = 2, numbers[2] = 15
// key = 3, numbers[3] = 16
// key = 4, numbers[4] = 23
// key = 5, numbers[5] = 42
// key = foo, numbers[foo] = I'm a property, not a number!

for…in loop also iterates over the foo property, which is why this loop type must be used with caution when working with arrays.

For Of

The for…of loop iterates over the values of an iterable object (such as an array, set, or map). It gives you the values of the iterable directly.

It can't be used directly on plain objects without using methods like Object.keys, Object.values, or Object.entries.

Example with an Array

const arr = [10, 20, 30];
for (const value of arr) {
  console.log(value); // Logs '10', '20', '30'

Example with an Object

The for…of loop is designed to work with iterable objects, such as arrays, strings, maps, sets, etc. Ordinary objects are not iterable by default, so attempting to use for…of with an object will lead to an error.

However, you can convert an object's keys, values, or key-value pairs into an array and then iterate over that array with a for…of loop.

const person = {
  name: 'Alice',
  age: 30,
  profession: 'Engineer',
// Iterate over keys
for (const key of Object.keys(person)) {
  console.log(`Key: ${key}`);
// Iterate over values
for (const value of Object.values(person)) {
  console.log(`Value: ${value}`);
// Iterate over key-value pairs
for (const [key, value] of Object.entries(person)) {
  console.log(`Key: ${key}, Value: ${value}`);
// Output:
// Key: name
// Key: age
// Key: profession
// Value: Alice
// Value: 30
// Value: Engineer
// Key: name, Value: Alice
// Key: age, Value: 30
// Key: profession, Value: Engineer

In this example, Object.keys(person), Object.values(person), and Object.entries(person) return arrays of the object's keys, values, and key-value pairs, respectively, allowing you to iterate over them with a for…of loop.


  • Use for…in to iterate over the keys of an object (including non-array objects).
  • Use for…of to iterate over the values of iterables like arrays, sets, and maps.
  • Be cautious using for…in with arrays, especially if you have added custom properties, as it can lead to unexpected results.
java-script/for-in-vs-for-of.txt · Last modified: 2023/08/11 12:22 by odefta