The difference between for .. in and for .. of in javascript

Jun 9, 2024·
Gert de Pagter
Gert de Pagter
· 1 min read

If you want to do a for loop over an array in javascript you generally want to do a for (const value of array) {. Because this will loop over all the items of an array. A for (const key in array) { will instead loop over the keys of an array (or of an object).

const letters = ['a', 'b', 'c'];

for (const value of letters) {
  console.log(value); // 'a', 'b', 'c'

for (const key in letters) {
  console.log(key); // 0, 1, 2

My personal mnemonic to remember this is that you can do an in check to see if a key exists in javascript. So you can do a for in loop to loop over the keys. e.g.:

const person = {
  age: 30,
  job: 'Software Engineer',
  name: 'Gert',

console.log('age' in person); // true
console.log('intellect' in person); // false

So in conclusion, use for .. of to loop over values, and for .. in to loop over keys.

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