for-in vs for-of loops in JavaScript

Created: 28 November 2020

const arr = [1, 1, 2, 3, 5];

const obj = {
  foo: 'bar',
  foo2: ['b', 'ba', 'bar']
};

for-in on arrays prints indices

for (let i in arr) {
  console.log(i);
}

for-of on arrays prints its elements

for (let i of arr) {
  console.log(i);
}

for-in on objects prints keys

for (let i in obj) {
  console.log(i);
}

foo
foo2

for-of on objects: EXCEPTION!

for (let i of obj) {
  console.log(i);
}

for (let i of obj) {
  ^

TypeError: obj is not iterable

Summary

Loop	Arrays	Objects
for-in	indices	keys
for-of	values	EXCEPTION!