Notes mostly about software engineering and what I’m working on.

19 Jun 2017

for...in vs for...of Loops in JavaScript


The for..in and for..of loops, gives us a clean and concise syntax to iterate on iterable items like arrays, strings, objects, and enumerables. Now the question is where to use which. Here’s a little reminder to get you you started.


Use this to iterate over the properties of an object:

let person = {
    name: 'Francis',
    alias: 'codehakase',
    eyeColour: 'brown'
for(let key in person) {
    console.log( `${key} => ${person[key]}` );

// name => Francis
// alias => codehakase

The for..in loop can also be used to iterate over indexed values of a string:

let text = 'JavaScript';
for (let i in text) {
    console.log(`Index of ${str[index]}: ${i}`);
// Index of J: 0
// Index of a: 1


Use this to iterate over the values in an iterable like an array:

let girls = ['Jane', 'Lilian', 'Tobi'];
let boys = ['John', 'Tom', 'Kelvin'];
for (let crush of girl_crush) {
    // lucky crush for them boys
    let index = Math.floor(Math.random() * names.length);
    console.log( `${boys[index} crushes on ${girls}` );
// Tom crushes on Tobi
// Kelvin crushes on Lilian
// John crushes on Jane

The for..of loop can also iterate over maps, generators, and DOM node collections

Now you know where and when to use the for..in and for..of loops. Did I miss anything? Let me know in comments section.