Node.js ships with the for…of and for…in loops. These two loops provide a convenient iteration besides the common for loop. Both loops give you a clean syntax for iterations and quick access to keys or values.

This tutorial explores both loops in more detail and shows you what to look out for when using them in your code. Also, check out the sample code for for…of and for…in loops (the sample code is also linked below the title).

Node.js Series Overview

Iterables in JavaScript

Before looking at the loops, you should know what an iterable in JavaScript is. An iterable is a JavaScript object returning a function that creates an iterator for its Symbol.iterator property.

Common iterables are arrays, typed arrays, maps, sets, and array-like objects (e.g., NodeLists). Strings are iterables as well, you can loop over each character.

for…of

The for…of loop iterates through the values of an iterable:

const types = [ 'object', 'array', 'string', 'integer', 'float', 'boolean' ]

for (const type of types) {  
  console.log(`A JavaScript type is: ${type}`)
}

// output:
// A JavaScript type is: object
// A JavaScript type is: array
// A JavaScript type is: string
// A JavaScript type is: integer
// A JavaScript type is: float
// A JavaScript type is: boolean

The assigned variable in a for…of loop receives an item’s value.

Another iterable in JavaScript is the Map type. A map offers different options for iterations. For example, you could loop through a map’s values or destructure the key-value pairs and access both simultaneously. Here’s an example showing you both ways:

const cache = new Map()

cache.set('posts:1', { id: 1, title: 'Post 1' })  
cache.set('posts:2', { id: 2, title: 'Post 2' })

for (const item of cache.values()) {  
  console.log(`Cache item: ${JSON.stringify(item)}`)
}

// output:
// Cache item: {"id":1,"title":"Post 1"}
// Cache item: {"id":2,"title":"Post 2"}

// or

for (const [ key, value ] of cache) {  
  console.log(`Cache item: "${key}" with values ${JSON.stringify(value)}`)
}

// output:
// Cache item: "posts:1" with values {"id":1,"title":"Post 1"}
// Cache item: "posts:2" with values {"id":2,"title":"Post 2"}

Remember, the for…of loop provides an iterable’s values.

for…in

The for…in loop iterates through the keys of an iterable. Iterating over arrays returns the item’s index:

const names = [ 'Marcus', 'Norman', 'Christian' ]

for (const index in names) {  
  console.log(`${names[index]} is at position ${index}`)
}

// output:
// Marcus is at position 0
// Norman is at position 1
// Christian is at position 2

The for…in loop also iterates through objects returning all the keys.

const user = { name: 'Marcus', likes: 'Node.js' }

for (const key in user) {  
  console.log(`${key}: ${user[key]}`)
}

// output:
// name: Marcus
// likes: Node.js

Remember, the for…in loop provides an iterable’s keys.

Enjoy looping & make it rock!

Mentioned Resources

• Sample Code for for…of and for…in loops
• Symbol.iterator on MDN