Node.js — for…of vs. for…in Loops

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

  1. String Replace All Appearances
  2. Remove All Whitespace From a String in JavaScript
  3. Generate a Random ID or String in Node.js or JavaScript
  4. Remove Extra Spaces From a String in JavaScript or Node.js
  5. Remove Numbers From a String in JavaScript or Node.js
  6. Get the Part Before a Character in a String in JavaScript or Node.js
  7. Get the Part After a Character in a String in JavaScript or Node.js
  8. How to Check if a Value is a String in JavaScript or Node.js
  9. Check If a String Includes All Strings in JavaScript/Node.js/TypeScript
  10. Check if a Value is a String in JavaScript and Node.js
  11. Limit and Truncate a String to a Given Length in JavaScript and Node.js
  12. Split a String into a List of Characters in JavaScript and Node.js
  13. How to Generage a UUID in Node.js
  14. Reverse a String in JavaScript or Node.js
  15. Split a String into a List of Lines in JavaScript or Node.js
  16. Split a String into a List of Words in JavaScript or Node.js
  17. Detect if a String is in camelCase Format in Javascript or Node.js
  18. Check If a String Is in Lowercase in JavaScript or Node.js
  19. Check If a String is in Uppercase in JavaScript or Node.js
  20. Get the Part After First Occurrence in a String in JavaScript or Node.js
  21. Get the Part Before First Occurrence in a String in JavaScript or Node.js
  22. Get the Part Before Last Occurrence in a String in JavaScript or Node.js
  23. Get the Part After Last Occurrence in a String in JavaScript or Node.js
  24. How to Count Words in a File
  25. How to Shuffle the Characters of a String in JavaScript or Node.js
  26. Append Characters or Words to a String in JavaScript or Node.js (Coming soon)
  27. Check if a String is Empty in JavaScript or Node.js (Coming soon)
  28. Ensure a String Ends with a Given Character in JavaScript or Node.js (Coming soon)
  29. Left-Trim Characters Off a String in JavaScript or Node.js (Coming soon)
  30. Right-Trim Characters Off a String in JavaScript or Node.js (Coming soon)
  31. Lowercase the First Character of a String in JavaScript or Node.js (Coming soon)
  32. Uppercase the First Character of a String in JavaScript or Node.js (Coming soon)
  33. Prepend Characters or Words to a String in JavaScript or Node.js (Coming soon)
  1. Get Number of Seconds Since Epoch in JavaScript
  2. Get Tomorrow’s Date in JavaScript
  3. Increase a Date in JavaScript by One Week
  4. Add Seconds to a Date in Node.js and JavaScript
  5. Add Month(s) to a Date in JavaScript or Node.js
  6. Add Week(s) to a Date in JavaScript or Node.js
  7. Get the Current Year in JavaScript or Node.js
  8. How to Get a UNIX Timestamp in JavaScript or Node.js
  9. How to Convert a UNIX Timestamp to a Date in JavaScript or Node.js
  10. Add Days to a Date in JavaScript or Node.js
  11. Get Yesterday's Date in JavaScript or Node.js
  12. Add Minutes to a Date in JavaScript or Node.js (Coming soon)
  13. Add Hours to a Date in JavaScript or Node.js (Coming soon)
  14. Check If a Date Is Today in JavaScript or Node.js (Coming soon)
  15. Check If a Date is Tomorrow in JavaScript or Node.js (Coming soon)
  16. Check If a Date is Yesterday in JavaScript or Node.js (Coming soon)
  17. How to Format a Date YYYY-MM-DD in JavaScript or Node.js (Coming soon)

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!


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