Sofia Kelly

Sofia Kelly


JavaScript: Difference between .forEach() and .map()

Some of the most loved functions in JavaScript might be map and forEach. They both started to exist since ECMAScript 5, ores5 in short.

In this post, I am going to talk about the main difference between each and show you some examples of their usages.

Before Reading

Basically, looping over an object in JavaScript counts on whether or not the object is an iterable. Arrays are iterable by default.

map and forEach are included in the Array.prototype, so we don’t need to think about iterable. If you want to study further, I recommend you check out what an iterable object is in JavaScript!

What Are map() and forEach()?

map and forEach are helper methods in array to loop over an array easily. We used to loop over an array, like below, without any helper functions.

var array = ['1', '2', '3'];
for (var i = 0; i < array.length; i += 1) {
// 1
// 2
// 3


The for loop has been with us since the very beginning of the JavaScript era. It takes 3 expressions: the initial value, condition, and final expression.

This is a classic way of looping an array. Since ECMAScript 5, new functions have appeared to make us happier.


map does exactly the same thing as what the for loop does, except that map creates a new array with the result of calling a provided function on every element in the calling array.

It takes two parameters: a callback function that will be invoked later when map or forEach is called, and the context variable called thisArg that a callback function will use when it’s invoked.

const arr = ['1', '2', '3'];
// callback function takes 3 parameters
// the current value of an array as the first parameter
// the position of the current value in an array as the second parameter
// the original source array as the third parameter
const cb = (str, i, origin) => {
  console.log(`${i}: ${Number(str)} / ${origin}`);
// 0: 1 / 1,2,3
// 1: 2 / 1,2,3
// 2: 3 / 1,2,3


The callback function can be used as below. => { console.log(Number(str)); })

The result of map is not equal to the original array.

const arr = [1];
const new_arr = => d);
arr === new_arr; // false

You can also pass the object to the map as thisArg.

const obj = { name: 'Jane' };

[1].map(function() {
  // { name: 'Jane' }
}, obj);

[1].map(() => {
  // window
}, obj);


The object obj became the thisArg for map. But the arrow callback function can’t get obj as its thisArg.

This is because arrow functions work differently from normal functions. Visit this article to see what’s different between arrow functions and normal functions.


forEach is another looping function for an array but there’s a difference between map and forEach in use. There are two parameters that map and forEach can take — a callback function and thisArg which they use as their this.

const arr = ['1', '2', '3'];
// callback function takes 3 parameters
// the current value of an array as the first parameter
// the position of the current value in an array as the second parameter
// the original source array as the third parameter
const cb = (str, i, origin) => {
  console.log(`${i}: ${Number(str)} / ${origin}`);
// 0: 1 / 1,2,3
// 1: 2 / 1,2,3
// 2: 3 / 1,2,3


Then, what’s different?

map returns a new array of its original array. forEach, however, does not. But both of them ensure the immutability of the original object.

[1,2,3].map(d => d + 1); // [2, 3, 4];
[1,2,3].forEach(d => d + 1); // undefined;

~~ Edit ~~

forEach doesn’t ensure the immutability of an array if you change values inside an array. This method only ensures immutability when you don’t touch any values inside.

[{a: 1, b: 2}, {a: 10, b: 20}].forEach((obj) => obj.a += 1);
// [{a: 2, b: 2}, {a: 11, b: 21}]
// The array has been changed!

The example above is from Kenny Martin Rguez. Thank you! 👏

When to Use map() and forEach()?

Since the main difference between them is whether or not there is a return value, you would want to use map to make a new array and use forEach just to map over the array.

This is a simple example.

const people = [
  { name: 'Josh', whatCanDo: 'painting' },
  { name: 'Lay', whatCanDo: 'security' },
  { name: 'Ralph', whatCanDo: 'cleaning' }

function makeWorkers(people) {
  return => {
    const { name, whatCanDo } = person;
    return <li key={name}>My name is {name}, I can do {whatCanDo}</li>



In React, for example, map is used very commonly to make elements because map creates and returns a new array after manipulating data of the original array.

const mySubjectId = ['154', '773', '245'];

function countSubjects(subjects) {
  let cnt = 0;
  subjects.forEach(subject => {
    if (mySubjectId.includes( {
      cnt += 1;
  return cnt;

  { id: '223', teacher: 'Mark' },
  { id: '154', teacher: 'Linda' }
// 1


On the other hand, forEach is useful when you want to do something with the data without creating a new array. By the way, the example could be refactored using filter.

subjects.filter(subject => mySubjectId.includes(;

To summarize, I recommend you use map when creating a new array that you will use, and use forEach when you don’t need to make a new array, but rather, there’s something you want to do with the data.

Speed Comparison

Some posts mentioned map is faster than forEach. So, I was curious if it’s for real. I found this comparison result:

This is image title

The code looks very similar but the results are the opposite. Some tests said forEach is faster and some said map is faster.

Maybe you are telling yourself that map/forEach is faster than the other, and you might be right. I’m not sure, honestly. I think readability is much more important than the speed between map and forEach when it comes to modern web development.

But one thing’s for sure — both of them are slower than the built-in feature of JavaScript, for loop.


map and forEach are handy functions for looping over an iterable object and might beautify your code and give you more readability.

But one really important keynote to keep in mind is to understand what each does and when to use each.

map would be good when you want to make a new array that does not affect the original array, and forEach would be nicer when you just want to map over an array.


#javascript #programming

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JavaScript: Difference between .forEach() and .map()
Coy  Roberts

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Javascript Array forEach() Method Example

Javascript array foreach is an inbuilt function that can be used to execute a function on each item in the array. The forEach() method is called on the array Object and is passed the function that is called on each item in the array.  The  callback function can also take the second parameter of the index in case you need to reference the index of the current element in the array.

Understanding Javascript Array forEach

In a nutshell, Javascript forEach() method executes a provided function once for each array element. Javascript forEach only be used on the Arrays, Maps, and  Sets. This article briefly describes how to use the forEach() method to iterate the items of the array in JavaScript.

What’s the usual thing you do with an  array? Add or remove items from an array. Iterate through its items! This is where the forEach() array method shines.

Before we dive into seeing how forEach() works, we need to take a look at how looping works. Looping is a fundamental computer science concept. If you want to be a sound programmer, mastering loops are amidst the first steps you need to take.

Here’s an example of a for loop in Javascript.

let languages = ['Python', 'Javascript', 'PHP', 'Golang'];

for (i = 0; i < languages.length; i++) {

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Array Foreach, Map, Filter, Reduce, Concat Methods in Javascript

In this tutorial, we will see Javascript Array Foreach, Map, Filter, Reduce, Concat Methods. I dedicate this article only for these methods because, in Pure Functional Programming, this kind of method is required to perform some operations on an Array.

If you do not know What Pure Functions is, then check out my Pure Functions in Javascript article on this website.


All the programming languages have this kind of Data Structure to hold and manipulate the data and Javascript is not different.

We all know Arrayscollection of variables, and we all have used to perform some operations like Creating an array, Removing an Item from an Array, Sorting the data of an Array and other manipulations.

In Functional Programming, we are using functions like foreach, map, filter, reduce, concatAll and other Higher Order Functions. So today I am describing these functions in deep and show you how you can use it in various scenarios.

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Who invented JavaScript?

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What is JavaScript?

JavaScript is a client-side scripting language used with HTML (Hypertext Markup Language). JavaScript is an Interpreted / Oriented language called JS in programming language JavaScript code can be run on any normal web browser. To run the code of JavaScript, we have to enable JavaScript of Web Browser. But some web browsers already have JavaScript enabled.

Today almost all websites are using it as web technology, mind is that there is maximum scope in JavaScript in the coming time, so if you want to become a programmer, then you can be very beneficial to learn JavaScript.

JavaScript Hello World Program

In JavaScript, ‘document.write‘ is used to represent a string on a browser.

<script type="text/javascript">
	document.write("Hello World!");

How to comment JavaScript code?

  • For single line comment in JavaScript we have to use // (double slashes)
  • For multiple line comments we have to use / * – – * /
<script type="text/javascript">

//single line comment

/* document.write("Hello"); */


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