Javascript Arrays - Immutable functions

by Janeth Kent Date: 09-08-2019 javascript coding functions webdev

In these years, we have been playing with React, which has allowed us to deeply explore Javascript and concepts such as functional programming.

This has led us to study concepts such as pure functions and immutability (both primary basis for libraries like Redux) and try to put them into practice whenever we can.

Also, of course, we have had to face bugs for having used a mutable function within a reducer without realizing it, therefore, we have decided to compile all the immutable functions that we can use when working with arrays in Javascript.

Remember that an immutable function is one that does not modify the value on which we are applying it.

For example, splice is a mutable function because it modifies the original array, while slice returns a new array.

Yes, you may think that there are already libraries such as ImmutableJS or Immutability Helper that solve this problem but we are in favour of using Vanilla javascript whenever possible instead of resorting to third party libraries in order not to overload the dependencies too much and so practice a little with basic Javascript, which is never too much.

So here we go.

Inmutable Push

function immutablePush(arr, newEntry){
return [ ...arr, newEntry ]      

Inmutable Pop

function immutablePop(arr){
return arr.slice(0, -1)     

Inmutable Unshift

function immutableUnshift(arr, newEntry){
return [ newEntry, ...arr ]

These were easy, uh, let's complicate it a little.


The default Javascript sort method modifies the original array, which will leave us two options: or write our own or the simplest, make a copy of the original array and apply that method, so that the original is not affected:

function immutableSort(arr, compareFunction) {
return [ ...arr ].sort(compareFunction)
// or...
function immutableSort(arr, compareFunction) {
return arr.slice().sort(compareFunction)


We have the same case as with sort , the reverse function of Javascript modifies the array on which it is applied so in order to obtain a reverse method that is immutable we will resort to making a copy:

function immutableSort(arr, compareFunction) {
return [ ...arr ].reverse()
// or...
function immutableSort(arr, compareFunction) {
return arr.slice().reverse()


The splice method allows us to add elements from a position inside an array (or replace them if we indicate how many we want to eliminate), but unfortunately it is mutable. However, in this case we can reimplement it to transform it into immutable.

// ES6
function immutableSplice(arr, start, deleteCount, ...items) {
return [ ...arr.slice(0, start), ...items, ...arr.slice(start + deleteCount) ]


If we want to remove an element from an array located in a certain index in an immutable way we can do it in the following way:

function immutableDelete (arr, index) {
return arr.slice(0,index).concat(arr.slice(index+1))

As you can see, almost all of them are quite simple (even if they abuse copying an array via slice ) but the important thing is to know them, to avoid the famous mutability bugs that are so complicated to detect.

People vector created by freepik -
by Janeth Kent Date: 09-08-2019 javascript coding functions webdev hits : 9061  
Janeth Kent

Janeth Kent

Licenciada en Bellas Artes y programadora por pasión. Cuando tengo un rato retoco fotos, edito vídeos y diseño cosas. El resto del tiempo escribo en MA-NO WEB DESIGN END DEVELOPMENT.


Related Posts

Dates in local format with Javascript

In the articles we have about dates in JavaScript we were missing one about how to create dates in local format with JavaScript. That is to say, being able to…

Formatting hours in Javascript

Continuing with the set of articles that talk about internationalisation elements, like the previous one where we talked about relative dates in JavaScript, we will see in this one how…

Request data with prompt in JavaScript

After having published several articles about how to manipulate arrays and dates, today I will publish a post that some of you will find too basic and others will find…

Relative dates in JavaScript

One of the interesting things about the internationalisation library represented in the Int object is the handling of relative dates in Javascript. This handling allows us to represent a date…

How to access webcam and grab an image using HTML5 and Javascript

We often use webcams to broadcast video in real time via our computer. This broadcast can be viewed, saved and even shared via the Internet. As a rule, we need…

The JavaScript forEach loop

We have already talked about how to handle some of loops  types in Javascript including for, for-in and for-off loops. In the case of today we are going to see how…

What are React Hooks and what problems they solve

Working with React, - and before the release of Hooks in version 16.8 -  there was always the possibility to create components in three different ways based on a number of…

Flattening arrays in JavaScript

When we are handling arrays that are arrays or have multiple dimensions it can be very useful to know how to flatten arrays in JavaScript. That is to say, to…

How to populate an array with random numbers in JavaScript

Some of you might think that what we explained in the article on how to populate an array with numbers, apart from the didactic part, would not have much applicability…

How to populate an array with numbers in JavaScript

Populate an array with numbers in JavaScript The first step is to initialise the array. So today we are going to see a simple way to do it and see how…

Top Javascript Libraries and Frameworks Part 2

What are JavaScript frameworks?   JavaScript frameworks are application frameworks that allow developers to manipulate code to meet their particular needs. Web application development is like building a house. You have the option…

How to Send Email from an HTML Contact Form

In today’s article we will write about how to make a working form that upon hitting that submit button will be functional and send the email (to you as a…

We use our own and third-party cookies to improve our services, compile statistical information and analyze your browsing habits. This allows us to personalize the content we offer and to show you advertisements related to your preferences. By clicking "Accept all" you agree to the storage of cookies on your device to improve website navigation, analyse traffic and assist our marketing activities. You can also select "System Cookies Only" to accept only the cookies required for the website to function, or you can select the cookies you wish to activate by clicking on "settings".

Accept All Only sistem cookies Configuration