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.

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by Janeth Kent Date: 09-08-2019 javascript coding functions webdev hits : 8238  
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.


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