Immutable.js is a library that provides immutable collections for JavaScript, inspired by clojure[script] immutable data structures. It has been developed by Facebook.

As they explain on their website:

Immutable data cannot be changed once created, leading to much simpler application development, no defensive copying, and enabling advanced memoization and change detection techniques with simple logic.

Persistent data presents a mutative API which does not update the data in-place, but instead always yields new updated data.

In this article, we are going to go over a basic use case where immutable.js is much much faster than javascript: pushing an element to an array without modifying the original array.

In javascript the only way of doing that is by first copying the array then pushing the element to it. With immutable.js, push returns a new list with the element appended to it; and this is super fast.


Immutable lists vs. javascript arrays

First, let’s load immutable.js in the browser:


Here is a benchmark function that calculates execution time of code:

function benchmark(iterations, f) {
var start = new Date();
for (var i = 0; i < iterations; i++) {
var end = new Date();
return "Elapsed time: " + (end - start) + " msec";
benchmark(1000000, function() {});

By the way, you see that javascript function call mechanism is super fast.

All the code snippets of this page are live and interactive powered by the klipse plugin:

  1. Live: The code is executed in your browser
  2. Interactive: You can modify the code and it is evaluated as you type

Now, let’s compare the performance of:

  1. creating a javascript array vs. creating an immutable list

  2. copying and pushing an element to a javascript array vs. pushing an element to an immutable.js list

Collection creation

Creating a javascipt array of size 100000 takes almost no time:

benchmark(1, function() {
var jsArr = [];
jsArr[100000] = 42;

While creating an immutable.js list of size 100000 takes a bit of time:

benchmark(1, function() {
      immutableList = Immutable.Range(0,100000).toList();

Javascript array slice is slow

Manipulation a javascript array with slice and push is quit expensive:

var jsArr = [];
jsArr[100000] = 42;
benchmark(100,  function() {

A few remarks:

  1. slice(0) is equivalent to copying the array

  2. push modifies the array it is called on

myArr = [];

Immutable push is super fast

Pushing an element to an immutable list is super fast:

immutableList = Immutable.Range(0, 100000).toList();

benchmark(100, function() {

If you see that the elapsed time is 0 msec, it’s not a bug: it is indeed very fast. Increase by 10x the number of iterations and see what happens.

On my computer push of immutable.js is about 100x faster than push of native javascript.

Note that when pushing an element to an immutable.js list, the list is not modified. A new list is returned with the element appended to it: this is how immutable collections work.

myList = Immutable.Range(0, 10).toList(); 

Go deeper

Here are a couple of interesting articles that describe in details what are persistent data structures and why they are so fast: