Jon's Programming Blog

Functional Immutability in JavaScript

This is taken from Michael Fogus’ book Functional JavaScript in the chapter Purity, Immutability, and Policies for Change.

Immutability is impossible to achieve in JavaScript without using Object.freeze but it only performs makes the object immutable shallowing, you have to roll your own deepFreeze if you would like to make the object truly immutable.

So there are some practices you can do to make your functions and variables perform in a more immutable manner.

var SaferQueue = function(elems) {
  this._q = _.clone(elems);

If you need to you can use a deepClone instead.

SaferQueue.prototype = {
  enqueue: function(thing) {
    return new SaferQueue(cat(this._q, [thing]));

Where cat concatenates two arrays without changing the values of the original arrays.

And don’t forget to encapsulate the object instatiation behind a function.

Prototypes & Compositions with Objects Functional or Object Oriented JavaScript?