FP & TS chapter 2: Immutability

Published: 04-01-2024 webdevelopment

The previous article explained what side effects are and why it is important to prevent them and to write pure functions.

The example of a function with a side effect was:

const add = (x: number, y: number): number => {
  const sum = x + y;
  window.z = sum;
  return sum;

Let’s look at a similar example, but without the global window variable. Instead, an array that is declared outside the function is updated:

const heroes: string[] = ['Dent', 'Prefect'];

const addHero = (h: string) => {


Although the heroes array is declared with const, that just means that it can’t be reassigned, e.g. with heroes = []. In fact, the array itself can still be mutated. This is the default behavior for data structures in JavaScript. That is why addHero causes a side-effect, it mutates the heroes array that was declared outside the scope of the function, so when the heroes array is accessed after calling addHero, it will contain the value Marvin.

Typescript offers the readonly modifier to make the compiler prevent calling mutating methods on a data structure:

const heroes: readonly string[] = ['Dent', 'Prefect'];

const addHero = (h: string) => {
  heroes.push(h); // TS2339 property 'push' does not exist on type 'readonly string[]'


Copying instead of mutating

Instead you should create a new array and return that inside of addHero. You could create an intermediary array in addHero and push to it, and then return it. In functional style you would do this without an intermediary variable. TypeScript has a very readable way to do this, with the spread operator:

type Heroes = readonly string[];
const heroes: Heroes = ['Dent', 'Prefect'];

const addHero = (hs: Heroes, h: string) => {
  return [...hs, h];

const newHeroes: Heroes = addHero(heroes, 'Marvin');

This copies the values of the heroes array and the new value to a new array, preventing side effects on the heroes array. This also applies to objects:

interface Hero {
  readonly name: string;

const dent: Hero = {
  name: 'Dent'

const updateName = (h: Hero, n: string) => {
  // h.name = n; // TS2540 cannot assign to 'name' because it is a read-only property
  // return h;
  return {
    name: n

const newDent = updateName(dent, 'Arthur');

Mutating methods

The mutating methods pop, shift, and unshift on arrays can all be solved with similar solutions to push:

const addHeroToStart = (hs: Heroes, h: string) => {
  // Instead of: hs.unshift(h); return hs;
  return [h, ...hs];
const lastHero = (hs: Heroes, h: string) => {
  // Instead of: return hs.pop();
  return hs.at(-1);
const firstHero = (hs: Heroes, h: string) => {
  // Instead of: return hs.shift();
  return hs.at(1);

It it also possible to write similar solutions for the mutating methods sort, splice, and reverse. But we don’t have to! Modern browsers benefit from new copying versions of these methods. It is just a matter of replacing them and returning directly:

// mutating method:
return hs;
// copying method:
return hs.toReversed();

There are also other mutating methods:


Besides marking properties on objects as readonly to let the TypeScript compliler notice unintended mutations, we can also add runtime protection by sealing or freezing objects with Object.seal() or Object.freeze(). Freezing offers the most protection of the two: existing properties are made immutable and can’t be reassigned. It will fail silently, or when called from a strict mode context, it will throw a TypeError.

This works for Objects as well as Arrays, for the complete documentation and examples see MDN.


Instead of freezing objects and arrays manually, there are libraries that provide immutable data structures. E.g. the aptly named Immutable provides several collection types, like Map. ECMAScript natively already has a collection type Map, but that one is mutable. Immutable’s Map returns a new Map when the original Map was modified, for instance by setting a property.

Another library is Immer. It is used internally in e.g. Redux Toolkit. It converts data structures to be immutable while preserving the mutable methods. To clarify:

import { produce } from 'immer';

const nextState = produce(baseState, (draft) => {
  draft[1].done = true;
  draft.push({ title: 'Tweet about it' });

In this example, draft is not mutable. It was made immutable by Immer. But the method push still updates it like you would expect from mutable object. It does this by internally copying the object.

While the library itself is solid, it introduces one drawback that you should be very wary of in my opinion: someone who is not familiar with Immer might be confused and think this object is mutable. At a bad moment, it may even be copy-pasted to a place that is not protected by Immer. That’s why I would not disregard the more verbose, but explicitly immutable, alternatives.

In the next chapter we will look at the functional operators.

Acknowledgement: this article was inspired by the course Functional-Light JavaScript, v3 by Kyle Simpson.