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Immutability JS: React

State immutability is very important when it comes to working with React. It's easy to follow when working with primitive data types, but an untrained user may have difficulty with composite ones, such as objects and arrays.

This lesson discusses the ways to update objects and arrays partially. In addition to examples in pure JavaScript, we'll also show how to use the immutability-helper library to facilitate such operations. It helps to work with upgrades where the JavaScript code is too complex.


Adding elements to an array

The simplest method is adding to an array:

const items = ['one', 'two', 'three'];
const item = 'four';
const newItems = [...items, item];
// ['one', 'two', 'three', 'four'];

If you want to add an element to the beginning, all you have to do is swap the elements of the array:

const newItems = [item, ...items];
// ['four', 'one', 'two', 'three'];

Using immutability-helper

import update from 'immutability-helper';

const state1 = ['x'];
const state2 = update(state1, { $push: ['y'] }); // ['x', 'y']

Deleting from an array

It is a more curious example. If you want to remove an element from an array, you should know what to remove. It means that each item in the collection must have an identifier. We can use good old-fashioned filtering to delete something:

const newItems = items.filter((item) => !== id);

You may be wondering where the identifier inside the handler came from. And this is where closure comes to our aid:

See the Pen js_react_immutability_array_remove_element by Hexlet (@hexlet) on CodePen.

Note the way the handler is set: removeItem = (id) => (e) => { and its use of onClick={this.removeItem(id)}.

Using immutability-helper

const index = 5;
const newItems = update(items, {$splice: [[index, 1]]});

In clean JavaScript, deleting using a filter is the best way. Using immutability-helper is too complicated.

Changing an array

Unfortunately, if we do not have any additional tools, the code for our solution will get too cumbersome. We're showing it here for reference, but you shouldn't do this in real projects:

const index = items.findIndex((item) => === id);
const newItem = { ...items[index], value: 'another value' };
const newItems = [...items.slice(0, index), newItem, ...items.slice(index + 1)];

We imagine we won't have to convince you that this is too much :)

Using immutability-helper

const collection = { children: ['zero', 'one', 'two'] };
const index = 1;
const newCollection = update(collection, { children: { [index]: { $set: 1 } } });
// { children: ['zero', 1, 'two'] }

As you can see, this method is much easier and cleaner, so we recommend to use it.


Adding to an object

It is just as simple as with arrays:

const items = { a: 1, b: 2 };
const newItems = { ...items, c: 3 };
// { a: 1, b: 2, c: 3 }

Or you can do this, if the key is calculated dynamically:

const items = { a: 1, b: 2 };
const key = 'c';
const newItems = { ...items, [key]: 3 };
// { a: 1, b: 2, c: 3 }

Removing from an object

Here destructuring comes to the rescue:

const { deletedKey, ...newState } = state;

Using immutability-helper

import update from 'immutability-helper';

const state = { a: 1, c: 3 };
const updatedState = update(state, {
  $unset: ['c'],
// { a: 1 }

Changing an object

It is the same as adding:

const items = { a: 1, b: 2 };
const newItems = { ...items, a: 3 };
// { a: 3, b: 2 }

Using immutability-helper

const data = { a: 1, b: 2 };
const key = 'a';
const newData = update(data, { [key]: { $set: 3 } });
// { a: 3, b: 2 }

Deep nesting

In the examples above, you can mostly make do with the standard JavaScript tools' it's only more convenient to use third-party solutions in certain situations. It is the same in real projects, especially if you consider React's recommendations and keep the state as flat as possible.

In some situations, the data that needs to be changed isn't on the surface but deep within the structures. Unfortunately, regular JavaScript code will be huge in these situations, so you can't do without additional libraries.

import update from 'immutability-helper';

const myData = {
  x: { y: { z: 5 } },
  a: { b: [1, 2] },

const newData = update(myData, {
  x: { y: { z: { $set: 7 } } },
  a: { b: { $push: [9] } }
// => { x: { y: { z: 7 } }, a: { b: [ 1, 2, 9 ] } }

Equivalent options

The library immutability-helper isn't the only one for such tasks. Here are a few more popular tools:

  • immutable-js, based on persistent data
  • updeep, making extensive use of mapping
  • immerjs, which is probably the most popular library in JavaScript for working with immutable data

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