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Cloning arrays and objects in vanilla JavaScript

Today, I wanted to look at copying arrays and objects, how JavaScript handles some things under-the-hood, and some modern approaches that make things a bit easier.

Let’s dig in!

Arrays, objects, and references

Let’s imagine you have an array of wizards.

let wizards = ['Gandalf', 'Radagast', 'Merlin'];

You want to make a copy of that array, so you assign the array to another variable.

let moreWizards = wizards;

Here, moreWizards is not a copy of wizards. Instead, it’s a reference to the original array. If you were to modify one or the other, both would look the same.

Here, both wizards and moreWizards have a new item, Ursula.


// logs ["Gandalf", "Radagast", "Merlin", "Ursula"]

This is true for objects as well. If you assign an existing object to a new variable, the new variable is a reference to the original, not a copy of its values.

let merlin = {
	job: 'Wizard',
	spells: ['Turn into an animal', 'Dancing teacups']

// This is a reference to merlin
let gandalf = merlin;

How to create a copy of an array or object in JavaScript

To copy an array, you can use the Array.from() method or Array.slice() method.

let moreWizards = Array.from(wizards);
let evenMoreWizards = wizards.slice();

To copy an object, you can use the Object.assign() method, with an empty object ({}) as the first argument.

let gandalf = Object.assign({}, merlin);

For both arrays and objects, you can also use the spread syntax operator.

let stillMoreWizards = [...wizards];
let radagast = {...merlin};

Deep copies of an array or object

The Array.from(), Array.slice(), and Object.assign() methods, as well as the spread operator, all create shallow copies of an array or object.

If the array or object has a nested or array or object in it (like the spells array in the merlin object), that item is actually a reference to the original rather than a unique copy.

Here, adding a spell to the merlin.spells array also adds it to the gandalf.spells array.

// Copy merlin
let gandalf = Object.assign({}, merlin);

// Add a spell to merlin

// gandalf.spells has the new spell, too
// logs ["Turn into an animal", "Dancing teacups", "Disappear"]

To create a deep copy, one in which any nested array or objects are also unique copies, we need to use a newer JavaScript method: structuredClone().

let gandalf = structuredClone(merlin);

Merging arrays and objects

Tomorrow, we’ll look at how to merge arrays and objects together.