Samantha Ming

Posted on Mar 4, 2019 • Updated on Mar 9, 2019

How to Deep Clone an Array in JavaScript

#javascript #webdev #beginners #codenewbie

There are 2 types of array cloning: shallow & deep. Shallow copies only cover the 1st level of the array and the rest are referenced. If you want a true copy of nested arrays, you’ll need a deep clone. For deep clones, go with the JSON way OR better yet use Lodash 👍

const numbers = [1, [2], [3, [4]], 5];

// Using JavaScript
JSON.parse(JSON.stringify(numbers));

// Using Lodash
_.cloneDeep(numbers);

Arrays are Reference Types

In order to understand why there are two types of cloning. Let's dig into the fundamentals and explains what are reference types.

Unlike your primitive types (ie. number or string), arrays are reference types. Which means when you assign an array to a variable, you're assigning a memory address and not the actual array itself. WTH 😱. I know this is a bit confusing. So let's explain with an example.

Copying a Value type

So no biggie here. We're creating a copy of value. And if we change the valueCopy, it doesn't affect the original value. Makes sense - when we change the copy it shouldn't affect the original at all. All good here 👍

let value = 3;
let valueCopy = value; // create copy

console.log(valueCopy); // 3

// Change valueCopy
valueCopy = 100
console.log(valueCopy); // 100

// ✅ Original NOT affected 
console.log(value); // 3

Copying a Reference type

Okay, things are about to get weird now. Let's copy our array using the same method as we did to copy a value type.

let array = [1,2,3];
let arrayCopy = array; // create copy

console.log(arrayCopy); // [1,2,3];

// Change 1st element of the array
arrayCopy[0] = '👻';
console.log(arrayCopy); // [ '👻', 2, 3 ]

// ❌Original got affected
console.log(array); // [ '👻', 2, 3 ]

Why did the original array also got affected? That's because what you copied over is not the array itself but the pointer to the memory space the array occupies. Reference types don't hold values, they are a pointer to the value in memory.

Solution to Copying Reference Types

So the solution is to copy over the value NOT the pointer. Like this:

let array = [1,2,3];
let arrayCopy = [...array]; // create TRUE copy

console.log(arrayCopy); // [1,2,3];

// Change 1st element of the array
arrayCopy[0] = '👻';
console.log(arrayCopy); // [ '👻', 2, 3 ]

// ✅ Original NOT affected 
console.log(array); // [ 1, 2, 3 ]

Shallow vs Deep Clone

When I used spread ... to copy an array, I'm only creating a shallow copy. If the array is nested or multi-dimensional, it won't work. Let's take a look:

let nestedArray = [1, [2], 3];
let arrayCopy = [...nestedArray]; 

// Make some changes
arrayCopy[0] = '👻'; // change shallow element
arrayCopy[1][0] = '💩'; // change nested element
console.log(arrayCopy); // [ '👻', [ '💩' ], 3 ]

// ❌ Nested array got affected
console.log(nestedArray); // [ 1, [ '💩' ], 3 ]

As you can see, the shallow or first layer is fine. However, once we change the nested element, the original array also got affected. So the solution is to do a deep clone:

let nestedArray = [1, [2], 3];
let arrayCopy = JSON.parse(JSON.stringify(nestedArray)); 

// Make some changes
arrayCopy[0] = '👻'; // change shallow element
arrayCopy[1][0] = '💩'; // change nested element
console.log(arrayCopy); // [ '👻', [ '💩' ], 3 ]

// ✅ Nested array NOT affected
console.log(nestedArray); //  1, [ 2 ], 3 ]

Community Input

Values Not Compatible with JSON

@tailcall : One has to be really careful with JSON solution! It doesn't work with values not compatible with JSON. Consider using a library function if you have to work with such data.

function nestedCopy(array) {
    return JSON.parse(JSON.stringify(array));
}

// undefineds are converted to nulls
nestedCopy([1, undefined, 2]) // -> [1, null, 2]

// DOM nodes are converted to empty objects
nestedCopy([document.body, document.querySelector('p')]) // -> [{}, {}]

// JS dates are converted to strings
nestedCopy([new Date()]) // -> ["2019-03-04T10:09:00.419Z"]

deepClone vs JSON

@alfredosalzillo : I'd like you to note that there are some difference between deepClone and JSON.stringify/parse.

JSON.stringify/parse only work with Number and String and Object literal without function or Symbol properties.
deepClone work with all types, function and Symbol are copied by reference.

Here's an example:

const lodashClonedeep = require("lodash.clonedeep");

const arrOfFunction = [() => 2, {
    test: () => 3,
}, Symbol('4')];

// deepClone copy by refence function and Symbol
console.log(lodashClonedeep(arrOfFunction));
// JSON replace function with null and function in object with undefined
console.log(JSON.parse(JSON.stringify(arrOfFunction)));

// function and symbol are copied by reference in deepClone
console.log(lodashClonedeep(arrOfFunction)[0] === lodashClonedeep(arrOfFunction)[0]);
console.log(lodashClonedeep(arrOfFunction)[2] === lodashClonedeep(arrOfFunction)[2]);

Using Recursion

Tareq Al-Zubaidi: There is another simple and more performant solution to this problem. I would use recursion to solve this.

const clone = (items) => items.map(item => Array.isArray(item) ? clone(item) : item);

See comparison test here

Resources

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Top comments (30)

Maria Zaitseva • Mar 4 '19

One has to be really careful with JSON solution! It doesn't work with values not compatible with JSON. Examples:

function nestedCopy(array) {
    return JSON.parse(JSON.stringify(array));
}

// undefineds are converted to nulls

nestedCopy([1, undefined, 2]) // -> [1, null, 2]

// DOM nodes are converted to empty objects

nestedCopy([document.body, document.querySelector('p')]) // -> [{}, {}]

// JS dates are converted to strings

nestedCopy([new Date()]) // -> ["2019-03-04T10:09:00.419Z"]

Consider using a library function if you have to work with such data.

Samantha Ming • Mar 8 '19

Yes! Very good point, let me add that to the code notes! Thanks for noting that 👏

Vipin Saini • May 28 '20

Wow.
This is really cool!
You have added a gem to the post!
Thanks!

V • Mar 4 '19 • Edited

Nice summary!

I'd like to add a tiny bit to the nested array example, that when you rest-spread an array, if that array has another nested array as its element, what gets shallow-copied is the reference to the original nested array:

let nestedArray = [1, [2], 3];
let arrayCopy = [...nestedArray]; // arrayCopy == [1, ref, 3];

thus creating the mutation effects.

Samantha Ming • Mar 8 '19

Yup! In those cases, if you want a true copy, you will need to do a deep clone. Thanks for sharing!

guico33 • Mar 4 '19

As pointed out in other comments, relying on JSON.parse and JSON.stringify to perform a deep cloning is suboptimal (edge cases, performances). Better use the method provided by some library (e.g lodash's cloneDeep).

worc • Mar 4 '19

pulling in an entire library for a single method can also be a kind of suboptimal (versioning, security auditing, vendoring vs. managed dependencies). it's a lot of overhead when there might be a better solution in just not doing deeply nested arrays.

guico33 • Mar 5 '19 • Edited

My point is to use an optimised method. Deep cloning arrays or objects may not be very common but if the need arises, I'd recommend using an exisiting solution.

Samantha Ming • Mar 8 '19

Totally! JSON is the quick&dirty way 😂Lodash is definitely the preferred optimal solution. Thanks for pointing that out 👍

alokz diptim! • May 7 '22 • Edited

This right here deep copies nested array too

    let cloneMatrix  = [];

    for(let m = 0; m < matrix.length; m++){
        cloneMatrix.push([...matrix[m]]);
    }

It pushes into a new array with [...matrix[m]]

Alfredo Salzillo • Mar 4 '19 • Edited

Hi,
nice post, but i'd like you to note that there are some difference between deepClone and JSON.stringify/parse.

JSON.stringify/parse only work with Number and String and Object litteral without function or Symbol properties.
deepClone work with all types, function and Symbol are copied by reference.

Here an example:

const lodashClonedeep = require("lodash.clonedeep");

const arrOfFunction = [() => 2, {
    test: () => 3,
}, Symbol('4')];

// deepClone copy by refence function and Symbol
console.log(lodashClonedeep(arrOfFunction));
// JSON replace function with null and function in object with undefined
console.log(JSON.parse(JSON.stringify(arrOfFunction)));

// function and symbol are copied by reference in deepClone
console.log(lodashClonedeep(arrOfFunction)[0] === lodashClonedeep(arrOfFunction)[0]);
console.log(lodashClonedeep(arrOfFunction)[2] === lodashClonedeep(arrOfFunction)[2]);

Samantha Ming • Mar 8 '19

Exactly! Thanks for sharing this! Let me add it to my code notes 👍

Rishi Setpal • Jun 20 '23 • Edited

There is no point in compairing: new Object with new Object
lodashClonedeep(arrOfFunction)[0] === lodashClonedeep(arrOfFunction)[0]

Instead Compare: old Object with new Object
arrOfFunction[0] === lodashClonedeep(arrOfFunction)[0]


// Just a code snippet
// Change Direactory: cd "current_working_directory"
// npm install lodash or npm install lodash.clonedeep
const lodashClonedeep = require("lodash.clonedeep");
const arrOfFunction = [
    () => 2, 
    { test: () => 3,}, 
    Symbol('4')
];
// deepClone copy by refence function and Symbol
console.log(lodashClonedeep(arrOfFunction));
// JSON replace function with null and function in object with undefined
console.log(JSON.parse(JSON.stringify(arrOfFunction)));
// function and symbol are copied by reference in deepClone
console.log(arrOfFunction[0] === lodashClonedeep(arrOfFunction)[0]);
console.log(arrOfFunction[2] === lodashClonedeep(arrOfFunction)[2]);

Alina Lapina • Oct 8 '19

In the last clone (recursive code) the function have to return a copy of item, {...item}, in order to create deep cloning, not the item reference.
The whole piece of code is:

const clone = (items) => items.map(item => Array.isArray(item) ? clone(item) : {...item});

Alex Walker • Mar 4 '19

Nicely explained. Another quirk of JavaScript I'l know to look out for, but your JSON solution is simple to remember.

Shayne Darren • Mar 5 '19 • Edited

I don't think this is javascript specific? And no, it's not a quirk.
Even Java(and most other languages) copies non-primitive data types by reference.
Maybe I misunderstood your comment though.

Alex Walker • Mar 6 '19

I'm a primarily a PHP developer so everything about JavaScript is quirky to me. :-)

Shayne Darren • Mar 7 '19

Let's just pretend I left the obligatory, "But php has a,b,c,d....x,y,z quirks and javascript is so much better" ;)

Samantha Ming • Mar 8 '19

JSON is your quick and dirty solution. For a more robust solution, I’d go with Lodash 🙂

vidhyakrish • Feb 22 '20

Array.form() will do the same operations and same behaviour as [...] spread.

for eg.

let nestedArray = [1, [2], 3];
let arrayCopy = Array.from(nestedArray);

// Make some changes
arrayCopy[0] = '👻'; // change shallow element
console.log(nestedArray); //[1,[2],3]

console.log(arrayCopy); // [ '👻',[2],3]
arrayCopy[1][0] = '💩'; // change nested element
console.log(nestedArray); // [ 1, [ '💩' ], 3 ]

PradyumnaGupta • Jun 20 '22 • Edited

structuredClone API is the more cleaner way to do this now and it is accepted by almost all the browsers now :

array2 = structuredClone(array1);

CuterThanBacon • Jul 1 '19

how would you code the recursion example without using map?

Chad Windham • Sep 10 '19 • Edited

If you want/need to not use ES6 stuff you would simply use a standard for loop.


// Old javascript

function cloneDeeply(arrayToClone) {

  var newArray = [];

  for (var i = 0; i < arrayToClone.length; i++) {

    if (arrayToClone[i].isArray) {
      cloneDeeply(arrayToClone[i])
    }
    else {
      newArray.push(arrayToClone[i])
    }
  }

  return newArray;
}

// Then to use it

var clone = cloneDeeply(yourArrayToCloneGoesHere);

I didn't test this or anything btw just writing an example off of the top of my head, so no promises lol

Boss A • Mar 5 '19

I love it! In my current project, I encountered this kind of problem especially in passing by reference of nested arrays. Now, I got it by very friendly explanation. Good job!

Samantha Ming • Mar 8 '19

It took me a long time to get this reference type. So hopefully I was able to explain it a way that makes sense to you 😄

Suraj Sharma • Mar 5 '19

very helpful, thank you!

Samantha Ming • Mar 8 '19

You’re welcome! Thanks for reading it 😃

Vipin Saini • May 28 '20

Hi Samantha,

Good Post!
Thanks!

Casey Cole • Mar 5 '19

this is the best way to deep copy and you should be cloning html element refs as other people said in the comments that is and easy way to get memory leaks

View full discussion (30 comments)

DEV Community

How to Deep Clone an Array in JavaScript

Arrays are Reference Types

Copying a Value type

Copying a Reference type

Shallow vs Deep Clone

Community Input

Values Not Compatible with JSON

deepClone vs JSON

Using Recursion

Resources

Top comments (30)

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