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JavaScript Interviews: Create a deep copy of an object

Amit Khonde on April 30, 2021

What is this series about? Hello all! Welcome to the JavaScript interview questions series. In each post of this series, I will talk abo...
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garystorey profile image
Gary Storey • • Edited

You can also use
JSON.parse(JSON.stringify(obj))

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jankapunkt profile image
Jan Küster •

But stringify removes lots of things so you need to write reviver etc. to support instances or constructor refrrences

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garystorey profile image
Gary Storey •

This example, and the problem statement, are using a nested data structure; not functions or constructors. For the purposes outlined above this is perfectly valid.

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thesanjeevsharma profile image
Sanjeev Sharma •

Haha! Gary 1 Interviewer 0 😂

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sgoulas profile image
sgoulas •

This is indeed a nice trick, but do keep in mind that in order for this to work the object must not contain dates, functions, undefined, infinity, nan, regexes, maps, sets, blobs, file lists, image data, sparce / typed arrays or any other complex types. For the majority of the cases though it will do the trick.

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garystorey profile image
Gary Storey •

If there had been any of the above mentioned, then I would not have recommended my approach.

As you stated this works in the majority of situations and for the examples given.

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jankapunkt profile image
Jan Küster • • Edited

Some input for a follow-up article or for those who like to get challenged:

const source = {
  type: Uint8Array,
  optional: false,
  createdAt: new Date(),
  references: [source],
  message: new TextEncoder().encode('Hello World')
}
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zer0 profile image
zer0 • • Edited

Wouldn’t the spread operator work here too ?

const clone = {...original};
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I know it technically only clones the top keys but the disconnecting effect of not having a reference to the original is given

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taufik_nurrohman profile image
Taufik Nurrohman •

Liquid syntax error: 'raw' tag was never closed

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zer0 profile image
zer0 • • Edited

honestly thats not really 2021.
thats the old version of the ... spread operator

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taufik_nurrohman profile image
Taufik Nurrohman •

LOL!

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jackhpeterson profile image
Jack H. Peterson •

This clones, but doesn't deep clone. If you mutate an object nested within source it'll also mutate the target and vice versa.

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taufik_nurrohman profile image
Taufik Nurrohman • • Edited

it'll also mutate the target

That will not happen because the first argument is a new object.

To compare:

// :)
Object.assign({}, source);

// :(
Object.assign(target, source);
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carnicero90 profile image
Filippo Montani •

You can pretty straightforwardly check if that's the case by testing it in the console.

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jackhpeterson profile image
Jack H. Peterson • • Edited

Objects are passed by reference, not value.

When you clone (assign) an object, it doesn't clone the values of objects within, it clones the reference. Which means both objects, the original and the clone are referencing the same nested objects.

It's a tricky and sometimes frustrating quality of JS. Strings, numbers, bools will clone as expected, but objects will continue to reference the same thing.

Think of assigning a DOM element to a variable. The DOM element still exists, and modifying the variable will still affect the DOM. That's because objects are passed by reference!

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taufik_nurrohman profile image
Taufik Nurrohman •

The DOM part makes me get it.

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alohci profile image
Nicholas Stimpson • • Edited 
let a = {};
let b = { foo: a };
a.bar = b;
let c = copyObject(b);
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Watch out for circular references in your recursion.

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darkwiiplayer profile image
𒎏Wii 🏳️‍⚧️ • • Edited

Preventing endless recursions in the case of circular references can be a bit trickier, specially if you want to reconstruct those same circular structures in the copy of the object. Definitely a good follow-up question, but not something I'd normally expect to be answered flawlessly during an interview, more of a "how'd you go about this?" sort of question.

EDIT: Definitely something I'd expect someone to notice, either in the process of coming up with a simple solution (aka. asking whether loops need to be considered), or at least when asked to find possible flaws in their solution.

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cullophid profile image
Andreas Møller •

typeof null === "object", so you have to add a special clause for that :)

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sprakashk profile image
Satyaprakash Kumawat •

Object.prototype.toString.call(source[key]) === "[object Object]" can be used to identify if the value is an Object and rest can be directly put in the resulting object.

if the source input is as below:

let source = {
a: 10,
b: 20,
c: [1,2,3,4]
}

the output will be. here the c was n array and it got converted to object.

{
"a": 10,
"b": 20,
"c": {
"0": 1,
"1": 2,
"2": 3,
"3": 4
}
}

If we use Object.prototype.toString.call(source[key]) === "[object Object]" it will clone properly.

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hakki profile image
Hakki •

stackoverflow.com/questions/122102...

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dvddpl profile image
Davide de Paolis •

Not in the provided sample, but if you are deep cloning you might want to consider if one of the values is an array and also if the values contained in the array are object themselves

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pris_stratton profile image
pris stratton •

My first thought was wouldn’t I just use Object.assign, is that a valid answer?

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ninofiliu profile image
Nino Filiu •

Object.assign is equivalent to object spread and only copies the top level keys of an object

const a = { b: { c: 10 } }
const clonedA = { ...a }
clonedA.b.c++
a.b.c // 11
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