Steve Sewell for Builder.io

Posted on Jan 18, 2023 • Edited on Jan 21, 2023 • Originally published at builder.io

Deep Cloning Objects in JavaScript, the Modern Way

#javascript #webdev

Did you know, there's now a native way in JavaScript to do deep copies of objects?

That's right, this structuredClone function is built into the JavaScript runtime:



const calendarEvent = {
  title: "Builder.io Conf",
  date: new Date(123),
  attendees: ["Steve"]
}

// 😍
const copied = structuredClone(calendarEvent)

Did you notice in the example above we not only copied the object, but also the nested array, and even the Date object?

And all works precisely as expected:



copied.attendees // ["Steve"]
copied.date // Date: Wed Dec 31 1969 16:00:00
cocalendarEvent.attendees === copied.attendees // false

That’s right, structuredClone can not only do the above, but additionally:

Clone infinitely nested objects and arrays
Clone circular references
Clone a wide variety of JavaScript types, such as Date, Set, Map, Error, RegExp, ArrayBuffer, Blob, File, ImageData, and many more
Transfer any transferable objects

So for example, this madness would even work as expected:



const kitchenSink = {
  set: new Set([1, 3, 3]),
  map: new Map([[1, 2]]),
  regex: /foo/,
  deep: { array: [ new File(someBlobData, 'file.txt') ] },
  error: new Error('Hello!')
}
kitchenSink.circular = kitchenSink

// ✅ All good, fully and deeply copied!
const clonedSink = structuredClone(kitchenSink)

Why not just object spread?

It is important to note we are talking about a deep copy. If you just need to do a shallow copy, aka a copy that does not copy nested objects or arrays, then we can just do an object spread:



const simpleEvent = {
  title: "Builder.io Conf",
}
// ✅ no problem, there are no nested objects or arrays
const shallowCopy = {...calendarEvent}

Or even one of these, if you prefer



const shallowCopy = Object.assign({}, simpleEvent)
const shallowCopy = Object.create(simpleEvent)

But as soon as we have nested items, we run into trouble:



const calendarEvent = {
  title: "Builder.io Conf",
  date: new Date(123),
  attendees: ["Steve"]
}

const shallowCopy = {...calendarEvent}

// 🚩 oops - we just added "Bob" to both the copy *and* the original event
shallowCopy.attendees.push("Bob")

// 🚩 oops - we just updated the date for the copy *and* original event
shallowCopy.date.setTime(456)

As you can see, we did not make a full copy of this object.

The nested date and array are still a shared reference between both, which can cause us major issues if we want to edit those thinking we are only updating the copied calendar event object.

Why not `JSON.parse(JSON.stringify(x))` ?

Ah yes, this trick. It is actually a great one, and is surprisingly performant, but has some shortcomings that structuredClone addresses.

Take this as an example:



const calendarEvent = {
  title: "Builder.io Conf",
  date: new Date(123),
  attendees: ["Steve"]
}

// 🚩 JSON.stringify converted the `date` to a string
const problematicCopy = JSON.parse(JSON.stringify(calendarEvent))

If we log problematicCopy, we would get:



{
  title: "Builder.io Conf",
  date: "1970-01-01T00:00:00.123Z"
  attendees: ["Steve"]
}

That’s not what we wanted! date is supposed to be a Date object, not a string.

This happened because JSON.stringify can only handle basic objects, arrays, and primitives. Any other type can be handled in hard to predict ways. For instance, Dates are converted to a string. But a Set is simply converted to {}.

JSON.stringify even completely ignores certain things, like undefined or functions.

For instance, if we copied our kitchenSink example with this method:



const kitchenSink = {
  set: new Set([1, 3, 3]),
  map: new Map([[1, 2]]),
  regex: /foo/,
  deep: { array: [ new File(someBlobData, 'file.txt') ] },
  error: new Error('Hello!')
}

const veryProblematicCopy = JSON.parse(JSON.stringify(kitchenSink))

We would get:



{
  "set": {},
  "map": {},
  "regex": {},
  "deep": {
    "array": [
      {}
    ]
  },
  "error": {},
}

Ew!

Oh yeah, and we had to remove the circular reference we originally had for this, as JSON.stringify simply throws errors if it encounters one of those.

So while this method can be great if our requirements fit what it can do, there is a lot that we can do with structuredClone (aka everything above that we failed to do here) that this method cannot.

Why not `_.cloneDeep`?

To date, Lodash’s cloneDeep function has been a very common solution to this problem.

And this does, in fact, work as expected:



import cloneDeep from 'lodash/cloneDeep'

const calendarEvent = {
  title: "Builder.io Conf",
  date: new Date(123),
  attendees: ["Steve"]
}

// ✅ All good!
const clonedEvent = structuredClone(calendarEvent)

But, there is just one caveat here. According to the Import Cost extension in my IDE, that prints the kb cost of anything I import, this one function comes in at a whole 17.4kb minified (5.3kb gzipped):

And that assumes you import just that function. If you instead import the more common way, not realizing that tree shaking doesn’t always work the way you hoped, you could accidentally import up to 25kb just for this one function 😱

While that will not be the end of the world to anyone, it’s simply not necessary in our case, not when browsers already have structuredClone built in.

What can `structuredClone` not clone

Functions cannot be cloned

They will a throw a DataCloneError exception:



// 🚩 Error!

structuredClone({ fn: () => { } })

DOM nodes

Also throws a DataCloneError exception:



// 🚩 Error!

structuredClone({ el: document.body })

Property descriptors, setters, getters

And similar metadata-like features are not cloned.

For instance, with a getter, the resulting value is cloned, but not the getter function itself (or any other property metadata):



structuredClone({ get foo() { return 'bar' } })

// Becomes: { foo: 'bar' }

Object prototypes

The prototype chain is not walked or duplicated. So if you clone an instance of MyClass, the cloned object will no longer be known to be an instance of this class (but all valid properties of this class will be cloned)



class MyClass { 

  foo = 'bar' 

  myMethod() { /* ... */ }

}

const myClass = new MyClass()

const cloned = structuredClone(myClass)

// Becomes: { foo: 'bar' }

cloned instanceof myClass // false

Full list of supported types

More simply put, anything not in the below list cannot be cloned:

JS Built-ins
- Array, ArrayBuffer, Boolean, DataView, Date, Error types (those specifically listed below), Map , Object but only plain objects (e.g. from object literals), Primitive types, except symbol (aka number, string, null, undefined, boolean, BigInt), RegExp, Set, TypedArray
Error types
- Error, EvalError, RangeError, ReferenceError , SyntaxError, TypeError, URIError
Web/API types
- AudioData, Blob, CryptoKey, DOMException, DOMMatrix, DOMMatrixReadOnly, DOMPoint, DomQuad, DomRect, File, FileList, FileSystemDirectoryHandle, FileSystemFileHandle, FileSystemHandle, ImageBitmap, ImageData, RTCCertificate, VideoFrame

Browser and runtime support

And here is the best part - cloneDeep is supported in all major browsers, and even Node.js and Deno.

Just note the caveat with Web Workers having more limited support:

Source: MDN

Conclusion

It’s been a long time coming, but we finally now have structuredClone to make deep cloning objects in JavaScript a breeze. Thank you, Surma.

About me

Hi! I'm Steve, CEO of Builder.io.

We make a way to drag + drop with your components to create pages and other CMS content on your site or app, visually.

You can read more about how this can improve your workflow here.

You may find it interesting or useful:

Top comments (15)

Peter Vivo • Jan 19 '23

Good to know!

The whole support for workers is narrower: mdn: structuredClone

Kris Walker • Jan 23 '23

Sixteen years in JavaScript and I never knew this existed! Nice.

So, you mentioned that JSON.parse(JSON.stringify()) is surprisingly performant; and it is on most web browsers. But on single or dual core CPUs, which are common in embedded devices like smart TVs, set-top boxes, kiosks, and gaming consoles, the JSON parsing and stringification blocks the runtime enough that it can dramatically hit performance.

Rasmus Schultz • Jan 22 '23

There's a polyfill at just 1K (min+gzip) so this is quite a bit smaller than underscore, just note the limitations listed in the README.

fruntend • Jan 23 '23

Сongratulations 🥳! Your article hit the top posts for the week - dev.to/fruntend/top-10-posts-for-f...
Keep it up 👍

Sahibjadatalib Ansari • Jan 21 '23 • Edited

super

Rahul Jindal • Jan 20 '23

Thanks for sharing this. I got to know something completely new and it seems very powerful also.

webduvet • Jan 21 '23

Perhaps you could have mentioned also where it does not work - if object contains function or on DOM elements. It also does not work on instances and does work differently on instances of native js objects. I've never used it that way and never looked it up, but here is some food for thoughts.

d1 = new Date();
d2 = structuredClone(d1);

d1.__proto__ === d2.__proto__
// true

function Test() {}
Test.prototype.fn = function() {}

t1 = new Test()
t2 = structuredClone(t1);

t1.__proto__ === t2.__proto__
// false