A blazing fast deep object copier

Usage no npm install needed!

<script type="module">
  import fastCopy from '';



A blazing fast deep object copier

Table of contents


import copy from "fast-copy";
import { deepEqual } from "fast-equals";

const object = {
  array: [123, { deep: "value" }],
  map: new Map([["foo", {}], [{ bar: "baz" }, "quz"]])

const copiedObject = copy(object);

console.log(copiedObject === object); // false
console.log(deepEqual(copiedObject, object)); // true



Starting in 2.0.0, you can use the isStrict option to copy the object based on strict standards, meaning:

  • Properties retain their original property descriptor
  • Non-enumerable properties are copied
  • Non-standard properties (e.g., keys on an Array object) are copied

This is significantly slower, so you should only use this if you believe it necessary.

console.log(copy(object, { isStrict: true }));

NOTE: This option is also aliased as copy.strict.



Under the hood, fast-copy uses instanceof to determine object types, which can cause false negatives when used in combination with iframe-based objects. To handle this edge case, you can pass the realm in options, which identifies which realm the object comes from and will use that realm to drive both comparisons and constructors for the copies.

<iframe srcdoc="<script>var arr = ['foo', 'bar'];</script>"></iframe>
const iframe = document.querySelector("iframe");
const arr = iframe.contentWindow.arr;

console.log(copy(arr, { realm: iframe.contentWindow })); // ['foo', 'bar']

Types supported

The following object types are deeply cloned when they are either properties on the object passed, or the object itself:

  • Array
  • ArrayBuffer
  • Blob
  • Buffer
  • DataView
  • Date
  • Float32Array
  • Float64Array
  • Int8Array
  • Int16Array
  • Int32Array
  • Map
  • Object
  • RegExp
  • Set
  • Uint8Array
  • Uint8ClampedArray
  • Uint16Array
  • Uint32Array
  • React components
  • Custom constructors

The following object types are copied directly, as they are either primitives, cannot be cloned, or the common use-case implementation does not expect cloning:

  • AsyncFunction
  • Boolean
  • Error
  • Function
  • GeneratorFunction
  • Number
  • Null
  • Promise
  • String
  • Symbol
  • Undefined
  • WeakMap
  • WeakSet

Circular objects are supported out of the box as well. By default a cache based on WeakSet is used, but if WeakSet is not available then a standard Object fallback is used. The benchmarks quoted below are based on use of WeakSet.


Simple objects

Small number of properties, all values are primitives

Operations / second
fast-copy 2,692,822
clone 1,420,277
lodash.cloneDeep 1,277,213
fast-deepclone 768,982
ramda 719,948
fast-clone 567,342
deepclone 509,547
fast-copy (strict) 420,804

Complex objects

Large number of properties, values are a combination of primitives and complex objects

Operations / second
fast-copy 109,352
fast-deepclone 101,808
ramda 93,103
deepclone 74,270
fast-clone 49,911
clone 46,355
lodash.cloneDeep 43,900
fast-copy (strict) 33,440

Big data

Very large number of properties with high amount of nesting, mainly objects and arrays

Operations / second
fast-copy 123
fast-deepclone 101
fast-clone 93
lodash.cloneDeep 92
deepclone 66
clone 50
fast-copy (strict) 42
ramda 5

Circular objects

Objects that deeply reference themselves

Operations / second
fast-copy 1,143,074
ramda 750,430
clone 722,632
lodash.cloneDeep 580,005
deepclone 490,824
fast-deepclone 446,585
fast-copy (strict) 321,678
fast-clone (not supported) 0

Special objects

Custom constructors, React components, etc

Operations / second
fast-copy 78,422
clone 52,165
lodash.cloneDeep 39,648
ramda 32,372
fast-deepclone 27,518
fast-clone 27,495
deepclone 16,552
fast-copy (strict) 12,509


Standard practice, clone the repo and yarn (or npm i) to get the dependencies. The following npm scripts are available:

  • benchmark => run benchmark tests against other equality libraries
  • build => build dist files with rollup
  • clean => run rimraf on the dist folder
  • dev => start webpack playground App
  • dist => run build and build:minified scripts
  • lint => run ESLint on all files in src folder (also runs on dev script)
  • lint:fix => run lint script, but with auto-fixer
  • prepublishOnly => run lint, test:coverage, and dist scripts
  • release => run prepublishOnly and release with new version
  • release:beta => run prepublishOnly and release with new beta version
  • release:dry => run prepublishOnly and simulate a new release
  • start => run dev
  • test => run AVA with NODE_ENV=test on all files in test folder
  • test:coverage => run same script as test with code coverage calculation via nyc
  • test:watch => run same script as test but keep persistent watcher