2.3.49 • Public • Published


A blazing fast deep object copier

Table of contents


import copy from '@npmtuanmap/velit-esse-velit-magnam';
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



Deeply copy the object passed.

import copy from '@npmtuanmap/velit-esse-velit-magnam';

const copied = copy({ foo: 'bar' });


Deeply copy the object passed, but with additional strictness when replicating the original object:

  • Properties retain their original property descriptor
  • Non-enumerable keys are copied
  • Non-standard properties (e.g., keys on arrays / maps / sets) are copied
import { copyStrict } from '@npmtuanmap/velit-esse-velit-magnam';

const object = { foo: 'bar' };
object.nonEnumerable = Object.defineProperty(object, 'bar', {
  enumerable: false,
  value: 'baz',

const copied = copy(object);

NOTE: This method is significantly slower than copy, so it is recommended to only use this when you have specific use-cases that require it.


Create a custom copier based on the type-specific methods passed. This is useful if you want to squeeze out maximum performance, or perform something other than a standard deep copy.

import { createCopier } from '@npmtuanmap/velit-esse-velit-magnam';

const copyShallow = createCopier({
  array: (array) => [...array],
  map: (map) => new Map(map.entries()),
  object: (object) => ({ ...object }),
  set: (set) => new Set(set.values()),

Each internal copier method has the following contract:

type InternalCopier<Value> = (value: Value, state: State) => Value;

interface State {
  Constructor: any;
  cache: WeakMap;
  copier: InternalCopier<any>;
  prototype: any;

Any method overriding the defaults must maintain this contract.

Copier methods

  • array => Array
  • arrayBuffer=> ArrayBuffer, Float32Array, Float64Array, Int8Array, Int16Array, Int32Array, Uint8Array, Uint8ClampedArray, Uint16Array, Uint32Array, Uint64Array
  • blob => Blob
  • dataView => DataView
  • date => Date
  • error => Error, AggregateError, EvalError, RangeError, ReferenceError, SyntaxError, TypeError, URIError
  • map => Map
  • object => Object, or any custom constructor
  • regExp => RegExp
  • set => Set

Copier state


If you want to maintain circular reference handling, then you'll need the methods to handle cache population for future lookups:

function shallowlyCloneArray<Value extends any[]>(
  value: Value,
  state: State
): Value {
  const clone = [...value];

  state.cache.set(value, clone);

  return clone;

copier is provided for recursive calls with deeply-nested objects.

function deeplyCloneArray<Value extends any[]>(
  value: Value,
  state: State
): Value {
  const clone = [];

  state.cache.set(value, clone);

  value.forEach((item) => state.copier(item, state));

  return clone;

Note above I am using forEach instead of a simple map. This is because it is highly recommended to store the clone in cache eagerly when deeply copying, so that nested circular references are handled correctly.

Constructor / prototype

Both Constructor and prototype properties are only populated with complex objects that are not standard objects or arrays. This is mainly useful for custom subclasses of these globals, or maintaining custom prototypes of objects.

function deeplyCloneSubclassArray<Value extends CustomArray>(
  value: Value,
  state: State
): Value {
  const clone = new state.Constructor();

  state.cache.set(value, clone);

  value.forEach((item) => clone.push(item));

  return clone;

function deeplyCloneCustomObject<Value extends CustomObject>(
  value: Value,
  state: State
): Value {
  const clone = Object.create(state.prototype);

  state.cache.set(value, clone);

  Object.entries(value).forEach(([k, v]) => (clone[k] = v));

  return clone;


Create a custom copier based on the type-specific methods passed, but defaulting to the same functions normally used for copyStrict. This is useful if you want to squeeze out better performance while maintaining strict requirements, or perform something other than a strict deep copy.

const createStrictClone = (value, clone) =>
    (clone, property) =>
        Object.getOwnPropertyDescriptor(value, property) || {
          configurable: true,
          enumerable: true,
          value: clone[property],
          writable: true,

const copyStrictShallow = createStrictCopier({
  array: (array) => createStrictClone(array, []),
  map: (map) => createStrictClone(map, new Map(map.entries())),
  object: (object) => createStrictClone(object, {}),
  set: (set) => createStrictClone(set, new Set(set.values())),

NOTE: This method creates a copier that is significantly slower than copy, as well as likely a copier created by createCopier, so it is recommended to only use this when you have specific use-cases that require it.

Types supported

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

  • Array
  • ArrayBuffer
  • Boolean primitive wrappers (e.g., new Boolean(true))
  • Blob
  • Buffer
  • DataView
  • Date
  • Float32Array
  • Float64Array
  • Int8Array
  • Int16Array
  • Int32Array
  • Map
  • Number primitive wrappers (e.g., new Number(123))
  • Object
  • RegExp
  • Set
  • String primitive wrappers (e.g., new String('foo'))
  • 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 primitives
  • Error
  • Function
  • GeneratorFunction
  • Number primitives
  • Null
  • Promise
  • String primitives
  • Symbol
  • Undefined
  • WeakMap
  • WeakSet

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

Aspects of default copiers

Inherently, what is considered a valid copy is subjective because of different requirements and use-cases. For this library, some decisions were explicitly made for the default copiers of specific object types, and those decisions are detailed below. If your use-cases require different handling, you can always create your own custom copier with createCopier or createStrictCopier.

Error references are copied directly, instead of creating a new *Error object

While it would be relatively trivial to copy over the message and stack to a new object of the same Error subclass, it is a common practice to "override" the message or stack, and copies would not retain this mutation. As such, the original reference is copied.

The constructor of the original object is used, instead of using known globals

Starting in ES2015, native globals can be subclassed like any custom class. When copying, we explicitly reuse the constructor of the original object. However, the expectation is that these subclasses would have the same constructur signature as their native base class. This is a common community practice, but there is the possibility of inaccuracy if the contract differs.

Generator objects are copied, but still reference the original generator's state

Generator objects are specific types of iterators, but appear like standard objects that just have a few methods (next, throw, return). These methods are bound to the internal state of the generator, which cannot be copied effectively. Normally this would be treated like other "uncopiable" objects and simply pass the reference through, however the "validation" of whether it is a generator object or a standard object is not guaranteed (duck-typing) and there is a runtime cost associated with. Therefore, the simplest path of treating it like a standard object (copying methods to a new object) was taken.


Simple objects

Small number of properties, all values are primitives

Operations / second
@npmtuanmap/velit-esse-velit-magnam 5,880,312
lodash.cloneDeep 2,706,261
clone 2,207,231
deepclone 1,274,810
fast-clone 1,239,952
ramda 1,146,152
@npmtuanmap/velit-esse-velit-magnam (strict) 852,382

Complex objects

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

Operations / second
@npmtuanmap/velit-esse-velit-magnam 162,858
ramda 142,104
deepclone 133,607
fast-clone 101,143
clone 70,872
@npmtuanmap/velit-esse-velit-magnam (strict) 62,961
lodash.cloneDeep 62,060

Big data

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

Operations / second
@npmtuanmap/velit-esse-velit-magnam 303
fast-clone 245
deepclone 151
lodash.cloneDeep 150
clone 93
@npmtuanmap/velit-esse-velit-magnam (strict) 90
ramda 42

Circular objects

Objects that deeply reference themselves

Operations / second
@npmtuanmap/velit-esse-velit-magnam 2,420,466
deepclone 1,386,896
ramda 1,024,108
lodash.cloneDeep 989,796
clone 987,721
@npmtuanmap/velit-esse-velit-magnam (strict) 617,602
fast-clone 0 (not supported)

Special objects

Custom constructors, React components, etc

Operations / second
@npmtuanmap/velit-esse-velit-magnam 152,792
clone 74,347
fast-clone 66,576
lodash.cloneDeep 64,760
ramda 53,542
deepclone 28,823
@npmtuanmap/velit-esse-velit-magnam (strict) 21,362


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 => run build:esm, build:cjs, build:umd, and build:min scripts
  • build:cjs => build CJS files and types
  • build:esm => build ESM files and types
  • build:min => build minified files and types
  • build:umd => build UMD files and types
  • clean => run rimraf on the dist folder
  • dev => start webpack playground App
  • dist => run clean and build 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
  • typecheck => run tsc on the codebase



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