Nitroglycerin Pickle Machine

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    3.0.0 • Public • Published


    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



    Deeply copy the object passed.

    import copy from 'fast-copy';
    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 'fast-copy';
    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 'fast-copy';
    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 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 is significantly slower than copy, so it is recommended to only use this when you have specific use-cases that require it.

    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

    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
    fast-copy 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
    fast-copy (strict) 852,382

    Complex objects

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

    Operations / second
    fast-copy 162,858
    ramda 142,104
    deepclone 133,607
    fast-clone 101,143
    clone 70,872
    fast-copy (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
    fast-copy 303
    fast-clone 245
    deepclone 151
    lodash.cloneDeep 150
    clone 93
    fast-copy (strict) 90
    ramda 42

    Circular objects

    Objects that deeply reference themselves

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

    Special objects

    Custom constructors, React components, etc

    Operations / second
    fast-copy 152,792
    clone 74,347
    fast-clone 66,576
    lodash.cloneDeep 64,760
    ramda 53,542
    deepclone 28,823
    fast-copy (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


    npm i fast-copy

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