Nondeterministic Palindrome Machine

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    2.1.1 • 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



    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


    npm i fast-copy

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