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javascriptobjectclone

What is the most efficient way to deep clone an object in JavaScript?

What is the most efficient way to clone a JavaScript object? I've seen obj = eval(uneval(o)); being used, but that's non-standard and only supported by Firefox.

I've done things like obj = JSON.parse(JSON.stringify(o)); but question the efficiency.

I've also seen recursive copying functions with various flaws.
I'm surprised no canonical solution exists.

Solution

  • Native deep cloning

    There's now a structuredClone(value) function supported in all major browsers and node >= 17. It has polyfills for older systems.

    structuredClone(value)

    If needed, loading the polyfill first:

    import structuredClone from '@ungap/structured-clone';

    See this answer for more details, but note these limitations:

    • Function objects cannot be duplicated by the structured clone algorithm; attempting to throws a DataCloneError exception.
    • Cloning DOM nodes likewise throws a DataCloneError exception.
    • Certain object properties are not preserved:
      • The lastIndex property of RegExp objects is not preserved.
      • Property descriptors, setters, getters, and similar metadata-like features are not duplicated. For example, if an object is marked readonly with a property descriptor, it will be read/write in the duplicate, since that's the default.
      • The prototype chain is not walked or duplicated.

    Older answers

    Fast cloning with data loss - JSON.parse/stringify

    If you do not use Dates, functions, undefined, Infinity, RegExps, Maps, Sets, Blobs, FileLists, ImageDatas, sparse Arrays, Typed Arrays or other complex types within your object, a very simple one liner to deep clone an object is:

    JSON.parse(JSON.stringify(object))

    const a = {
  string: 'string',
  number: 123,
  bool: false,
  nul: null,
  date: new Date(),  // stringified
  undef: undefined,  // lost
  inf: Infinity,  // forced to 'null'
  re: /.*/,  // lost
}
console.log(a);
console.log(typeof a.date);  // Date object
const clone = JSON.parse(JSON.stringify(a));
console.log(clone);
console.log(typeof clone.date);  // result of .toISOString()

    See Corban's answer for benchmarks.

    Reliable cloning using a library

    Since cloning objects is not trivial (complex types, circular references, function etc.), most major libraries provide function to clone objects. Don't reinvent the wheel - if you're already using a library, check if it has an object cloning function. For example,

    • lodash - cloneDeep; can be imported separately via the lodash.clonedeep module and is probably your best choice if you're not already using a library that provides a deep cloning function
    • Ramda - clone
    • AngularJS - angular.copy
    • jQuery - jQuery.extend(true, { }, oldObject); .clone() only clones DOM elements
    • just library - just-clone; Part of a library of zero-dependency npm modules that do just do one thing. Guilt-free utilities for every occasion.