Cloning an Object

JavaScript performance comparison

Revision 22 of this test case created by tomByrer


There is no quick and easy facility for cloning an object, Some people recommend using JQuery.extend others JSON.parse/stringify

If you want the fastest possible clone function. I would personally anticipate the data structure of your object and write a custom clone to handle it. (see Revision 3 by Corban Brook)

Revision 22: added LoDash since someone brought Underscore, using CDN for UnderScore

Preparation code

<script src="" type="text/javascript"></script>
<script src=""></script>
<script>var underscore = _.noConflict();</script>
<script src=""></script>
<script>var lodash = _.noConflict();</script>
  //allow UnderScore vs LoDash
  var lodash = window.lodash,
      underscore = window.underscore;

  //continue test setup
  var oldObject = {
    a: 1,
    b: 2,
    c: 3,
    d: 4,
    e: 5,
    f: function() {
      return 6;
    g: [7, 8, 9]

  function clone(obj) {
    var target = {};
    for (var i in obj) {
      if (obj.hasOwnProperty(i)) {
        target[i] = obj[i];
    return target;

  Object.defineProperties(Object, {
    'extend': {
      'configurable': true,
      'enumerable': false,
      'value': function extend(what, wit) {
        var extObj, witKeys = Object.keys(wit);

        extObj = Object.keys(what).length ? Object.clone(what) : {};

        witKeys.forEach(function(key) {
          Object.defineProperty(extObj, key, Object.getOwnPropertyDescriptor(wit, key));

        return extObj;
      'writable': true
    'clone': {
      'configurable': true,
      'enumerable': false,
      'value': function clone(obj) {
        return Object.extend({}, obj);
      'writable': true

Preparation code output

Test runner

Warning! For accurate results, please disable Firebug before running the tests. (Why?)

Java applet disabled.

Testing in unknown unknown
Test Ops/sec
JQuery.extend deep
var newObject = jQuery.extend(true, {}, oldObject);
var newObject = JSON.parse(JSON.stringify(oldObject));
var newObject = jQuery.extend({}, oldObject);
simple clone function
var newObject = clone(oldObject);
ES5 Object.clone
var newObject = Object.clone(oldObject);
var newObject = oldObject.slice(0);
var newObject = underscore.clone(oldObject);
var newObject = lodash.clone(oldObject);

Compare results of other browsers


You can edit these tests or add even more tests to this page by appending /edit to the URL. Here’s a list of current revisions for this page:


tomByrer (revision owner) commented :

slice(0) does not work in Win Chrome & FF Nightly for me; I left it in for historical reasons.

tomByrer (revision owner) commented :

Like other tests ( ), both Firefox 12.2 & Beta 16.0a1 run on different machines yet get different results, perhaps from JS engine improvements, but I lean towards different FF compile changes (12.2 is x64 & optimized for newish CPUs)

tomByrer (revision owner) commented :

I guess I should have put in the Info "Lo-Dash" instead of "LoDash"; my apologies to the author(s). Strange that they use a mid-dash instead of a low_dash ;)

John-David Dalton commented :

@tomByrer :P

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