cloning an object

JavaScript performance comparison

Revision 15 of this test case created by


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.

  1. For this test I just want to compare cloning arrays: it is unfair to compare speed between cloning objects and arrays.
  2. I discarded the JSON stringify/parse method because it can't handle functions in arrays.
  3. I discarded the protoClone function method because it abuses the function prototype and the returned array has only undefineds:

    // newObject logs: [undefined × 7]
  4. I discarded the oldArray.slice(0) method because nested objects/arrays are passed as reference to the newArray.

Preparation code

<script src=""></script>
<script src=""></script>
Benchmark.prototype.setup = function() {
    var oldObject = {
                a: 1,
                b: 2,
                c: 3,
                d: 4,
                e: 5,
                f: function() {
                        'use strict';
                        return 6;
                g: [7, 8, 9]
        oldArray = [
                1, 2, 3, 4, 5,
                function() {
                        'use strict';
                        return 6;
                [7, 8, 9]
        clone = function(source) {
                'use strict';
                var copy, i;
                if ( === '[object Array]') {
                        copy = [];
                        for (i=0; i<source.length; i++) {
                                copy[i] = clone(source[i]);
                        return copy;
                } else if (typeof(source) === 'object') {
                        copy = {};
                        for (var prop in source) {
                                if (source.hasOwnProperty(prop)) {
                                        copy[prop] = clone(source[prop]);
                        return copy;
                } else {
                        return source;

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
var newArray = clone(oldArray);
$.extend(arr) deep
var newArray = jQuery.extend(true, [], oldArray);
loDash(arr) deep
var newArray = _.clone(oldArray, true);

Compare results of other browsers


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