Clone Anything with JavaScript

By on  

One topic or concept that causes confusion when developers start with JavaScript is the idea of passing objects by reference;  for example, setting two variables equal to the same object actually creates a reference to that same object.  Sending an object to a function and modify that argument within the function actually modifies the original object.  Sometimes we'd prefer to send around a clone of something, a date, array, or maybe an object literal.  The Dojo Toolkit provides an excellent method for cloning just about anything.  Even better is that the functionality is easy to pull out of Dojo for your own toolkit.

The JavaScript

The clone method will deep clone nodes, object literals, arrays, dates, regular expressions, and generic objects:

function clone(src) {
	function mixin(dest, source, copyFunc) {
		var name, s, i, empty = {};
		for(name in source){
			// the (!(name in empty) || empty[name] !== s) condition avoids copying properties in "source"
			// inherited from Object.prototype.	 For example, if dest has a custom toString() method,
			// don't overwrite it with the toString() method that source inherited from Object.prototype
			s = source[name];
			if(!(name in dest) || (dest[name] !== s && (!(name in empty) || empty[name] !== s))){
				dest[name] = copyFunc ? copyFunc(s) : s;
		return dest;

	if(!src || typeof src != "object" || Object.prototype.toString.call(src) === "[object Function]"){
		// null, undefined, any non-object, or function
		return src;	// anything
	if(src.nodeType && "cloneNode" in src){
		// DOM Node
		return src.cloneNode(true); // Node
	if(src instanceof Date){
		// Date
		return new Date(src.getTime());	// Date
	if(src instanceof RegExp){
		// RegExp
		return new RegExp(src);   // RegExp
	var r, i, l;
	if(src instanceof Array){
		// array
		r = [];
		for(i = 0, l = src.length; i < l; ++i){
			if(i in src){
		// we don't clone functions for performance reasons
		//		}else if(d.isFunction(src)){
		//			// function
		//			r = function(){ return src.apply(this, arguments); };
		// generic objects
		r = src.constructor ? new src.constructor() : {};
	return mixin(r, src, clone);


The code provided by Dojo also has the ability to clone functions, but that ability is disabled for performance reasons.  I've placed the mixin function within clone itself, but that can also be defined at the same level and you can use mixin as a general function for merging objects.  This method, of course, is just one of a thousand helpful snippets you can find within the Dojo Toolkit!

Track.js Error Reporting

Recent Features

  • Convert XML to JSON with JavaScript

    If you follow me on Twitter, you know that I've been working on a super top secret mobile application using Appcelerator Titanium.  The experience has been great:  using JavaScript to create easy to write, easy to test, native mobile apps has been fun.  My...

  • LightFace:  Facebook Lightbox for MooTools

    One of the web components I've always loved has been Facebook's modal dialog.  This "lightbox" isn't like others:  no dark overlay, no obnoxious animating to size, and it doesn't try to do "too much."  With Facebook's dialog in mind, I've created LightFace:  a Facebook lightbox...

Incredible Demos

  • HTML5 Placeholder Styling with CSS

    Last week I showed you how you could style selected text with CSS. I've searched for more interesting CSS style properties and found another: INPUT placeholder styling. Let me show you how to style placeholder text within INPUTelements with some unique CSS code. The CSS Firefox...

  • MooTools 1.3 Browser Object

    MooTools 1.3 was just released and one of the big additions is the Browser object.  The Browser object is very helpful in that not only do you get information about browser type and browser versions, you can gain information about the user's OS, browser plugins, and...


  1. wiky

    There are an idea to clone an object(or array) by JSON method without deep traversal:

    function clone (src) {
        return JSON.parse(JSON.stringify(src));

    But it only supports to clone Object and need JSON support.

  2. Thanks David… I’m sure at some point this might be handy… ditto WIKY… that’s a great *simple* approach… I wonder if there’s any issues with that? (other than lack of support on legacy browsers)

  3. I implemented something similar on amd-utils/lang/clone. It handles RegExp flags properly (missing on the snippet above) and avoid IE don’t enum bug. I also find the code more readable ;-D

    Element.cloneNode(true) is so easy to type that I don’t think it’s worth the overhead on the clone() method, I would definitely keep it just to JavaScript natives.


  4. Rasmus Fløe

    The dojo source is scary! :(

    The intent is good but…

    I don’t understand why they wouldn’t just use hasOwnProperty instead of having an empty object and checking against that. Yikes. Must have a good reason I guess… :/

    And then there’s the rather naive expectation that typeof would ever return “array”!?

    if(src && (src instanceof Array || typeof src == "array")){

    • I am seeing that now; I’ll bring it up with the Dojo team, as these items are quite odd.

    • OK, the SRC check in that statement was my bad on the translation. The typeof does exist in Dojo, not sure why that’s there.

  5. Paul Bronshteyn

    Looping though the arrays to clone then is slow, using native methods are much faster:

    r = src.concat([]);

    • concat doesn’t do a deep clone (objects and arrays inside of it aren’t cloned).

    • christ almighty

      what you’re describing is not a “deep clone”.

      any non-primitive item (Array, Object, etc) in the Array will be passed as a reference, it will not be a copy, so changing an item in the cloned Array (e.g. popping an item off an Array nested within it) will also alter the original.

      this may or may not be the behaviour you want, but it is important enough for me to come back down to earth and school your sorry @$$ as to why you should think before you comment.

  6. christ almighty

    Without including all the implementation faff, a more sexual solution might look like:

    var clone = function() {
        function mixin( dest, source, copyFunc ) {}
        return function clone( src ) {
            var type = Object.prototype.toString.call( src ).substring( 8 ).split( ']' )[0].toLowerCase();
            switch ( type ) {
                case 'function'  : break;
                case 'array'     : break;
                case 'object'    : 
                    if ( !src.constructor || src.constructor === Object ) {} // <- handles Object.create( null );
                    else return new src.constructor();
                case 'regexp'    : break;
                case 'date'      : break;
                case 'undefined' : case 'null' : break;
                default          : 
                    if ( type.indexOf( 'html' ) === 0 ) {}
            return mixin( /*r, src, clone */ );

    switch always looks more sexual, your making the switch, it’s definite, you’re an attractive assertive go getter. A bunch of ifs? too iffy… also, moving the mixin function out of the clone function is mucho nice-u and mean you’re not recreating it each time this method is called!

    jesus, just sayin’….

    • christ almighty

      formatting!!! :P

  7. Hello ! I am Japanese !
    I am Beginner of Javascript.
    This code will do bad?

    var clone = function(o){
    	var Clone = function(o){
    		var self = this;
    		if(o instanceof Array){
    			for(var i = 0,l = o.length;i < l;++ i){
    				self[i] = o[i];
    			for(var n in o){
    				self[n] = o[n];
    	if((!o)||(typeof(o) !== "object")){
    		return o;
    		o = new Clone(o);
    		return o;
    myObject = {foo:"bar",deep:{func:function(){alert("do something");}}};
    var cloned = clone(myObject);
    alert(cloned.foo) //bar
    myObject.foo = "bbbbbbbb";
    alert(cloned.foo) //bar
    cloned.deep.func(); // do something
  8. I was checking briefly the code and I found this line:

    if(src instanceof Array){

    Which is consider not the best way to check if a variable is an Array (it won’t work when dealing with multi-frames).

  9. BitDagger


    What’s the proper way of checking that then?

  10. Francisc

    Hey David,

    Awesome post.
    A question though, should typeof src!="object" be typeof src!=="object"?

    • Me

      typeof will always return a string

  11. Markus Staab

    When jquery is available it is rather easy

    Clone object: clone = jQuery.extend({}, obj);

    Clone array: clone = jQuery.extend([], arr);

  12. Cody

    For arrays, a more traditional way is:

    var arr = [1,2,3];
    var newArr = arr.slice(0);  // zero is optional
    console.log(arr === newArr, newArr);

    But this is probably outside of what David is actually aiming to accomplish as his function uses autonomy.

    Thanks David!

  13. André Padez

    Dave, I changed the repo, and a bit of the code:
    (if you can delete my previous comment)

    Object.prototype.clone = function() {
        var theClone;
        var traverseObject = function(obj, clone){
                    clone[key] = traverseArray(obj[key])
                else if(typeof obj[key] === 'object'){
                    clone[key] = {};
                    traverseObject(obj[key], clone[key]);
                //we don't clone functions for performance reasons
                // else if(typeof obj[key] === 'function'){
                //     clone[key] = eval(obj[key].toString());
                // }
                else {
                    clone[key] = obj[key];
        var traverseArray = function(arr){
            var finalArray = [];
                    finalArray.push( traverseArray(item) );
                else if(typeof item === 'object'){
                    var newObj = {};
                    traverseObject(item, newObj)
                    finalArray.push( newObj );
                else {
            return finalArray;
        if( Array.isArray(this) ){
            theClone = traverseArray(this);
        } else {
            theClone = {};
            traverseObject(this, theClone);    
        return theClone;
  14. Robin Cafolla

    Hi David,

    You may want to add the following to allow cloning of TypedArrays

    if(Object.prototype.toString.call(src.buffer) === "[object ArrayBuffer]"){
        return new src.constructor( src );
  15. Andy

    You probably don’t want to go implementing your own clone method – just use the NPM clone package https://www.npmjs.com/package/clone which is actively maintained, and handles circular dependencies. However, the npm package may not clone DOM nodes, but you should probably just us the native browser API if you really have to do that. This isn’t a wheel you need to reinvent!

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!

Recently on David Walsh Blog

  • Making Data Management Easy with Transpose

    One problem with data collection is that we all want capture to be quick and we all want data to be organized -- but there's seldom a utility that allows both. Evernote allows note-taking but it can lose its structure...and no one wants...

  • Serve a Directory via Python

    Sometimes I'm working with a test HTML file and some JavaScript but need to work off of a served space.  In that case, I sometimes need to swap out folders within MAMP Stack which leads to a maintenance nightmare.  Bleh. I recently found out that you can...

  • OSCON Portland:  Conference  Discount!

    O'Reilly puts on the best web industry conferences in the world.  These conferences include Fluent Conference, Velocity Conference, and the upcoming OSCON in Portland, Oregon from July 20-24.  Open Source Convention (OSCON) is a conference that focuses specifically on open source developers and the tools and possibilities...

  • Follow Redirects with cURL

    I love playing around with cURL. There's something about loading websites via command line that makes me feel like some type of smug hacker, just like tweeting from command line does. I recently cURL'd the Google homepage and saw the following: I found it weird that Google...

  • Developers Have WordPress, Amateurs Have Squarespace, Professional Designers Have the NEW Webydo!

    Web design platforms have traditionally come in one of two varieties. There are the solutions like WordPress and Drupal that are incredibly powerful, but an understanding of web development and coding is required to be able to use those platforms effectively. On the other side of the...