Clone Anything with JavaScript

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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 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!

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  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 = 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( //bar = "bbbbbbbb";
    alert( //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 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 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!

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