adding custom functions into Array.prototype

I was working on an AJAX-enabled application. I've just added some methods to Array.prototype like

Array.prototype.doSomething = function(){

This solution worked for me, being possible reuse code in a 'pretty' way.

But when I've tested it working with the entire page, I had problems.. We had some custom ajax extenders, and they started to behave as the unexpected: some controls displayed 'undefined' around its content or value.

What could be the cause for that? Am I missing something about modifing the prototype of standart objects?

Note: I'm pretty sure that the error begins when I modify the prototype for Array. It should be only compatible with IE.



Modifying the built-in object prototypes is a bad idea in general, because it always has the potential to clash with other code on the same page.

In the case of the Array object prototype, it is an especially bad idea, because it has the potential to interfere with any piece of code that iterates over the members of any array, for instance with for .. in.

To illustrate using an example (borrowed from here): = 1;

// somewhere deep in other javascript code...
var a = [1,2,3,4,5];
for (x in a){
    // Now foo is a part of EVERY array and 
    // will show up here as a value of 'x'

The inverse is true - you should avoid in case some n00b has modified the Array prototype, and you should avoid modifying the Array prototype in case some n00b has used on an array.

It would be better for you to create your own type of object constructor complete with doSomething function, rather than extending the built-in Array.

What about Object.defineProperty?

There now exists Object.defineProperty as a general way of extending object prototypes without the new properties being enumerable, though I still wouldn't use this as justification for extending the built-in types, because even besides there is still the potential for other conflicts with other scripts. Consider someone using two Javascript frameworks that both try to extend the Array in a similar way and pick the same method name. Or, consider someone forking your code and then putting both the original and forked versions on the same page. Will the custom enhancements to the Array object still work?

This is the reality with Javascript, and why you should avoid modifying the prototypes of built-in types, even with Object.defineProperty. Define your own types with your own constructors.


While the potential for clashing with other bits o' code the override a function on a prototype is still a risk, if you want to do this with modern versions of JavaScript, you can use the Object.defineProperty method, turning off the enumerable bit, e.g.

// functional sort
Object.defineProperty(Array.prototype, 'sortf', {
    value: function(compare) { return [].concat(this).sort(compare); }

There is a caution! Maybe you did that: fiddle demo

Let us say an array and a method foo which return first element:

var myArray = ["apple","ball","cat"];

foo(myArray) // <- 'apple'

function foo(array){
    return array[0]

The above is okay because the functions are uplifted to the top during interpretation time.

But, this DOES NOT work: (Because the prototype is not definned) // <- 'undefined function foo' = function(){
    return this[0]

For this to work, simply define prototypes at the top: = function(){
    return this[0]
} // <- 'apple'

And YES! You can override prototypes!!! It is ALLOWED. You can even define your own own add method for Arrays.


In general messing with the core javascript objects is a bad idea. You never know what any third party libraries might be expecting and changing the core objects in javascript changes them for everything.

If you use Prototype it's especially bad because prototype messes with the global scope as well and it's hard to tell if you are going to collide or not. Actually modifying core parts of any language is usually a bad idea even in javascript.

(lisp might be the small exception there)


You augmented generic types so to speak. You've probably overwritten some other lib's functionality and that's why it stopped working.

Suppose that some lib you're using extends Array with function Array.remove(). After the lib has loaded, you also add remove() to Array's prototype but with your own functionality. When lib will call your function it will probably work in a different way as expected and break it's execution... That's what's happening here.


Using Recursion

function forEachWithBreak(someArray, fn){
   let breakFlag = false
   function breakFn(){
       breakFlag = true
   function loop(indexIntoSomeArray){

       if(!breakFlag && indexIntoSomeArray<someArray.length){

Test 1 ... break is not called

forEachWithBreak(["a","b","c","d","e","f","g"], function(element, breakFn){

Produces a b c d e f g

Test 2 ... break is called after element c

forEachWithBreak(["a","b","c","d","e","f","g"], function(element, breakFn){
    if(element =="c"){breakFn()}

Produces a b c


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