the important difference is that in c# there is no performance penalty as it is just translated into a static method call by the compiler for you. In javascript every array object you subsequently create will have that extension whether it is used or not. I think this is why you are advised to be careful about extending the prototype of basic types in javascript.  I think it's OK if it is a function that actually makes sense for all Array objects (such as your example) but it can be easily abused (what can't?)

The bizarre bit is that JScript doesen't have contains() built in....oh well its only 2008.....

@julian Since the method was defined on the prototype, I believe the implementation is shared between Array instances, rather than copied as you suggest.

Jeremy, my favourite JS feature is the ability to use map, filter and reduce on arrays.  Nice to be able use functional paradigms and also, a great way to optimise js file size too.

Actually there is Array.indexOf() which you can use - but it's still not supported in all browsers!

What does the light bulb indicate in your code snippet?

It is an emoticon representation of '( i )'

Why not use the extend function of JQuery:

jQuery.fn.extend({

 check: function() {

   return this.each(function() { this.checked = true; });

 },

 uncheck: function() {

   return this.each(function() { this.checked = false; });

 }

});

$("input[@type=checkbox]").check();

$("input[@type=radio]").uncheck();

and why are we comparing javascript/C# again?

@Jeremy,

Why are nested javascript functions to be avoided?

The functions in question are added to all JQuery objects which might be much more useful.

Did somebody say ruby?  No?  Oh, then I'll say it.  :)

Really though, open classes that allow monkey patching are awesome as hell, but can also be dangerous in the wrong hands.  Just don't let yourself be the wrong hands.

take what you know about c# and throw it out the window.

javascript != c#.  different programming style.  nested functions are everywhere.  closures are fun and useful (see module pattern, private member variables, loops and creating event handlers on dom elements).

@Scott

JavaScript has been around as long as Ruby ... so not sure what you are getting at.

@Jeremy

Spend some serious time in JavaScript and you will see the true power of JavaScript.  Most people can't because they lose their intellisense or simply don't understand JavaScript.

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A post by Jeremy Miller caught my eye this morning in regards to extension methods in Javascript . While

You should probably avoid modifying the prototype of system objects, here's why (run it in the firebug console for best results):

Array.prototype.contains = function(value){

    for (var i = 0; i < this.length; i++){

        if (this[ i] == value) return true;

    }

    return false;

};

var r = ['p','q'];

for(var i in r) {

   console.log(i);

}

@Si - Great point dude, I had no idea that would happen!!

@Si - don't loop your arrays that way and the problem goes away - www.prototypejs.org/.../array

@julian - Nope, they're shared amongst all instances - see the comments about the respondTo method msdn.microsoft.com/.../cc163419.aspx

@Jeremy - Javascript is wonderful :-)

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Modifying systems objects will bite you.  What happens when the next version of JavaScript (EcmaScript)  or a browser implementation has a 'contains' function with a different signature?  Or another library you are using modifies the prototype differently?  I want my code to be as future proof as possible.  Not polluting the system objects or global namespace is the safest way to code.  I use the Dojo Toolkit which follows the no pollution rule.  

Here is a library comparison:

dojotoolkit.org/.../why-dojo

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Monkey patch!

It is very good feature for JS!

I like to use it for custom Array list.