Extension Methods in Javascript

Can we create Extension Methods in Javascript?

One of the biggest development boosts is having a decent boilerplate code base you can use when starting new projects. One way to encourage re-use of code is the use of .NET extension methods. I like them so much, me and Fons Sonnemans made a website dedicated to them. My favorite methods are Chris Meijers IsFilled() and IsEmpty(). I use them all the time to avoid checking for !String.IsNullOrEmpty() which is a lot harder to read.

I am currently working on a HTML5 website that uses KnockoutJS, jQuery and a bit of MVC3 and need to check if a string has a value. I know Javascript should just support:

if (value) {
   // you are here if value != null and value.length>0

but for some reason it did not work for me in combination with a Knockout Observable so I created a function that returns true when the string is filled. Then I wondered if Javascript has support for extension methods. And it has!

Javascript Extension Methods

The following code is a js file (/ExtensionMethods/String.js) that you can include in your project:

String.prototype.IsFilled = function () {
    if (this && this.length > 0) {
        return true;
    return false;

String.prototype.IsEmpty = function () {
    return !this.IsFilled();
Include and use the extensionmethod like this:
<!DOCTYPE html>
    <title>Javascript ExtensionMethods</title>
    <script src="Extensionmethods/String.js"></script>
    <script type="text/javascript">
        var name="Loek van den Ouweland";
        document.write("name.IsFilled(): "+name.IsFilled());
        document.write("name.IsEmpty(): "+name.IsEmpty());
  <h1>ExtensionMethods in Javascript</h1>
  <h2><a href="http://loekvandenouweland.com/">http://loekvandenouweland.com/</a></h2>
Written by Loek van den Ouweland on May 15, 2012. Questions regarding this artice? You can send them to the address below.
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