Possible Duplicate:

How does prototype extend on typescript?

I am currently learning TypeScript, and would like to know how it is possible to add functionality to existing objects. Say I want to add an implementation for Foo to the String object. In JavaScript I would do this:

String.prototype.Foo = function() {

    // DO THIS...

}

Understanding that TypeScript classes, interfaces and modules are open ended led me to try the following, without success

1. Reference the JavaScript implementation from TypeScript

    JavaScript:



    String.prototype.Foo = function() {

        // DO THIS...

    }



    TypeScript:



    var x = Hello World;

    x.Foo(); //ERROR, Method does not exist

2. Extend the interface

interface String {

    Foo(): number;

}



var x = Hello World;

x.Foo(); //Exists but no implementation.

3. Extend the class

class String {

    Foo(): number {

        return 0;

    }

}



// ERROR: Duplicate identifier 'String'

As you can see from these results, so far I have been able to add the method via an interface contract, but no implementation, so, how do I go about defining AND implementing my Foo method as part of the pre-existing String class?