Extending String in typescript

gistfile1.js

interface String {
    format(): string;
}

String.prototype.format = function () { return this + "-formatted"; }


// Module
module Test {

    // Class
    export class Hello {
        // NOTE: x is a string, not String
        constructor (public x: string) { }

        say() {
            //note: .format method is visible by the language service
            var val = this.x.format();
            alert(val);
        }
    }

}

var hello = new Test.Hello("alex");
hello.say();