Is there a way to add methods on the fly to a class using typescript?

This solution has the benefit of giving you type checking when you dynamically add a method:

class MyClass {
    start() {

    }
}
var example = new MyClass();
// example.stop(); not allowed


interface MyClass {
  stop(): void;
}

MyClass.prototype['stop'] = function () {
    alert('Stop');
}
var stage2 = example;
stage2.stop();