Implementing an indexer in a class in TypeScript

You cannot implement a class with an indexer. You can create an interface, but that interface cannot be implemented by a class. It can be implemented in plain JavaScript, and you can specify functions as well as the indexer on the interface:

class MyType {
    constructor(public someVal: string) {

    }
}

interface MyCollection {   
   [name: string]: MyType;
}

var collection: MyCollection = {};

collection['First'] = new MyType('Val');
collection['Second'] = new MyType('Another');

var a = collection['First'];

alert(a.someVal);