Very interesting. The Apache 2.0 Licence is comforting, makes it worth having a look. The docs are here for anyone interested in the spec:

http://typescript.codeplex.com/documentation

I like what they've done. Nothing too revolutionary, mostly adding static typing to Javascript without straying to far from the existing (or future) language or increasing the noise significantly.

Here's some highlights:

* Type inference

    function f() {
        return "Hello World!";
    }

Will do the right thing

* Explicit Typing

    function f(s: string) {
        return s;
    }

* 'Ambient Types'

To facilitate integration into existing JS libraries or the DOM, TypeScript lets you specify 'placeholders' for variables / classes that the runtime would expect to see e.g.

    declare var document;

Almost a kind of dependency injection but also a neat way to manage talking to existing code.

* Structural Typing

* Classes and Interfaces

    interface BankAccount {
        balance: number;
        deposit(credit: number): number;
    }
    
    class CheckingAccount extends BankAccount {
        constructor(balance: number) {
            super(balance);
        }
        writeCheck(debit: number) {
            this.balance -= debit;
        }
    }

* Modules

    module M {
        var s = "hello";
        export function f() {
            return s;
        }
    }

* Arrow function expressions

    (x) => { return Math.sin(x); }