The nice thing about TypeScript is that it compiles to EcmaScript 3, so you can target older browsers and still use these goodies.

Also, TypeScript 0.9 is released, which includes niceties such as generics and type info overloading based on constant values passed in function parameters.

It's pretty slick, if only it could output code optimized for Google Closure's advanced mode, for tree-shaking awesomeness.