It's developed by Microsoft, who adds:
TypeScript offers classes, modules, and interfaces to help you build robust components.
We've long wanted to add TypeScript to CodePen, but were particularly motivated to get it done when it was revealted that Angular 2 will be written in TypeScript. There are a heck of a lot of Angular developer out there, so perhaps there we be a lot more people interested in working with TypeScript. Now you can, enjoy!
Here's their Hello World:
Here's the To Do demo they offer: