Go to the link and read about it. It's not for web. It's a way for web dev to create WinStore/Metro apps.

Jesus f Christ! 100% of people commenting here don't even fucking bother to read what the fuck WinJs is about before shooting off their mouth.

How does that invalidate parent's question? TypeScript compiles to JS. Couldn't MS have developed "WinTS" instead?

Though I haven't checked, I would guess that WinJS has strongly-typed TypeScript bindings, for those who choose to use TypeScript. But releasing the library itself in JS makes it available to a wider audience and doesn't prevent it from being used in TypeScript.