Typescript is licenced under the Apache 2 licence and is a standard. So there's no non-technical reason why they couldn't use it.
As to the technical reasons why you would use it... The annotations perhaps...? I'm struggling to see much if any need for Runtime Introspection in a language designed to run in the browser. I've seen some sophisticated client-side applications these days so... maybe? But I can't think of any real reason why you'd want it and I can see it mainly being used by some far too clever programmers to write code that everyone who inherits their project stares at and backs away from.
So chiefly I see its use being impregnable job security. No doubt and Objective C programmer will appear to tell me why it's actually useful here. ;)
Biting the hand that feeds IT
