[hackerboos]

Typescript looks great until you realize you need special files that match types in third-part libraries:

http://www.typescriptlang.org/Handbook#writing-dts-files

Will your third-party-lib that doesn't use Typescript keep a dts file? Who knows...

[radicalbyte]

..and for that work, you get a whole class of errors removed from your code..

Luckily most of the big libraries are covered; it's mainly smaller stuff - think random jQuery plugins - that aren't covered.

These are libraries which are often hacked together, lack tests, clear documentation and anything approaching support. So things that you shouldn't really rely on.