The TypeScript compiler can be used as a library, I believe. The Closure Compiler has a whole type system that lives in doc comments: https://developers.googl

The TypeScript compiler can be used as a library, I believe. The Closure Compiler has a whole type system that lives in doc comments: https://developers.google.com/closure/compiler/docs/js-for-compiler#types https://developers.google.com/closure/compiler/docs/js-for-c... and I believe there's a command line option for doing type checking. Alas, Closure Compiler is written in Java rather than JavaScript so you can't use it quite as seamlessly as TypeScript's compiler.