All of our existing JavaScript libraries were already typed, but with the awkward "types in comments" system: https://github.com/google/closure-compiler/wiki/Ty

All of our existing JavaScript libraries were already typed, but with the awkward "types in comments" system: https://github.com/google/closure-compiler/wiki/Types-in-the-Closure-Type-System https://github.com/google/closure-compiler/wiki/Types-in-the... Their team only (though it's a big "only") needed to do the work to make this existing type info available to TS