Or we can just drop TypeScript completely and type-check JavaScript directly.

With HM-style type inference, row-polymorphism, sum types, type classes for overloading + operations, recursive types for object methods, and type annotations for edge cases.

I've been working on this since before TypeScript existed. Latest incarnation: https://github.com/sinelaw/inty

Demo: https://sinelaw.github.io/inty/ (you may need to reload once)