TypL is a next generation tool for analyzing and improving your JS code, a very different take on language types than TypeScript or Flow.
With TypL, you just write completely standard JS, and the tool figures out your types via powerful inferencing. You can optionally annotate value-types (not variables!), again with completely standard JS.
Unlike TypeScript/Flow, TypL has both a compile-time linter and a runtime assertion library, for complete coverage of your types while still maintaining the essence of dynamic-typing in JS.