TypeScript works reasonably well for this approach; it's how I tend to do things. It has some gaps when you want to do really advanced stuff, but it gives you tools like typecasting for crossing those gaps. One of the great benefits of JavaScript type systems is that since they're overlaid on a dynamic language, you can just "turn them off" as needed (when the type system fails to understand that what you're doing is safe). You can comment out type checking for a particular line, or even let your whole project successfully build while type errors remain. The type system then becomes purely a tool, and never a roadblock.