Flow is a static type checker for JavaScript, and flow-runtime is a separate project that turns your static types into runtime assertions. You can even enable t

Flow is a static type checker for JavaScript, and flow-runtime is a separate project that turns your static types into runtime assertions. You can even enable this on a file by file basis. We use this at my job to statically type all of our front-end code, and then check API responses (the bane of static type systems: external data) against those same types at runtime. It works really well.