[WorldMaker]

A good comparison here is the Python approach: Python 3 made the syntax for type annotations a first-class part of the language but only to the point of parsing that syntax and providing an AST, it did not (and at least for now will not) define "compile time" or "run time" meanings for them.

The immediate benefit to adding Typescript/Flow-like annotations to the ES spec would be that you could run Typescript files unchanged (without transpilation) in the browser, even if the Browser didn't do any type checks for you. You could still use TS or Flow to do the type checks in a separate process, but you could potentially drop the type-stripping steps in TS or Babel. The possibility exists that eventually the browser could also start to enforce basic type checks, but the immediate benefit of slightly faster build processes with no type-stripping step shouldn't be overlooked.

[Vinnl]

That makes sense. I can also imagine not wanting to start too quickly with that though, to learn what such a syntax would have to support from Flow and TypeScript.