I just want to note that it seems incorrect to call this a superficial resemblance in terms of syntax. I went and read through the whole thing, and the syntax for the type hints seems to be as close to 100% TypeScript as possible, which is to say, it is very close. There may be some features missing that TS supports, but the bulk of it is there, and what's there is basically Typescript, which is the best thing that could possibly be accomplished here.

Typescript has been doing a fantastic job, and this proposal is continuing in that same vein, truly absorbing as much of that as possible back into JS!

Kudos to everyone involved, great effort!

And re "[the type hints] do nothing": The section at the end of the proposal clearly explains why that must necessarily be the case: Evolutions of JS must not break the web (especially) for the users.

Quoting: "TypeScript's model -- which has been highly successful for JS developers -- is around non-local, best-effort checks. (...) Additionally, defining a type system to run directly in the browser means that improved type analyses would become breaking changes for the users of JavaScript applications, rather than for developers. This would violate goals around web compatibility (i.e. "don't break the web"), so type system innovation would become near-impossible. Allowing other type systems to analyze code separately provides developers with choice, innovation, and freedom for developers to opt-out of checking at any time."

So this proposal provides the best possible path forward for JS, based on what folks are voting for with their feet by using TS: Making JS compatible with TS-style type hints that can be used by external tools (i.e. not the JS engines executing the code at runtime) to validate the code in a best-effort manner while the developer is looking at it, while not changing JS runtime semantics and thus never breaking the code while the user is running it.