But this is just an on-the-fly transpile(natively supported). It would be much better if JavaScript would go the Python route. For that they would have to extend the EcmaScript spec (to be safe) I guess.

Just using JSDoc is imho not a good alternative. The big benefit of TypeScript is that your compiler checks this all for you. I've seen to much incomplete or wrong JSDoc in my life.. also a nightmare to maintain.