You mean in TS?

Yes. ∀ x: I can do x in JS → I can do x in TS

But, let's assume for a second that you couldn't: this thread is about TS being future-proof. I.e.: ∃ x: I can do x in TS & I cannot do x in JS? No, false.

This implies TS = JS. And that's the point: semantically, TS = JS.