[hn_throwaway_99]

FWIW Flow actually let you write type annotations as comments so that the code was syntactically valid JavaScript. I'm not sure if TS ever had something like this.

https://flow.org/en/docs/types/comments

[mikewhy]

For a while, TypeScript has been able to check types using JSDoc. Since 2.3 I think, back in 2017.