And you could transition off of Typescript just as gradually as you could transition on to it.
JS isn't going to get compile time anything. There was an attempt at adding "strong mode" by one of the google / chrome teams iirc and that went nowhere.
I think there were discussions of doing that at some point, but it seems to have gone nowhere.
On the other hand Flow is much less cool than TypeScript these days. Plus using it for development under Windows have issues, although FB has started paying attention to them.
Although I still hit enough annoyances that I'd go straight to `.ts` files for my next project.
You have to be aware these features would need to be manually converted back to JS were you ever to switch back. Honestly this would likely not be a significant amount of work, especially if you don't overuse them, but just to make you (GP) aware.