I used AS3 at that time and completely hated it. It would have been a terrible approach for JavaScript to adopt going forward. TS is much more suitable for the dynamic nature of JavaScript, IMO. I know that AS3 had dynamic features, but they tended to have a lot of very strange limitations in actual use (it actually felt more limiting than pure Java code in some cases).

I haven't tried Babel, though. It easily could be a better path forward. Does it have some equivalent of the typescript type descriptor files?