The libraries underneath will still allow nonsense at runtime
Only if you use a badly written library. Most libraries have types provided by DefinitelyTyped. Those who don’t are (in my experience) so tiny that you probably aren’t using them; or, if you really wanted, can check yourself.
In the end, if you encounter a bug, it’ still 99% of the time not a library’s fault, even if it’s written in plain JS.
Like I said to the other person, those are just types over top of JavaScript that can still fail if/when coercion happens under the hood.
I don’t even know how to search it now, but a specific example came up on here of a time when JavaScript libraries will cause problems, and problems you can’t even see very well if you’re expecting it to act strictly-typed.
