js opinion
@f0x just like every other tool, it has situations where it's useful and others where it's not.
I think for public library development, I'll go with typescript in the future because then it's easier for IDEs to tell developers how to use the lib without them having to read and memorize the docs. (Docstrings get lost in compilation sadly)
For regular frontends, I so far haven't seen the upside of it and I like to keep my bundling and external dependency stack as small as possible