A really unknown and underrated language for how good it is right now is rescript. It's ocaml's type & module systems and runtime semantics grafted onto JS. Ocaml's tooling, ecosystem, and standard lib situation are all ...fine... but quirky in ways that can be barriers when learning it.

Rescript lets you skip most of those, plus the (overblown but also real) weird syntax. I like ocaml and use it for projects still but if you just want to play around with what's unique and strong about it, rescript is what I would recommend right now.