Python is probably a good next language given that it's a very marketable skill right now. And it should be quite easy to pick up if you know JS already. After

Python is probably a good next language given that it's a very marketable skill right now. And it should be quite easy to pick up if you know JS already. After that I'd probably learn Rust - it's kind of a longer term bet. Seems to be getting some critical mass so it'll likely be around for a while, but that's by no means a sure thing at this point. Still, there are lots of good concepts in Rust that are worth learning that would be transferable. You might also look into ReasonML (https://reasonml.github.io/ https://reasonml.github.io/) a strongly-typed, functional alternative to JS.