I wish ReasonML took off more. It's an OCaml dialect which reminds me of Rust with a garbage collector, which is unsurprising given the Rust compiler's history being initially written in OCaml and where a lot of its ideas stem from, such as exhaustive pattern matching, the Result type, functional programming being as performant as imperative programming ("zero cost abstractions") and so on. ReasonML was developed at Facebook by the creator of React and it was supposed to be used as an alternative language than JS for writing React.
However, it didn't have much support after TypeScript took the JS world by storm. If it had, we might have seen the same great Rust-like benefits on the frontend as well, and perhaps the backend too instead of NodeJS.