ReasonML is a powerful functional language built by Facebook, and created by Jordan Walke who initially created React. It is not a brand new language, but a syntax over the language OCaml. OCaml is a very mature, battle tested language with extremely fast native compilation.
OCaml provides a functional syntax, with immutable data structures and 100% type safety. The first prototypes of React were actually built in a language called SML, a distant cousin of OCaml.