Facebook Messenger's backend was/is OCaml... React was originally written in SML, then OCaml, then whatever it is now. And a bunch of places use it for various things.
A version of React was built to run in ReasonML, which is a flavor of Ocaml for the web, but Reason didn't even exist before React was fairly well established.
Hell, Facebook's own XHP's interface (plus PHP/Hack's execution model) is more conceptually relatable to React, and its initial development predates Jordan's time at Facebook. It wasn't JavaScript, but at the very least it defined rails for writing applications that used the DOM.
> Yes, the first prototype of React was written in SML; we then moved onto OCaml.
> Jordan transcribed the prototype into JS for adoption; the SML version of React, however great it might be, would have died in obscurity. The Reason project's biggest goal is to show that OCaml is actually a viable, incremental and familiar-looking choice. We've been promoting this a lot but I guess one blog post and testimonial helps way more.