Just a quick heads-up, if you want to truly write functional code in the browser there is js_of_ocaml available in the OCaml realm. With Bonsai [0] a somewhat usable framework for webapps is available now, too. There are drawbacks like filesize, but if you don't have to serve the webapp on the first pageload it shouldn't be a problem.

[0]: https://bonsai.red