[lpw25]

OCaml has excellent compile-to-JavaScript support. Facebook use this to compile their Hack type-checker for an in-browser IDE. I imagine they do something similar for Flow.

[avsm]

They do indeed use js_of_ocaml in the Flow test suite to compile the full Flow parser to JavaScript and then test that.

See the related discussion on packaging it in the OPAM pull request: https://github.com/ocaml/opam-repository/pull/3083