[nevir]

IMO OCaml is the wrong tool for the job in this case (even if it is a better language for this sort of tool).

JavaScript has a weird ecosystem where it is extremely helpful to have all of your tools in the same language. browser-based IDEs, Node, portability, etc, and just one fewer runtime to juggle.

Same reasons why closure is awkward as a Java program.

[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