Reasonml is an alternative syntax for OCaml. From the user's perspective, there is just one language OCaml, with two alternative syntax OCaml or Reason. If you want to interface to Js you need additional annotation for FFI, no matter which syntax you choose. The Reason syntax includes this FFI https://reasonml.github.io/docs/en/external adapted from Bucklescript, while Bucklescript itself is just OCaml plus the annotation and maybe other compiler directives. The Reason syntax also includes support for React which is not part of OCaml. I don't know what goes under the ReScript umbrella.