"Cup of water" isn't a great example because it would imply then cup was made from water. This makes more sense when you realize "cup of water" is lazy speech for "cup full of water".
"of" usually implies a state of being while "from" implies a transformation.
"JS of Ocaml" implies that the JS is Ocaml, but that's not true as it is transformed. "JS from Ocaml" would be more accurate.
* "used as a function word to indicate origin or derivation, eg. a man of noble birth" (js code comes from ocaml code)
* "used as a function word to indicate the component material, parts, or elements or the contents, eg. throne of gold / cup of water" (js application made of ocaml code)