Its written mostly in ReasonML (syntax for OCaml) with a bunch of C-libs wrapped in OCaml (Skia/Harfbuzz/SDL2 etc), so it compiles properly to native in the same way that normal OCaml code would.

The only JS in Revery is for some utility scripts and some aroud an option to compile to JS (which isn't functional at the moment, but the idea being you could compile to JS if you wished, with stubs to fill in the blanks for API calls that would normally hit Skia etc).