JavaScript Compilation With Js_of_ocaml

Js_of_ocaml is a compiler from OCaml to JavaScript. The compiler works by translating OCaml bytecode to JS files. The compiler can be installed with opam:

$ opam install js_of_ocaml-compiler

Compiling to JS

Dune has full support building Js_of_ocaml libraries and executables transparently. There’s no need to customize or enable anything to compile OCaml libraries/executables to JS.

To build a JS executable, just define an executable as you would normally. Consider this example:

$ echo 'print_endline "hello from js"' >

With the following dune file:

(executable (name foo) (modes js))

And then request the .js target:

$ dune build ./foo.bc.js
$ node _build/default/foo.bc.js
hello from js

Similar targets are created for libraries, but we recommend sticking to the executable targets.

If you’re using the Js_of_ocaml syntax extension, you must remember to add the appropriate PPX in the preprocess field:

 (name foo)
 (modes js)
 (preprocess (pps js_of_ocaml-ppx)))

Separate Compilation

Dune supports two modes of compilation:

  • Direct compilation of a bytecode program to JavaScript. This mode allows Js_of_ocaml to perform whole-program deadcode elimination and whole-program inlining.

  • Separate compilation, where compilation units are compiled to JavaScript separately and then linked together. This mode is useful during development as it builds more quickly.

The separate compilation mode will be selected when the build profile is dev, which is the default. It can also be explicitly specified in an env stanza. See env for more information.