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"' > foo.ml

With the following dune file:

(executable (name foo))

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.

js_of_ocaml field

In library and executables stanzas, you can specify js_of_ocaml options using (js_of_ocaml (<js_of_ocaml-options>)).

<js_of_ocaml-options> are all optional:

  • (flags <flags>) to specify flags passed to js_of_ocaml. This field supports (:include ...) forms
  • (javascript_files (<files-list>)) to specify js_of_ocaml JavaScript runtime files.

<flags> is specified in the Ordered set language.

The default value for (flags ...) depends on the selected build profile. The build profile dev (the default) will enable sourcemap and the pretty JavaScript output.

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. There is currently no other way to control this behaviour.