Wasm_of_ocaml is a fork of Js_of_ocaml which compiles OCaml bytecode to WebAssembly.

The generated code works with Chrome 11.9, Node.js 22 and Firefox 122 (or more recent versions of these applications).

In particular, the output code requires the following Wasm extensions to run:

• the GC extension, including functional references and 31-bit integers
• the tail-call extension
• the exception handling extension

OCaml 5.x code using effect handlers can be compiled in two different ways: One can enable the CPS transformation from js_of_ocaml by passing the --enable=effects flag. Without the flag wasm_of_ocaml will instead emit code utilizing

• the JavaScript-Promise Integration extension

The following commands will perform a minimal installation:

git clone https://github.com/ocaml-wasm/wasm_of_ocaml
cd wasm_of_ocaml
opam pin add -n --with-version 3.17 https://github.com/ocaml/dune.git
opam pin add -n --with-version 5.8.2-wasm .
opam install wasm_of_ocaml-compiler

You may want to install additional packages. For instance:

opam install js_of_ocaml-ppx js_of_ocaml-lwt

You can try compiling the program in examples/cubes. Your program must first be compiled using the OCaml bytecode compiler ocamlc. JavaScript bindings are provided by the js_of_ocaml package. The syntax extension is provided by js_of_ocaml-ppx package. Package js_of_ocaml-lwt provides Javascript specific Lwt functions.

ocamlfind ocamlc -package js_of_ocaml,js_of_ocaml-ppx,js_of_ocaml-lwt -linkpkg -o cubes.byte cubes.mli cubes.ml

Then, run the wasm_of_ocaml compiler to produce WebAssembly code:

wasm_of_ocaml cubes.byte

This outputs a file cubes.js which loads the WebAssembly code from file cube.wasm. For debugging, we currently also output the generated WebAssembly code in text file to cube.wat. Since Chrome does not allow loading from the filesystem, you need to serve the files using some Web server. For instance:

python3 -m http.server 8000 --directory .

As a larger example, you can try CAMLBOY. You need to install a forked version of Brr. Once the Js_of_ocaml UI is compiled (with dune build --profile release), you can generate WebAssembly code instead with the following command:

wasm_of_ocaml _build/default/bin/web/index.bc-for-jsoo

A large part of the runtime is implemented. File-related functions and dynamic linking are not supported yet.

Since the value representation is different, some adaptations are necessary.

The most notable change is that, except for integers, OCaml numbers are no longer mapped to JavaScript numbers. So, explicit conversions Js.to_float and Js.float are now necessary to convert between OCaml floats and JavaScript numbers. The typing of JavaScript Typed Arrays has also been changed to deal with this.