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Js_of_ocaml is a compiler from OCaml bytecode programs to JavaScript. It makes it possible to write OCaml programs that run on Web browsers. It is easy to install as it works with an existing installation of OCaml, with no need to recompile any library. It comes with bindings for a large part of the browser APIs. According to our benchmarks, the generated programs runs typically faster than with the OCaml bytecode interpreter. We believe this compiler will prove much easier to maintain than a retargeted OCaml compiler, as the bytecode provides a very stable API.


  • Findlib
  • Lwt (version 2.3.0 at least)
  • Menhir



  • run make to compile everything
  • run make install as root to install the compiler and its libraries
  • run make uninstall as root to uninstall them


Your program must first be compiled using the OCaml bytecode compiler ocamlc. JavaScript bindings, as well as a corresponding syntax extension, are provided by the js_of_ocaml package.

      ocamlfind ocamlc -package js_of_ocaml -package js_of_ocaml.syntax \
          -syntax camlp4o -linkpkg -o cubes.byte cubes.ml

Then, run the js_of_ocaml compiler to produce JavaScript code:

      js_of_ocaml cubes.byte

See also the plugin Ocamlbuild_js_of_ocaml.


Most of the OCaml standard library is supported. However,

  • Weak semantic cannot be implemented using JavaScript. A dummy implemtation is available (use `+weak.js` option)
  • Most of Sys module is not supported.

Extra libraries distributed with Ocaml (such as Thread or Str) are not supported in general. However,

  • Bigarray: bigarray are supported using Typed Arrays
  • Num: supported using `+nat.js` option
  • Graphics: partially supported using canvas (see also js_of_ocaml.graphics)
  • Unix: time related functions are supported

Tail call is not optimized in general. However, mutually recursive functions are optimized:

  • self recursive functions (when the tail calls are the function itself) are compiled using a loop.
  • trampolines are used otherwise.

Data representation differs from the usual one. Most notably, integers are 32 bits (rather than 31 bits or 63 bits), wich is their natural size in JavaScript, and floats are not boxed. As a consequence, marshalling, polymorphic comparison, and hashing functions can yield results different from usual:

  • marshalling of floats is not supported (unmarshalling works);
  • the polymorphic hash function will not give the same results on datastructures containing floats;
  • these functions may be more prone to stack overflow.

Contents of the distribution

LICENSE license and copyright notice
README this file
compiler/ compiler
examples/ small examples
lib/ library for interfacing with JavaScript APIs
runtime/ runtime system

Debugging programs

To help debugging your programs, use the "-pretty" and "-noinline" options of Js_of_ocaml. If the bytecode is build with ocaml "-g" option, Js_of_ocaml will attempt to preserve variable names as well. You can also use Js.debugger() which inserts a JavaScript debugger statement. When the JavaScript debugger is active, this statement pauses the execution of the program.