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Js_of_ocaml is a compiler from OCaml bytecode programs to JavaScript. It makes it possible to run pure OCaml programs in JavaScript environment like browsers and Node.js. 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.

Js_of_ocaml is composed of multiple packages:

  • js_of_ocaml-compiler, the compiler.
  • js_of_ocaml-ppx, a ppx syntax extension.
  • js_of_ocaml, the base library.
  • js_of_ocaml-ppx_deriving_json
  • js_of_ocaml-lwt, lwt support.
  • js_of_ocaml-tyxml, tyxml support.
  • js_of_ocaml-toplevel, lib and tools to build an ocaml toplevel to javascript.

Note: All code examples in this manual use Js_of_ocaml's ppx syntax. It is, however, possible to use Js_of_ocaml purely as a compiler while using a different package (e.g. gen_js_api, brr) to provide bindings to the browser APIs.


The easiest way to install js_of_ocaml is to use opam. opam install js_of_ocaml js_of_ocaml-ppx js_of_ocaml-lwt

For alternatives, see Install.


Your program must first be compiled using the OCaml bytecode compiler ocamlc. JavaScript bindings are provided by the js_of_ocaml package and the syntax extension by the js_of_ocaml-ppx package

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

Then, run the js_of_ocaml compiler to produce JavaScript code:

      js_of_ocaml cubes.byte

JavaScript files generated by js_of_ocaml are UTF-8 encoded.

with ocamlbuild and oasis

Js_of_ocaml also provide an ocamlbuild plugin. See js_of_ocaml-ocamlbuild>>.

with dune

Dune has native support for js_of_ocaml. It support both standard and separate compilation of javascript files. See https://dune.readthedocs.io/en/latest/jsoo.html


You can find an OCaml toplevel running in the browser here.

Supported features

Most of the OCaml standard library is supported. However,

  • Most of Sys module is not supported.

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

  • Bigarray: bigarray are supported using Typed Arrays
  • Num: supported
  • Str: supported
  • Graphics: partially supported using canvas (see also js_of_ocaml-lwt.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. More about tail call optimization.

Effect handlers are fully supported with the --enable=effects flag. This is not the default for now since effects are not widely used at the moment and the generated code can be slower, larger and less readable.

Data representation differs from the usual one. Most notably, integers are 32 bits (rather than 31 bits or 63 bits), which 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 floats might generate different output. Such output should not be unmarshalled using the standard ocaml runtime (native or bytecode);
  • the polymorphic hash function will not give the same results on datastructures containing floats;
  • these functions may be more prone to stack overflow.

Note that float rounding is slightly different between native and JavaScript. Both round to nearest but resolve the tie differently. Javascript resolves the tie away from zero while libc resolves the tee to even.