package js_of_ocaml-lwt

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Compiler from OCaml bytecode to JavaScript


Dune Dependency





Js_of_ocaml (jsoo)

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

  • It is easy to install and use 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.


See opam file for version constraints.


Toplevel requirements

  • tyxml, reactiveData

  • ocp-indent: needed to support indentation in the toplevel

  • higlo: needed to support Syntax highlighting in the toplevel

  • cohttp: needed to build the toplevel webserver



opam install js_of_ocaml js_of_ocaml-compiler js_of_ocaml-ppx


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.syntax package.

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

Then, run the js_of_ocaml compiler to produce JavaScript code:

js_of_ocaml cubes.byte


Most of the OCaml standard library is supported. However,

  • Weak semantic cannot be implemented using JavaScript. A dummy implementation is provided.

  • Most of the Sys module is not supported.

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

  • Bigarray: bigarrays are supported using Typed Arrays

  • Num: supported

  • Str: supported

  • Graphics: partially supported using canvas (see

  • 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.

Data representation

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 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.

Ocaml javascript
int number (32bit int)
int32 number (32bit int)
nativeint number (32bit int)
int64 Object (MlInt64)
float number
string string or object (MlBytes)
bytes object (MlBytes)
"immediate" (e.g. true, false, None, ()) number (32bit int)
"block" array with tag as first element (e.g. C(1,2) => [tag,1,2])
array block with tag 0 (e.g. [\|1;2\|] => [0,1,2])
tuple block with tag 0 (e.g. (1,2) => [0,1,2])
record block (e.g. {x=1;y=2} => [0,1,2])
contructor with arguments block (e.g. C (1, 2) => [tag,1,2])
module block
exception and extensible variant block or immediate
function function


Contents of the distribution

Filename Description
LICENSE license and copyright notice
README this file
compiler/ compiler
examples/ small examples
lib/ library for interfacing with JavaScript APIs
ppx/ ppx syntax extensions
runtime/ runtime system
toplevel/ web-based OCaml toplevel
ocamlbuild/ ocamlbuild plugin for js_of_ocaml

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