Warning: Reason support is experimental. We are looking for beta-tester and contributors.

Link with JavaScript code

The Js_of_ocaml compiler accepts JavaScript files provided on the command-line. The main purpose is to provide (external) primitives needed by the bytecode program.

Most of the primitives from the standard library are already implemented and loaded by default (located in "runtime.js").

Additionally, some other primitives are installed but not loaded by default:

  • "+toplevel.js" : when compiling toplevel


Pass the JavaScript file (must have a ".js" extension)

js_of_ocaml jsfile.js a.byte

Or load it from a findlib package

js_of_ocaml +mypackage/jsfile.js a.byte

The file jsfile.js will be looked up inside mypackage lib directory. When no package is provided, the compiler will look inside js_of_ocaml-compiler lib directory.

Provide your own JavaScript

You may need to provide extra JavaScript files to provide missing primitives or to override existing ones. Primitive code must be annotated with the primitive name and primitive requirements. The linker uses these information to only include the primitive actually used in the program and to perform better deadcode elimination.


//Provides: primitive_name [const|mutable]
//Requires: primitive_name[,primitive_name]*
//Version: version_constraint[,version_constraint]*

function primitive_name(..){
 ... JavaScript code ...
  • //Provides is used to declare a primitive; an annotation can be used to specify the possible side-effects of the primitive:
    • const means no side-effect; mutable indicates that the primitive has no side-effect but that other primitives might affect the returned value of the primitive; when no annotation is provided, the linker assumes that the primitive may have side-effects.
  • //Requires is used if other primitives need to be loaded first
  • version_constraint looks like "< 4.02.1"
  • //Version is optional and is rarely used All JavaScript code following a //Provides annotation is associated to this annotation, until the next //Provides annotation.