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gen_js_api: easy OCaml bindings for JavaScript libraries

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gen_js_api aims at simplifying the creation of OCaml bindings for JavaScript libraries. It must currently be used with the js_of_ocaml compiler, although other ways to run OCaml code "against" JavaScript might be supported later with the same binding definitions (for instance, Bucklescript, or direct embedding of a JS engine in a native OCaml application).

gen_js_api is based on the following ideas:

  • Authors of bindings write OCaml signatures for JavaScript libraries and the tool generates the actual binding code with a combination of implicit conventions and explicit annotations.

  • The generated binding code takes care of translating values between OCaml and JavaScript and of dealing with JavaScript calling conventions.

  • All syntactic processing is done by authors of bindings: the client code is normal OCaml code and does not depend on custom syntax nor on JS-specific types.

gen_js_api can be used in two complementary ways:


The repository contains some examples of OCaml bindings to JavaScript libraries created with gen_js_api:


Related projects

  • js_of_ocaml: The compiler and runtime system on which gen_js_api relies. (Note: gen_js_api doesn't depend on js_of_ocaml's OCaml library, nor on its language extension.)

  • goji: A DSL to describe OCaml bindings for JavaScript libraries.

  • DefinitelyMaybeTyped: A project to parse DefinitelyTyped interfaces and produce OCaml interfaces.

  • ReScript: Another compiler from OCaml to JavaScript, featuring the genType ppx for generating TS / Flow types and runtime converters.


gen_js_api has been created by LexiFi for porting a web application from JavaScript to OCaml. The tool has been used in production since 2015.

This gen_js_api package is licensed by LexiFi under the terms of the MIT license.

See see Changelog



  • Alain Frisch
  • Sebastien Briais