js_of_ocaml
js_of_ocaml compiles OCaml bytecode programs into JavaScript for use in browsers and Node.js.
What js_of_ocaml is
js_of_ocaml compiles OCaml bytecode programs into JavaScript for use in browsers and Node.js. It works with an existing OCaml installation without requiring libraries to be recompiled, and Dune supports it natively, using separate compilation in development and whole-program compilation for release builds. The whole-program mode performs global dead-code elimination and cross-module optimization. Its library provides a typed interface between OCaml and JavaScript values, along with browser API bindings such as the DOM, Fetch, WebSockets, WebGL, workers, and typed arrays. Optional packages add Lwt support, TyXML, JSON derivation, and an OCaml toplevel in the browser. Documented runtime targets include Node.js 18 or later, QuickJS-NG, and modern ES2020 browsers. Requirements include OCaml 4.13 to 5.5, Dune 3.20 or later, and Menhir; installation is through opam. The project is free and open source, with GPL-2.0-or-later and LGPL-2.1-or-later licenses with the OCaml linking exception. Most of the standard library is supported, but most of the Sys module is not.
Good
- Works without recompiling existing OCaml libraries.
- Dune supports development and release compilation modes.
- Includes bindings for a broad set of browser APIs.
- Free and open source, with packages available through opam.
Watch out
- Most of the Sys module is unsupported.
- Extra libraries such as Thread are generally unsupported.
- Tail calls are not optimized in general.
Who it suits
It suits OCaml developers who want to target JavaScript environments while using browser bindings and typed OCaml-to-JavaScript interoperability.
Verdict
js_of_ocaml offers a route from OCaml bytecode to JavaScript, with Dune integration and browser bindings. Check its runtime and library limits against the application you plan to build.
Plans and prices
From js_of_ocaml's own pricing page, checked 9 Oct 2026.
| Plan | Price | What it gives you |
|---|---|---|
| js_of_ocaml | Free | No paid plans or usage limits stated on the pages reviewed |
The facts
Each one read from js_of_ocaml's own pages, with the date it was checked.
What it does
- Async integration
- The optional js_of_ocaml-lwt package provides Lwt support and bindings including DOM events, XMLHttpRequest, and JavaScript promises.checked 9 Oct 2026
- Build integration
- Dune supports js_of_ocaml natively and selects separate compilation in dev and whole-program compilation in release builds.checked 9 Oct 2026
- Installation
- The compiler and related packages are installable through opam, the OCaml package manager.checked 9 Oct 2026
- JavaScript interop
- Its library provides a typed interface for bridging differences between OCaml and JavaScript values.checked 9 Oct 2026
- JavaScript interoperability
- The library provides a typed interface for bridging OCaml and JavaScript values, with PPX syntax for JavaScript properties, methods, constructors, and objects.checked 9 Oct 2026
- Libraries
- The distribution includes packages for Lwt support, TyXML, JSON derivation, and running an OCaml toplevel in the browser.checked 9 Oct 2026
- License
- The repository lists GPL-2.0-or-later and LGPL-2.1-or-later with the OCaml linking exception as its licenses.checked 9 Oct 2026
- Limits
- Most of the Sys module is unsupported, extra OCaml libraries such as Thread are generally unsupported, and tail calls are not optimized in general.checked 9 Oct 2026
- Node support
- The installation guide lists Node.js 18 or later as a supported environment.checked 9 Oct 2026
- No recompiling libraries
- It works with an existing OCaml installation and does not require recompiling libraries.checked 9 Oct 2026
- Optimization
- Whole-program compilation performs global dead-code elimination and cross-module optimizations to produce smaller and faster output.checked 9 Oct 2026
- Performance
- The project says its benchmarks show generated programs typically run faster than with the OCaml bytecode interpreter.checked 9 Oct 2026
- Purpose
- Js_of_ocaml compiles OCaml bytecode programs to JavaScript so they can run in browsers and Node.js.checked 9 Oct 2026
- Requirements
- The documented compiler requirements include OCaml 4.13 to 5.5, Dune 3.20 or later, and Menhir.checked 9 Oct 2026
- Runtime targets
- Generated JavaScript is documented to work with Node.js 18 or later, modern ES2020 browsers, and QuickJS-NG.checked 9 Oct 2026
- Security and compliance
- The project pages reviewed do not state a security certification or compliance claim.checked 9 Oct 2026
- Support
- The project asks users to report problems or questions through GitHub issues and include version details and a minimal reproducible example when possible.checked 9 Oct 2026
- Support channel
- The contribution guide directs users to open a GitHub issue to report problems or ask questions.checked 9 Oct 2026
- Supported OCaml libraries
- Most of the OCaml standard library is supported, while most of the Sys module is not supported and Graphics is partially supported.checked 9 Oct 2026
Where it runs
- Browser APIs
- The base library includes bindings for browser APIs such as DOM, Fetch, WebSockets, WebGL, workers, and typed arrays.checked 9 Oct 2026
- Browser bindings
- The project includes bindings for a large part of browser APIs.checked 9 Oct 2026
- Browser support
- Generated JavaScript supports modern browsers that implement ECMAScript 2020, including Chrome 80+, Firefox 74+, Safari 13.4+, and Edge 80+.checked 9 Oct 2026
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