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** This repository is not maintained anymore. It moved here : **

An (unofficial) ECMAScript Reference Interpreter and Double-debugger.



The minimum system dependencies are:

  • OCaml (any working version, correct version will be installed via OPAM)
  • OPAM 2.0.2
  • Nodejs >=8 (Carbon LTS release) and the corresponding npm
  • Make
  • Git

Optional dependencies:

  • rsync, for uploading builds to deployment servers
  • xdg-open, for shortcut Make targets for opening built pages
  • Perl / PHP interpreters, for single-use scripts within the tools directory
  • Python, for the runtests distributed test execution tool in tools/runtests

Installation and test with OPAM

opam switch create jsexplain 4.04.2
eval $(opam env)

opam pin -yn add JS_Parser ""
opam install -y JS_Parser

opam repository add jsexplain

opam install -y jsexplain

firefox .opam/jsexplain/share/jsexplain/driver.html 

If you intend to run the test262 test suite:

make test_init


To build all default targets:


You can run the tool locally by opening the driver.html page. (make open is a shortcut for this).

To test jsjsref using test262:

make test

Toplevel Make Targets

  • Initialisation targets
    • test_init: Initialises the test262 submodule and any other dependencies required for testing the interpreter.
    • .merlin: Builds the .merlin IDE configuration file. (Automatically built by init).
  • Build Targets
    • all (default): Builds the generator, mljsref, jsjsref targets.
    • generator: Builds the generator subdirectory.
    • jsjsref: Builds the jsref interpreter using the ml-to-js generator.
    • mljsref: Builds the jsref interpreter using the standard OCaml compiler.
  • Testing targets
    • test: Tests the generator and jsjsref.
    • test_generator: Runs the generator testsuite. (See generator directory for detail.
    • test_jsjsref: Tests jsjsref against the test262 test suite.
  • Documentation targets
    • doc: Builds jsref documentation using ocamldoc with a custom 'esdocgen' output driver.
    • esdocgen: Builds the custom ocamldoc output driver.
  • Distribution targets
    • dist: Performs a build of jsjsref and documentation and places all files required for distribution into the dist directory.
    • publish: Publishes the dist directory to the inria-gforge server (requires authentication).
    • publish-github: Publishes the dist directory to the gh-pages branch. This is executed on successful CI build.
  • Browser shortcuts
    • open: Builds jsjsref and opens the driver.html page in your browser when done.
    • opendoc: Builds the jsref documentation and opens the documentation index in your browser when done.
  • clean: Cleans all built files for the entire project, recursively.


To select subsets of tests to run you can use the -g flag with the pattern to grep for in the test name. Each test262 test case's path is a part of the test name. For example, to select only the interpreter run test cases for the if statement, you could use: npx mocha -g 'test/language/statements/if.*interprets'

Latest version of jsjsref & debugger is automatically published at:


The source code for the interpreter is primarily written in a subset of OCaml with supporting runtime libraries written in both OCaml and JS.

The source code is located in the jsref directory. It can either be built using the standard OCaml compiler (to produce a result we term mljsref), or using a custom OCaml-to-JS compiler which is located in the generator directory (we term the resulting product jsjsref).

Details about the custom compilation are provided in the generator directory.

To simplify presentation of the code, a monadic binder syntax extension is used. This is also described in the generator directory.


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