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JSCaml is a compiler for typed JavaScript, using OCaml as the target runtime.

The compiler is an extension of the Flow static typechecker for JavaScript. To find out more about Flow, check out

Why would you want to use JSCaml?

JSCaml assumes that your JavaScript files are fully type checked with Flow and that the Flow types are correct and can be relied on. That implies that you have to forgo using "JavaScript the incredibly bad parts" such as eval.

Moreover, if you are interested in running benchmarks, you'll probably find that existing JavaScript JIT compilers are more than a match for the OCaml code produced by JSCaml.

So why would you use JSCaml at all? Two reasons come to mind:

  1. You want to run your JavaScript code on small devices where JIT compilers are not available or do a bad job.
  2. If you are actually writing your code in Reason but you want to make use of JavaScript libraries and you also want to run on small devices.

In either case you can translate all of your JavaScript code to Reason and then you won't have a use for JSCaml. You might, however, find it a bit easier to just clean up the JavaScript code and make it fully type checked with Flow. Once you've done that you can use JSCaml to produce precompiled machine code that starts up quickly and uses far less memory than a high end JavaScript VM.

That said, JSCaml is still incomplete and more work is needed before realistic programs can routinely be compiled and expected to just run without problems. So, unless you are interested to work on the compiler and its runtime, hang on for a bit and check back later.

Getting the sources

After cloning the JSCaml repository you also need to get hold of the submodules:

git submodule init
git submodule update --recursive --init

Next, copy the relevant Webkit files to the runtime/regexp subdirectory and patch them to make them self contained.

cd regexp
cd ..

Building the JSCaml compiler

First set up your environment as instructed in Flow repository, making sure that you can build and test Flow.

To build JSCaml use the script in the compiler directory. It copies some files over into the flow sub directory, runs its make, copies the resulting extended flow binary back into the JSCaml bin directory and removes the copied files.

Building the JSCaml runtime

The JSCaml runtime is a library, written in OCaml, that provides the builtin object model for translated JavaScript programs. The library makes use of some C/C++ helper files that come from Webkit as mentioned in the section on getting the sources.

To build the runtime run the script in the runtime directory. This creates a bunch of files in the runtime/_build and runtime/_cobj directories, most of which are later required to be referenced by the ocamlopt call that compiles the translated (into OCaml) JS files together with the builtin object model binaries into a single executable.

It would be nice the JSCaml runtime could be packaged into a single file and if there were a simple way for ocamlopt get everything from the unified file, but this seems to be impossible. If you know how to do this, or can make it possible somehow, please help.

Running the JSCaml tests

As yet, there is no support for running a large test suite such as and quite a bit of work is needed to deal with the lack of Flow type annotations and the use of features from "JavaScript the very bad parts".

In the early development of JSCaml modified versions of these tests have been actually been run and about 5000 of them pass, so there is hope.

In this repository, right now, there is a single "smoke alarm" test. To compile this test run the script. (This will actually compile all of the JS files in the directory.)

To run the test file, just run the .out/test binary.


JSCaml is BSD-licensed. We also provide an additional patent grant.


A compile time transformation from JavaScript to OCaml, along with an OCaml implementation of the JavaScript builtin library.







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