Haskell to JavaScript compiler, based on GHC
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Haskell to JavaScript compiler

GHCJS is a Haskell to JavaScript compiler that uses the GHC API.

GHCJS supports many modern Haskell features, including:

  • All type system extensions supported by GHC
  • Lightweight preemptive threading with blackholes, MVar, STM, asynchronous exceptions
  • Weak references, CAF deallocation, StableName, StablePtr
  • Unboxed arrays, emulated pointers
  • Integer support through JSBN, 32 and 64 bit signed and unsigned arithmetic (Word64, Int32 etc.)
  • Cost-centres, stack traces
  • Cabal support, GHCJS has its own package database

And some JavaScript-specific features:

  • new JavaScriptFFI extension, with convenient import pattens, asynchronous FFI and a JSRef FFI type,
  • synchronous and asynchronous threads.


First install GHC 7.8.2 or later and check with ghc --version that it's the compiler in your PATH. Next, make sure that you have all the prerequisites for your platform.

Platform-specific requirements

Linux / OS X

  • a recent version of alex and happy need to be in your PATH
  • git, make, cpp, autoreconf, patch need to be in your PATH
  • by default, ghcjs-boot will try to use the system GMP library, see ghcjs-boot --help for more info.


  • virus scanners often interfere with configure scripts (permission denied errors), disable on-access scanning before running ghcjs-boot.
  • no extra programs need to be installed, ghcjs-boot will download an archive (around 100MB) with the required programs.

Installation steps

Run the following script to install an updated Cabal and cabal-install with GHCJS support. Note that this will overwrite the cabal executable in your cabal executable installation path (typically ~/.cabal/bin), you might want to backup your current version.

git clone https://github.com/ghcjs/cabal.git
cd cabal
git checkout ghcjs
cabal install ./Cabal ./cabal-install

Make sure that you're now running the new cabal-install, GHCJS support must be listed under the compiler flags:

$ cabal install --help
                                build files (default dist)
 -g --ghc                           compile with GHC
    --ghcjs                         compile with GHCJS
    --nhc98                         compile with NHC

Next, install ghcjs and its ghcjs-prim dependency:

$ git clone https://github.com/ghcjs/ghcjs-prim.git
$ git clone https://github.com/ghcjs/ghcjs.git
$ cabal install ./ghcjs ./ghcjs-prim

If cabal install ./ghcjs ./ghcjs-prim fails because cabal cannot resolve dependencies, try adding --reorder-goals --max-backjumps=-1. Sometimes the transformers package causes problms, since GHC ships with an older version. Try --with-constraint=transformers== (or the version that came with your GHC) if the problem looks related to this package.

Build the base libraries for GHCJS:

if you used the Git repository to install:
$ ghcjs-boot --dev

if you are using a package from hackage that includes the libraries:
$ ghcjs-boot

Some distros install node.js as nodejs instead of node. Add --with-node nodejs to the ghcjs-boot command in that case.


ghcjs can be invoked with the same command line arguments as ghc. The generated programs can be run directly from the shell with Node.js and SpiderMonkey jsshell. for example:

$ ghcjs -o helloWorld helloWorld.hs
$ node helloWorld.jsexe/all.js
Hello world!

Cabal support

Use cabal install --ghcjs packageName to install a package

Most packages from hackage should work out of the box. The main exception is packages with foreign (non-Haskell) dependencies. For these packages a JavaScript implementation of the dependencies must be provided. If a package you want to use does not work, please create a ticket.


You can create a new sandbox or add GHCJS support to an existing sandbox with:

$ cabal sandbox init ghcjs

If you also want to set GHCJS as the default compiler in the sandbox, run:

$ cabal sandbox init ghcjs
$ echo "compiler: ghcjs" >> cabal.config

Package databases

Use ghcjs-pkg to manipulate the GHCJS package database

The package database and runtime files from the shims repository are kept in the GHCJS application data directory, typically ~/.ghcjs/. Remove this directory to reset your GHCJS installation, you will need to run ghcjs-boot --init again.

See GHCJS introduction for more examples.

Hacking GHCJS

If you want to hack on GHCJS, please join our friendly community on IRC at #ghcjs on freenode (You're also welcome if you only use the compiler or just want to chat about it!). Read the HACKING.markdown document to get started. The wiki may also contain useful information.

JSC and webkit

Applications that use the following libraries should compile with GHCJS and run in a modern web browser and interface with DOM and JavaScript in the browser.

  • webkit - Bindings for WebKitGTK+ that provide a low level DOM interface.
  • webkit-javascriptcore - Low level bindings for JavaScriptCore
  • jsc - Higher level interface for JavaScriptCore

You can use these libraries without GHCJS to build a native version of your application (it will use WebKitGTK+ to run without a browser). If you want to find out more about making GHCJS compatible Haskell applications check out the GHCJS Examples