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Using GHC's JavaScript Backend in the Browser

ยท 9 min read

In a previous post we introduced GHC's new JavaScript backend, which allows the compilation of Haskell code into JavaScript. This is the first tutorial in a new series about the JavaScript backend. In this post, we'll build GHC as a JavaScript cross-compiler and run a trivial Haskell program in the browser.

We plan to write more of those blog post in the coming weeks and months as we add new features (e.g. support for "foreign exports" that will allow JavaScript code to call into Haskell code, support for Template Haskell, etc.). For now it relies on our "insider" knowledge (e.g. how the FFI works) that isn't well documented elsewhere. We do plan to add a chapter about the JavaScript backend in GHC's user guide, but for now your best chance is to look at GHCJS's documentation or at the source code.

Please note: this is a technology preview of the in-development JavaScript backend for GHC. Not all Haskell features are implemented, and bugs are expected. It is currently rather complicated for JavaScript code to call into Haskell code ("foreign exports" aren't implemented). GHC isn't a multi-target compiler yet, so a GHC executable built for a native platform (Linux/x86-64, Windows/x86-64, Darwin/AArch64...) as currently distributed (via ghcup, Stack, binary distributions, etc.) won't be able to produce JavaScript. Official prebuilt binary distributions are likely to remain unavailable until GHC gains multi-target support - requiring the JavaScript backend to be built from source even after the backend matures. That's why we start this post with the required steps to build yourself a GHC compiler capable of producing JavaScript.

Building GHC as a Cross Compiler to JavaScriptโ€‹

Installing Dependenciesโ€‹

First we need to install all the typical dependencies for GHC plus Emscripten, so our final list is:

  • GHC version 9.2 or later
  • Cabal
  • Alex
  • Happy
  • Emscripten to configure with
  • (Optional) NodeJS to run JavaScript locally

Let's take these in order, a standard GHC distribution with Cabal is needed so we can boot our new compiler. We recommend using GHCUP (, or your system's package manager to install the this.

We need Alex and Happy to build GHC, these can be installed through Cabal:

cabal install alex happy -j

We need Emscripten during the configure step of the build. Emscripten should be available in most package managers, but you can also build and install it from source:

git clone
cd emsdk
./emsdk install latest
./emsdk activate latest
source ./

After installing Emscripten, emconfigure should be available on your system path. Use which emconfigure to check that it is on your $PATH. If you built from source, then the output should point to a location within the emsdk git project like so:

$ which emconfigure

For more detailed installation instructions, see

That's all we need to build GHC as a cross compiler. NodeJS can be installed via your system's package manager if you want to run the JavaScript programs locally. We'll assume it's in your $PATH for the rest of the blog post.

Building GHCโ€‹

With all the dependencies installed, we can clone GHC HEAD and build the cross compiler:

git clone --recursive

You should notice quite a few submodules being cloned as well as the main repo; expect this to take a while. Once this has completed, navigate to the ghc directory and run the following configuration commands:

cd ghc
emconfigure ./configure --target=javascript-unknown-ghcjs

emconfigure ./configure --target=javascript-unknown-ghcjs will finish by outputting a screen that looks like:

Configure completed successfully.

Building GHC version : 9.5.20221219
Git commit id : 761c1f49f55afc9a9f290fafb48885c2033069ed

Build platform : x86_64-unknown-linux
Host platform : x86_64-unknown-linux
Target platform : javascript-unknown-ghcjs

Bootstrapping using : /home/josh/.ghcup/bin/ghc
which is version : 9.4.2
with threaded RTS? : YES

Using (for bootstrapping) : gcc
Using clang : /home/josh/emsdk/upstream/emscripten/emcc
which is version : 15.0.0
linker options :
Building a cross compiler : YES
Unregisterised : NO
TablesNextToCode : YES
Build GMP in tree : NO
hs-cpp : /home/josh/emsdk/upstream/emscripten/emcc
hs-cpp-flags : -E -undef -traditional -Wno-invalid-pp-token -Wno-unicode -Wno-trigraphs
ar : /home/josh/emsdk/upstream/emscripten/emar
ld : /home/josh/emsdk/upstream/emscripten/emcc
nm : /home/josh/emsdk/upstream/bin/llvm-nm
objdump : /usr/bin/objdump
ranlib : /home/josh/emsdk/upstream/emscripten/emranlib
otool : otool
install_name_tool : install_name_tool
windres :
dllwrap :
genlib :
Happy : /home/josh/.cabal/bin/happy (1.20.0)
Alex : /home/josh/.cabal/bin/alex (
sphinx-build :
xelatex :
makeinfo :
git : /usr/bin/git
cabal-install : /home/josh/.cabal/bin/cabal

Using LLVM tools
clang : clang
llc : llc-14
opt : opt-14

HsColour was not found; documentation will not contain source links

Tools to build Sphinx HTML documentation available: NO
Tools to build Sphinx PDF documentation available: NO
Tools to build Sphinx INFO documentation available: NO

If everything is correct, you'll see that the Target platform is set to javascript-unknown-ghcjs, and the build tools will be set to their Emscripten counterparts: ar becomes emar, nm becomes llvm-nm, etc.

Finally, to build GHC:

./hadrian/build --bignum=native -j --docs=none

Expect this to take around a half hour or longer. If all goes well you should see:

| Successfully built library 'ghc' (Stage1, way p). |
| Library: _build/stage1/compiler/build/libHSghc-9.5_p.a |
| Library synopsis: The GHC API. |
| Copy package 'ghc'
# cabal-copy (for _build/stage1/lib/package.conf.d/ghc-9.5.conf)
| Run GhcPkg Recache Stage1: none => none
| Copy file: _build/stage0/bin/javascript-unknown-ghcjs-ghc => _build/stage1/bin/javascript-unknown-ghcjs-ghc
Build completed in 1h00m

Take note of _build/stage1/bin/javascript-unknown-ghcjs-ghc path. This is the GHC executable that we'll use to compile to JavaScript. To make life easier on ourselves we can alias it:

alias ghc-js=`pwd`/_build/stage1/bin/javascript-unknown-ghcjs-ghc

First Haskell to JavaScript Programโ€‹

Now that we have a version of GHC that can output JavaScript, let's compile a Haskell program and run it with NodeJS. Make a file named "HelloJS.hs", with the following contents:

-- HelloJS.hs
module Main where

main :: IO ()
main = putStrLn "Hello, JavaScript!"

Now we can compile it using the alias we defined earlier:

ghc-js HelloJS.hs

You should see the following output, and a HelloJS executable.

[1 of 2] Compiling Main             ( HelloJS.hs, HelloJS.o )
[2 of 2] Linking HelloJS.jsexe

If you have NodeJS is on your Path, then this executable can be run just like any other command line program:

Hello, JavaScript!

Notice that a directory called HelloJS.jsexe was created. This directory contains all the final JavaScript code, including a file named all.js, and a minimal index.html HTML file that wraps all.js. For now, we'll only care about all.js and return to index.html later. all.js is the payload of our HelloJS exectuable. The executable is simply a copy of all.js, with a call to node added to the top. We could have equivalently run our program with:

node HelloJS.jsexe/all.js

Haskell in the Browserโ€‹

We saw in the previous example that GHC's JavaScript backend allows us to write Haskell and run the output JavaScript with NodeJS. This produces a portable executable, but otherwise doesn't enable anything we couldn't do before; GHC can already compile Haskell to run on most platforms! So let's do something novel, and run Haskell in the browser.

In this example, we'll use Haskell to draw a simple SVG circle to our browser window. Put the following code in a file named HelloBrowser.hs:

-- HelloBrowser.hs
module Main where

import Foreign.C.String

foreign import javascript "((arr,offset) => document.body.innerHTML = h$decodeUtf8z(arr,offset))"
setInnerHtml :: CString -> IO ()

circle :: String
circle = "<svg width=300 height=300><circle cx=50% cy=50% r=50%></circle></svg>"

main :: IO ()
main = withCString circle setInnerHtml

Notice that we've encountered a Haskell feature that's only available in the JavaScript backend: JavaScript foreign imports. This feature allows our Haskell program to call JavaScript functions. In our example we use this feature to call a JavaScript arrow function that updates the body of the page with our HTML snippet containing a drawing of a circle. Alternatively, we could have set the foreign import to a function symbol like so:

foreign import javascript "setInnerHTML"
setInnerHtml :: CString -> IO ()

where setInnerHTML is defined in a .js file that is then loaded by passing the JavaScript file to GHC along with the Haskell sources.

Next, we can compile our program to JavaScript, again with our built GHC:

ghc-js HelloBrowser.hs

Or ghc-js HelloBrowser.hs foo.js if setInnerHTML is defined in foo.js.

Recall the index.html file inside the HelloBrowser.jsexe directory. This HTML file has our compiled JavaScript already included, so if you open it in your browser, you'll find it loads our SVG circle in the top-left of the page!

Example webpage screenshot

index.html contains the minimal HTML code required to load the generated JavaScript code. It simply loads the all.js file mentioned above with the following script tag that you can reuse in your own HTML files:

<script language="javascript" src="all.js" defer></script>

As the JS backend still lacks support for some FFI features (foreign exports, foreign "wrapper" imports...), JavaScript codes can't easily interact with Haskell codes. It reduces the amount of advanced/interesting examples we can present for now. We'll publish new blog posts illustrating these features when they will be implemented.


In this post, we've seen how to build a first Haskell program to run in the browser using a preview of GHC's in-development JavaScript backend. This program used "foreign imports" to make a JavaScript function available within the Haskell code, which allows a limited interaction between Haskell and the browser. We also saw the structure of the outputs of the JavaScript backend, in the .jsexe directory, and how this allows our Haskell program to be invoked by a custom HTML wrapper. This was all enabled by building a version of GHC from source, with the build process having been configured with Emscripten to produce a GHC exectuable that targets JavaScript.