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Compiling R packages for WebAssembly

Building binary R packages for WebAssembly (Wasm) requires cross-compiling packages containing C/C++/Fortran source code using a Wasm development toolchain. As such, the Emscripten C/C++ toolchain and a version of LLVM flang configured to output Wasm must be available in the environment for the rwasm package to function.

There are two supported ways to ensure that the required environment and toolchain is available. Either install and build a local development build of webR, or use the rwasm package inside a pre-prepared Docker container to build Wasm R packages.

Setting up the WebAssembly toolchain

First, ensure that Docker is installed on your machine1. Then, in a terminal, pull the latest version of the webR development container:

docker pull

Next, create a new directory named output to store the built R packages,

mkdir -p output

Then start R in the Docker container, mounting the new output directory into place and setting it as the working directory2:

docker run -it --rm -v ${PWD}/output:/output -w /output R

You may now continue by working in this R session. The Docker container will provide the required development environment and tools for building binary R packages for Wasm. R packages and repositories built in this way will be written in the directory output.

Note that building webR from source takes a long time and a lot of disk space, mostly due to the requirement of building LLVM flang from source.

Follow the webR build instructions to download and build webR from source. If you are planning to build R packages that depend on system libraries, ensure that you also perform the optional step to additionally build all WebAssembly libraries.

Once webR has been built, we need to configure your environment so that the native R process can find your webR development installation. Make a note of your webR development directory and your Emscripten installation directory3.

Edit or create the file ~/, and ensure it contains the following lines, replacing the values with your own installation directories:


The settings above can also be made available to R by exporting them as environment variables.

Installing the rwasm package

The rwasm package builds R binary packages for WebAssembly, organising the output into CRAN-like repositories. The pak package can be used to install rwasm from GitHub.


Once installed, load the rwasm package. If rwasm is able to find the Wasm development environment, it will print the directories it discovered and the version of webR that it is targeting. This might be different to the version of R installed on your native system.

#> Targeting Wasm packages for R 4.3.0
#> With `WEBR_ROOT` directory: /opt/webr
#> With `EMSCRIPTEN_ROOT` directory: /opt/emsdk/upstream/emscripten

Building an R package

R package binaries can be built for WebAssembly using the build() function. The resulting Wasm R package binary and an Emscripten filesystem image containing the R package are written to the directory given by the out_dir argument, defaulting to the current working directory.


Wasm R package binaries must be made available to the webR filesystem in some way before they can be used. Read on for more details on how to build an R package library for static web hosting of Wasm R packages. Alternatively, see vignette("mount-fs-image.Rmd") for details on how to mount Emscripten filesystem images.

Adding an R package to a package repository

Use add_pkg() to build an R package (along with its dependencies) for Wasm and add it to an R package repository. A new directory named repo will be created for the repository if it does not already exist, otherwise the existing repository will be updated to include the new package. By default, hard package dependencies will also be built for Wasm and added to the repository.


See the ?pkgdepends::pkg_refs article to see which kind of package references can be used to add packages to the repository.

Managing and using the repository

The CRAN-like R package repository in the output directory repo should be hosted by a web server so that it is available at some URL. Such a URL can then be passed to webr::install() as a repository from which to install Wasm R packages.

Local testing

The R command given below starts a local web server to serve your package repository for testing4. The Access-Control-Allow-Origin: * HTTP header is included, required for loading R packages from a cross-origin server through the CORS mechanism.

If you are using a Docker container to build Wasm R packages, be sure to run the following command in a new R session outside Docker so that the web server is accessible to the host machine. You might also want to set dir = "./output" to serve the contents of the output directory mounted in the Docker container.

  dir = ".",
  port = 9090,
  browse = FALSE,
  headers = list("Access-Control-Allow-Origin" =  "*")

Once the web server is running start a webR session in your browser, such as the console at Install a package from your local repository using your test server URL as the repos argument5:

webr::install("cli", repos = "")
#> Downloading webR package: cli

Deployment to static hosting

Once you are happy that your R package repository is working and sufficient, it should be deployed to the web via a static file hosting service of your choice. In this example we will use GitHub Pages.

First, create a new directory for your GitHub repository and copy your webR binary repo into place.

mkdir -p my-wasm-repo
cp -r ./path/to/output/repo my-wasm-repo/repo
cd my-wasm-repo

Next, initialise a new GitHub repository to host your binary R packages, and run the commands given by GitHub to initialise and push an initial commit of your Wasm binary packages.

git init
git add repo
git commit -m "First commit"
git branch -M main
git remote add origin
git push -u origin main

Now, in your web browser, refresh your GitHub project and click Settings. If you cannot see the “Settings” tab, click the dropdown menu, then click Settings.

In the “Code and automation” section of the sidebar, click Pages.

Under “Build and deployment”, under “Source”, select Deploy from a branch.

Under “Branch”, use the branch dropdown menu and select main as the publishing source, then click Save.

GitHub will then start to prepare your GitHub Pages site to contain your CRAN-like Wasm package repository.

After a little while6, your GitHub Pages website will be ready and webR should be able to install your package from the GitHub Pages repo URL.

webr::install("cli", repos = "")
#> Downloading webR package: cli