R Packages

WebR supports the loading of R packages that have been pre-compiled targeting WebAssembly and for use with webR.

WebR packages must be installed before use, which in practice means copying binaries into the webR virtual filesystem in some way, either as part of the webR build process or during runtime by downloading packages from a repository.

Downloading packages from a webR binary repository

A collection of supported packages are publicly hosted via a CDN in a CRAN-like repository with URL,


The public CDN build of webR has been distributed with a pre-installed webr support package. This R package provides a helper function install(), which can be used to install further R packages from a compatible webR binary repository.

The repository URL is set using the repos argument and the public build of webR defaults the repo argument to the repository URL shown above.

Example: Installing the Matrix package

Run the following R code in a running webR session, such as the webR REPL demo,


If the package is available for webR and downloads successfully, you can then load the package in the usual way.


Interactively installing packages

As part of startup, the webR REPL demo website runs the following R code:


This function, also provided by the pre-installed webr support package, installs a global handler that will show an interactive prompt whenever loading a library fails, asking the user if they would like to try downloading the package from the configured default webR binary package repository.

Once the global handler has been installed in this way, it is possible to install and load packages interactively via a prompt. The process is as follows,

Failed to load package "Matrix". Do you want to try downloading
it from the webR binary repo?
  1: Yes
  2: No
Selection: 1
Downloading webR package: lattice
Downloading webR package: Matrix

Once the package has been downloaded in this way, the next call to library() will load the package as usual.

Installing packages from JavaScript

The webR JavaScript API provides the function WebR.installpackages(). This convenience function takes an array of packages as its only argument and for each calls install() with the default webR binary package repository.

await webR.installPackages(['Matrix', 'cli'])

Once the promise returned by WebR.installPackages() has resolved, the packages can be loaded in the usual way using library().

Building R packages for webR


It is not possible to install packages from source in webR. This is not likely to change in the near future, as such a process would require an entire C and Fortran compiler toolchain to run inside the browser. For the moment, providing pre-compiled WebAssembly binaries is the only supported way to install R packages in webR.

Many R packages contain C or C++ code and so must be compiled for WebAssembly (along with any prerequisite system libraries) before they can be loaded in webR. One way to do this is by using the Makevars system to instruct R to compile R packages with the Emscripten compiler toolchain, setting variables such as CC and CXX to point to the relevant Emscripten compilers.

Once the R package has been compiled and binaries targeting WebAssembly are available, they can be loaded into webR by providing a CRAN-like repo to the webr::install() command.

More detailed information about the repository and package building process will be made available in the future, when the infrastructure is more stable.