Introduction
When running under Node.js, the Emscripten WebAssembly environment can make available the contents of a directory on the host filesystem. In addition to providing webR access to external data files, a pre-prepared R package library can be mounted from the host filesystem. This avoids the need to download potentially large R packages or filesystem images over the network.
See the webR documentation for more details on mounting host directories under Node.js.
Building an R package library
To build an R package library, we must first build one or more Wasm R
packages using add_pkg(). As an example, let’s build a
package with a few hard dependencies. Ensure that you are running R in
an environment with access to Wasm development tools1, then run:
rwasm::add_pkg("dplyr")After the build process has completed, the new repo
directory contains a CRAN-like package repository with R packages build
for Wasm.
Next, run the following to build an R package library:
rwasm::make_library()By default, this function will create a new directory named
lib if it does not already exist. This directory will
contain an R package library consisting of all the packages previously
added to the CRAN-like repository in repo using
add_pkg().
Mounting host directories
In your node application, use the webR JS API to create a new directory on the VFS and mount the host directory containing your R packages:
await webR.init();
await webR.FS.mkdir("/my-library");
await webR.FS.mount('NODEFS', { root: '/path/to/lib' }, "/my-library");Once mounted, the contents of your host R library directory are
available at /my-library in the virtual filesystem.
list.files("/my-library")
#> [1] "R6" "cli" "dplyr" "fansi" "generics" "glue"
#> [7] "lifecycle" "magrittr" "pillar" "pkgconfig" "rlang" "tibble"
#> [13] "tidyselect" "utf8" "vctrs" "withr"This new directory should be added to R’s .libPaths(),
so that R packages may be loaded from the new library.