It's not obvious why a “linker” is needed in nodejs. After all, programs use dynamic lookups at runtime so we expect no need for static linking.
However, in the monorepo case, you develop a package and also reference it by name in the same repo. This means you need a workflow like npm link to symlink the package from the node_modules/name directory to packages/name or wherever the sources live. lerna does a similar thing, but at a wider scale: it links together a bunch of packages using a descriptor file to understand how to map from the source tree to the runtime locations.
Under Bazel, we have exactly this monorepo feature. But, we want users to have a better experience than lerna: they shouldn't need to run any tool other than bazel test or bazel run and they expect programs to work, even when they require() some local package from the monorepo.
To make this seamless, we run a linker as a separate program inside the Bazel action, right before node. It does essentially the same job as Lerna: make sure there is a $PWD/node_modules tree and that all the semantics from Bazel (such as LinkablePackageInfo provider) are mapped to the node module resolution algorithm, so that the node runtime behaves the same way as if the packages had been installed from npm.
Note that the behavior of the linker depends on whether the package to link was declared as:
In the future the linker should also generate package.json files so that things like main and typings fields are present and reflect the Bazel semantics, so that we can entirely eliminate custom loading and pathmapping logic from binaries we execute.
Update checked in generated linker index.js with
bazel run //internal/linker:linker_lib_check_compiled.update