Bazel rules for JavaScript
This ruleset is a high-performance alternative to the build_bazel_rules_nodejs
Bazel module and
accompanying npm packages hosted in https://github.com/bazelbuild/rules_nodejs.
- Only fetches/installs npm packages needed for the requested build/test targets.
- Works correctly with node resolution. For example there are no pathMapping issues with TypeScript
rootDirs
. - Supports npm workspaces (nested npm packages in a monorepo).
- Fast: Bazel's sandbox only sees packages as directories, not individual files.
Known issues:
- Doesn't support Remote Execution (RBE) due to bazelbuild/bazel#10298.
- Doesn't work with rules_docker due to bazelbuild/rules_pkg#115 (comment).
- Actions may follow symlinks out of the sandbox, leading to non-hermeticity. Fix planned for 1.0: aspect-build#114.
- No examples yet for proto/gRPC, though this may "just work".
- No examples yet for publishing npm packages.
rules_js is just a part of what Aspect provides:
- Need help? This ruleset has support provided by https://aspect.dev.
- See our other Bazel rules, especially those built for rules_js, such as rules_ts for TypeScript: https://github.com/aspect-build
Installation
From the release you wish to use:
https://github.com/aspect-build/rules_js/releases
copy the WORKSPACE snippet into your WORKSPACE
file.
Usage
See the API documentation in the docs folder and the example usage in the examples folder.
Note that the examples also rely on code in the
/WORKSPACE
file in the root of this repo.
Read our migration guide to adopt rules_js in an existing project.
Relationship to rules_nodejs
It is not a complete replacement for rules_nodejs, as the foundational layer is still used:
The common layer here is the rules_nodejs
Bazel module, documented as the "core" in
https://bazelbuild.github.io/rules_nodejs/:
It is currently useful for Bazel Rules developers who want to make their own JavaScript support.
That's what rules_js
does! It's a completely different approach to making JS tooling work under Bazel.
First, there's dependency management.
build_bazel_rules_nodejs
uses existing package managers by callingnpm install
oryarn install
on a wholepackage.json
.rules_js
uses Bazel's downloader to fetch only the packages needed for the requested targets, then mimicspnpm
to lay out anode_modules
tree.
Then, there's how a nodejs tool can be executed:
build_bazel_rules_nodejs
follows the Bazel idiom: sources in one folder, outputs in another.rules_js
follows the npm idiom: sources and outputs together in a common folder.
There are trade-offs involved here, but we think the rules_js
approach is superior for all users,
especially those at large scale. Read below for more in-depth discussion of the design differences
and trade-offs you should be aware of.
Also see the slides for our Bazel eXchange talk
Design
The authors of rules_js
spent four years writing and re-writing build_bazel_rules_nodejs
.
We learned a lot from that project, as well as from discussions with Rush maintainer @octogonz.
There are two core problems:
- How do you install third-party dependencies?
- How does a running nodejs program resolve those dependencies?
And there's a fundamental trade-off: make it fast and deterministic, or support 100% of existing use cases.
Over the years we tried a number of solutions and each end of the trade-off spectrum.
Installing third-party libraries
Downloading packages should be Bazel's job. It has a full featured remote downloader, with a content-address-cached (confusingly called the "repository cache"). We now mirror pnpm's lock file into starlark code, then use only Bazel repository rules to perform fetches and translate the dependency graph into Bazel's representation.
See the design doc
Running nodejs programs
Fundamentally, Bazel operates out of a different filesystem layout than Node. Bazel keeps outputs in a distinct tree outside of the sources.
Our first attempt was based on what Yarn PnP and Google-internal nodejs rules do:
monkey-patch the implementation of require
in NodeJS itself,
so that every resolution can be aware of the source/output tree difference.
The main downside to this is compatibility: many packages on npm make their own assumptions about
how to resolve dependencies without asking the require
implementation, and you can't patch them all.
Unlike Google, most of us don't want to re-write all the npm packages we use to be compatible.
Our second attempt was essentially to run npm link
before running a program, using a runtime linker.
This was largely successful at papering over the filesystem layout differences without disrupting
execution of programs. However, it required a lot of workarounds anytime a JS tool wanted to be
aware of the input and output locations on disk. For example, many tools like react-scripts (the
build system used by Create React App aka. CRA) insist on writing their outputs relative to the
working directory. Such programs were forced to be run with Bazel's output folder as the working
directory, and their sources copied to that location.
rules_js
takes a better approach, where we follow that react-scripts-prompted workaround to the
extreme. We always run JS tools with the working directory in Bazel's output tree.
We can use a pnpm
-style layout tool to create a node_modules
under bazel-out
, and all resolutions
naturally work.
This third approach has trade-offs.
- The benefit is that very intractable problems like TypeScript's
rootDirs
just go away. In that example, we filed microsoft/TypeScript#37378 but it probably won't be solved, so many users trip over issues like this and this. Now this just works, plus results like sourcemaps look like users expect: just like they would if the tool had written outputs in the source tree. - The downside is that Bazel rules/macro authors (even
genrule
authors) must re-path inputs and outputs to account for the working directory underbazel-out
, and must ensure that sources are copied there first. This forces users to pass aBAZEL_BINDIR
in the environment of every node action. bazelbuild/bazel#15470 suggests a way to improve that, avoiding that imposition on users.