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.

Lazy: only fetches/installs npm packages needed for the requested build/test targets.
Correct: works seamlessly with node.js module resolution. For example there are no pathMapping issues with TypeScript rootDirs.
Fast: Bazel's sandbox only sees npm packages as directories, not individual files.
Supports npm "workspaces": nested npm packages in a monorepo.

https://blog.aspect.dev/rulesjs-npm-benchmarks shows benchmarks for fetching, installing, and linking packages under rules_js as well as typical alternatives like npm and yarn.

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).
No examples yet for publishing npm packages.
ESM imports escape the runfiles tree and the sandbox due to aspect-build#362

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 documentation in the docs folder and the example usages 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

rules_js replaces some parts of bazelbuild/rules_nodejs and re-uses other parts:

Layer	Legacy	Modern
Custom rules	`npm:@bazel/typescript`, etc.	`aspect_rules_ts`, etc.
Package manager and Basic rules	`build_bazel_rules_nodejs`	`aspect_rules_js`
Toolchain and core providers	`rules_nodejs`	`rules_nodejs`

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 calling npm install or yarn install on a whole package.json.
rules_js uses Bazel's downloader to fetch only the packages needed for the requested targets, then mimics pnpm to lay out a node_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.

For historical context, we started thinking about this in February 2021 in a (now outdated) design doc and have been working through the details since then.

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 under bazel-out, and must ensure that sources are copied there first. This forces users to pass a BAZEL_BINDIR in the environment of every node action. bazelbuild/bazel#15470 suggests a way to improve that, avoiding that imposition on users.

tetsuok / rules_js