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Migrating to rules_js

This document contains some of the lessons we've learned at https://aspect.dev from doing consulting work, migrating some large client repos from rules_nodejs to rules_js.

This guide remains a work-in-progress as we find new patterns.

Upgrade to Bazel 5.1 or greater

We follow Bazel's LTS policy.

rules_js and the related rules depend on APIs that were introduced in Bazel 5.0.

However we recommend 5.1 because it includes a cache for MerkleTree computations, which makes our copy operations a lot faster.

Upgrade to rules_nodejs 5.0 or greater

rules_js depends on rules_nodejs, the core module from https://github.com/bazelbuild/rules_nodejs. We need at least version 5.0 of the core rules_nodejs module.

This does not require that you upgrade build_bazel_rules_nodejs to 5.x. build_bazel_rules_nodejs can remain at 4.x or older and work along side rules_nodejs 5.x and rules_js. See the rules_nodejs_to_rules_js_migration example for how to configure your WORKSPACE to build with both rules_nodejs and rules_js while migrating.

Install pnpm (optional)

rules_js is based on the pnpm package manager. Our implementation is self-contained, so it doesn't matter if Bazel users of your project install pnpm. However it's typically useful to create or manipulate the lockfile, or to install packages for use outside of Bazel.

You can follow the pnpm install docs.

Alternatively, you can skip the install. All commands in this guide will use npx to run the pnpm tool without any installation.

If you want to use a hermetic, Bazel-managed pnpm and node rather than use whatever is on your machine or is installed by npx, see the FAQ.

Translate your lockfile to pnpm format (optional)

rules_js uses the pnpm lockfile to declare dependency versions as well as a deterministic layout for the node_modules tree.

The node_modules tree laid out by rules_js should be bug-for-bug compatible with the node_modules tree that pnpm lays out with hoisting disabled (hoist=false set in your .npmrc).

We recommend adding hoist=false to your .npmrc so that your node_modules tree outside of Bazel is similar to the node_modules tree that rules_js creates:

echo "hoist=false" >> .npmrc

See npm_translate_lock documentation for more information on pnpm hoisting.

You can use the npm_package_lock/yarn_lock attributes of npm_translate_lock to keep using those package managers. When you do, we automatically run pnpm import on that lockfile to create the pnpm-lock.yaml that rules_js requires. This has the downside that hoisting behavior may result in different results when developers use npm or yarn locally, while rules_js always uses pnpm. It also requires passing the package.json file to npm_translate_lock which invalidates that rule whenever the package.json changes in any way. As a result, we suggest using this approach only during a migration, and eventually switch developers to pnpm.

If you're ready to switch your repo to pnpm, then you'll use the pnpm_lock attribute of npm_translate_lock. Create a pnpm-lock.yaml file in your project:

  1. Most migrations should avoid changing two things at the same time, so we recommend taking care to keep all dependencies the same (including transitive). Run npx pnpm import to translate the existing file. See the pnpm import docs
  2. If you don't care about keeping identical versions, or don't have a lockfile, you could just run npx pnpm install --lockfile-only which generates a new lockfile.

To make those commands shorter, we rely on the npx binary already on your machine. However you could use the Bazel-managed node and pnpm instead, like so: bazel run -- @pnpm//:pnpm install --dir $PWD --lockfile-only

The new pnpm-lock.yaml file needs to be updated by engineers on the team as well, so when you're ready to switch over to rules_js, you'll have to train them to run pnpm rather than npm or yarn when changing dependency versions or adding new dependencies.

If needed, you might have both the pnpm lockfile and your legacy one checked into the repo during a migration window. You'll have to avoid version skew between the two files during that time.

Please note that using the yarn_lock attributes of npm_translate_lock has caveat of not supporting the pnpm-workspace.yaml which is needed by pnpm to declare workspaces. Therefore, if your project need this, the only option is to migrate to pnpm immediately and use solely the pnpm_lock attribute of npm_translate_lock.

Test whether pnpm is working

A few packages have bugs which rely on "hoisting" behavior in yarn or npm, where undeclared dependencies can be loaded because they happen to be installed in an ancestor folder under node_modules.

In many cases, updating your dependencies will fix issues since maintainers are constantly addressing pnpm bugs.

You can also check if the bug exists outside of Bazel by setting hoist=false in your .npmrc. This disables pnpm's default behavior of hoisting one version of every package to a node_modules folder at the root of the virtual store (node_modules/.pnpm/node_modules) so can resolve undeclared "phantom" dependencies. rules_js doesn't support phantom dependencies as this would break the ability to lazy fetch & lazy link only what is needed for the target being built. Setting hoist=false in your .npmrc outside of Bazel more closely resembles how dependency resolution works in rules_js. Dependency issues can often be reproduced outside of Bazel in this way.

Another pattern which may break is when a configuration file references an npm package, then a library reads that configuration and tries to require that package. For example, this mocha json config file references the mocha-junit-reporter package, so mocha will try to load that package despite not having a declared dependency on it.

Useful pnpm resources for these patterns:

In our mocha example, the solution is to declare the expected dependency in package.json using the pnpm.packageExtensions key: https://github.com/aspect-build/rules_js/blob/main/package.json.

Another approach is to just give up on pnpm's stricter visibility for npm modules, and hoist packages as needed. pnpm has flags public-hoist-pattern and shamefully-hoist which can do this, however we don't support those flags in rules_js yet. Instead we have the public_hoist_packages attribute of npm_translate_lock. In the future we plan to read these settings from .npmrc like pnpm does; follow https://github.com/aspect-build/rules_js/issues/239.

As long as you're able to run your build and test under pnpm, we expect the behavior of rules_js should match.

Typically you just add a npm_link_all_packages(name = "node_modules") call to the BUILD file next to each package.json file:

load("@npm//:defs.bzl", "npm_link_all_packages")

npm_link_all_packages(name = "node_modules")

This macro will expand to a rule for each npm package, which creates part of the bazel-bin/[path/to/package]/node_modules tree.

Update WORKSPACE

The WORKSPACE file contains Bazel module dependency fetching and installation.

Add install steps from a release of rules_js, along with related rulesets you plan to use.

Account for change to working directory

rules_js spawns all Bazel actions in the bazel-bin folder.

  • If you use a chdir.js workaround for tools like react-scripts, you can just remove this.
  • If you use $(location), $(execpath), or $(rootpath) make variable expansions in an argument to a program, you may need to prefix with ../../../ to avoid duplicated bazel-out/[arch]/bin path segments.
  • If you spawn node programs, you'll need to pass the BAZEL_BINDIR environment variable.
    • In a genrule add BAZEL_BINDIR=$(BINDIR)
    • ctx.actions.run add env = { "BAZEL_BINDIR": ctx.bin_dir.path}

Update usage of npm package generated rules

  • the load point is now a bin symbol from package_json.bzl
  • this now produces different rules, which are explicitly referenced from bin
  • to run as a tool under bazel build you use [package] which is a js_run_binary
    • rename data to srcs
    • rename templated_args to args
  • as a program under bazel run you need to add a _binary suffix, you get a js_binary
  • as a test under bazel test you get a js_test

Example, before:

load("@npm//npm-check:index.bzl", "npm_check")

npm_check(
name = "check",
data = [
"//third_party/npm:package.json",
],
templated_args = [
"--no-color",
"--no-emoji",
"--save-exact",
"--skip-unused",
"third_party/npm",
],
)

Example, after:

load("@npm//:npm-check/package_json.bzl", "bin")

exports_files(["package.json"])

bin.npm_check(
name = "check",
srcs = [
"//third_party/npm:package.json",
],
args = [
"--no-color",
"--no-emoji",
"--save-exact",
"--skip-unused",
"third_party/npm",
],
)

Completing the migration

Once everything is migrated, we can remove the legacy rules.

In package.json you can remove usage of the following npm packages which contain Bazel rules, as they don't work with rules_js. Instead, look under https://github.com/aspect-build/ for replacement rulesets.

  • @bazel/typescript
  • @bazel/rollup
  • @bazel/esbuild
  • @bazel/create
  • @bazel/cypress
  • @bazel/concatjs
  • @bazel/jasmine
  • @bazel/karma
  • @bazel/terser

Some @bazel-scoped packages are still fine, as they're tools or JS libraries rather than Bazel rules:

  • @bazel/bazelisk
  • @bazel/buildozer
  • @bazel/buildifier
  • @bazel/ibazel (watch mode)
  • @bazel/runfiles

In addition, rules_js and associated rulesets can manage dependencies for tools they run. For example, rules_esbuild downloads its own esbuild packages. So you can remove these tools from package.json if you intend to run them only under Bazel.

In WORKSPACE you can remove declaration of the following bazel modules:

  • build_bazel_rules_nodejs

You'll need to remove build_bazel_rules_nodejs load() statements from BUILD files as well. We suggest using https://docs.aspect.build/ to locate replacements for the rules you use.