Bazel JavaScript


Bazel JavaScript rules

Bazel is Google’s build system. It powers our development at large scale by caching intermediate build artifacts, allowing build and test to be incremental and massively parallelizable. Read more at https://bazel.build

This JavaScript support lets you build and test code that targets a JavaScript runtime, including NodeJS and browsers.


First we create a new workspace, which will be in a new directory. We can use the @bazel/create npm package to do this in one command. This is the fastest way to get started for most use cases.

See the installation page for details and alternative methods.

$ npm init @bazel my_workspace
$ cd my_workspace

You could do the same thing with yarn:

$ yarn create @bazel my_workspace
$ cd my_workspace

Both of these commands are equivalent to npx @bazel/create which downloads the latest version of the @bazel/create package from npm and runs the program it contains. Run the tool with no arguments for command-line options and next steps.

Next we install some development tools. For this example, we’ll use Babel to transpile our JavaScript, Mocha for running tests, and http-server to serve our app. This is just an arbitrary choice, you probably already have some tools you prefer.

$ npm install @babel/core @babel/cli @babel/preset-env http-server mocha domino

Let’s run these tools with Bazel. There are two ways to run tools:

  • Use an auto-generated Bazel rule by importing from an index.bzl file in the npm package
  • Use a custom rule in rules_nodejs or write one yourself

In this example we use the auto-generated rules. First we need to import them, using a load statement. So edit BUILD.bazel and add:

load("@npm//@babel/cli:index.bzl", "babel")
load("@npm//mocha:index.bzl", "mocha_test")
load("@npm//http-server:index.bzl", "http_server")

This shows us that rules_nodejs has told Bazel that a workspace named @npm is available (think of the at-sign like a scoped package for Bazel). rules_nodejs will add index.bzl files exposing all the binaries the package manager installed (the same as the content of the node_modules/.bin folder). The three tools we installed are in this @npm scope and each has an index file with a .bzl extension.

Next we teach Bazel how to transform our JavaScript inputs into transpiled outputs. Here we assume that you have app.js and es5.babelrc in your project. See our example webapp for an example of what those files might look like. Now we want Babel to produce app.es5.js so we add to BUILD.bazel:

    name = "compile",
    data = [
    outs = ["app.es5.js"],
    args = [
        "$(execpath es5.babelrc)",
        "$(execpath app.es5.js)",

This just calls the Babel CLI, so you can see their documentation for what arguments to pass. We use the $(execpath) helper in Bazel so we don’t need to hardcode paths to the inputs or outputs.

We can now build the application: npm run build

and we see the .js outputs from babel appear in the dist/bin folder.

Let’s serve the app to see how it looks, by adding to BUILD.bazel:

    name = "server",
    data = [
    args = ["."],

Add a serve entry to the scripts in package.json:

  "scripts": {
    "serve": "ibazel run :server"

ibazel is the watch mode for bazel.

Note that on Windows, you need to pass --enable_runfiles flag to Bazel. That’s because Bazel creates a directory where inputs and outputs both appear together, for convenience.

Now we can serve the app: npm run serve

Finally we’ll add a test using Mocha, and the domino package so we don’t need a browser. Add to BUILD.bazel:

    name = "unit_tests",
    args = ["*.spec.js"],
    data = glob(["*.spec.js"]) + [

Run the tests: npm test

Next steps

Look through the /examples directory in this repo for many examples of running tools under Bazel.

You might want to look through the API docs for custom rules such as TypeScript, Rollup, and Terser which add support beyond what you get from calling the CLI of those tools.


Add the options in the Support for debugging NodeJS tests section from https://github.com/bazelbuild/rules_nodejs/blob/master/common.bazelrc to your project’s .bazelrc file to add support for debugging NodeJS programs.

Using the --config=debug command line option with bazel will set a number of flags that are specified there are useful for debugging. See the comments under Support for debugging NodeJS tests for details on the flags that are set.

Use --config=debug with bazel test as follow,

bazel test --config=debug //test:...

or with bazel run,

bazel run --config=debug //test:test1

to also turn on the NodeJS inspector agent which will break before any user code starts. You should then see,

Executing tests from //test:test1
Debugger listening on ws://
For help, see: https://nodejs.org/en/docs/inspector

when the test is run.

To inspect with Chrome DevTools 55+, open chrome://inspect in a Chromium-based browser and attach to the waiting process. A Chrome DevTools window should open and you should see Debugger attached. in the console.

See https://nodejs.org/en/docs/guides/debugging-getting-started/ for more details.

Debugging with VS Code

With the above configuration you can use VS Code as your debugger. You will first need to configure your .vscode/launch.json:

      "type": "node",
      "request": "attach",
      "name": "Attach nodejs_binary",
      "internalConsoleOptions": "neverOpen",
      "sourceMapPathOverrides": {
        "../*": "${workspaceRoot}/*",
        "../../*": "${workspaceRoot}/*",
        "../../../*": "${workspaceRoot}/*",
        "../../../../*": "${workspaceRoot}/*",
        "../../../../../*": "${workspaceRoot}/*",
        // do as many levels here as needed for your project

We use sourceMapPathOverrides here to rewrite the source maps produced by ts_library so that breakpoints line up with the source maps. Once configured start your process with

bazel run --config=debug //test:test1

Then hit F5 which will start the VS Code debugger with the Attach nodejs_binary configuration. VS Code will immediatenly hit a breakpoint to which you can continue and debug using all the normal debug features provided.


Bazel is generally only a build tool, and is unaware of your version control system. However, when publishing releases, you typically want to embed version information in the resulting distribution. Bazel supports this natively, using the following approach:

To stamp a build, you must pass the --stamp argument to Bazel.

Previous releases of rules_nodejs stamped builds always. However this caused stamp-aware actions to never be remotely cached, since the volatile status file is passed as an input and its checksum always changes.

Also pass the workspace_status_command argument to bazel build. We prefer to do these with an entry in .bazelrc:

# This tells Bazel how to interact with the version control system
# Enable this with --config=release
build:release --stamp --workspace_status_command=./tools/bazel_stamp_vars.sh

Then create tools/bazel_stamp_vars.sh.

This is a script that prints variable/value pairs. Make sure you set the executable bit, eg. chmod 755 tools/bazel_stamp_vars.sh. For example, we could run git describe to get the current tag:

#!/usr/bin/env bash
echo BUILD_SCM_VERSION $(git describe --abbrev=7 --tags HEAD)

For a more full-featured script, take a look at the bazel_stamp_vars in Angular

Finally, we recommend a release script around Bazel. We typically have more than one npm package published from one Bazel workspace, so we do a bazel query to find them, and publish in a loop. Here is a template to get you started:

#!/usr/bin/env bash

set -u -e -o pipefail

# Call the script with argument "pack" or "publish"
readonly NPM_COMMAND=${1:-publish}
# Don't rely on $PATH to have the right version
readonly BAZEL_BIN=./node_modules/.bin/bazel
# Use a new output_base so we get a clean build
# Bazel can't know if the git metadata changed
readonly TMP=$(mktemp -d -t bazel-release.XXXXXXX)
readonly BAZEL="$BAZEL_BIN --output_base=$TMP"
# Find all the npm packages in the repo
readonly PKG_NPM_LABELS=`$BAZEL query --output=label 'kind("pkg_npm", //...)'`
# Build them in one command to maximize parallelism
$BAZEL build --config=release $PKG_NPM_LABELS
# publish one package at a time to make it easier to spot any errors or warnings
for pkg in $PKG_NPM_LABELS ; do
  $BAZEL run --config=release -- ${pkg}.${NPM_COMMAND} --access public --tag latest

WARNING: Bazel can’t track changes to git tags. That means it won’t rebuild a target if only the result of the workspace_status_command has changed. So changes to the version information may not be reflected if you re-build the package or bundle, and nothing in the package or bundle has changed.

See https://www.kchodorow.com/blog/2017/03/27/stamping-your-builds/ for more background.

Making changes to rules_nodejs

One advantage of open-source software is that you can make your own changes that suit your needs.

The packages published to npm and GitHub differ from the sources in this repo. The packages have only runtime bazel dependencies, but the sources depend on development dependencies. For example, the @bazel_skylib library is a development-time transitive dependency, while an npm package would have that dependency statically linked in.

This differs from much of the Bazel ecosystem, where you are expected to build the whole transitive toolchain from sources.

If you have a small change, it’s easiest to just patch the distributed artifacts rather than build from source. However if you’re doing active development in rules_nodejs or have a policy of not depending on release artifacts, it’s possible to depend directly on sources. This is not yet documented; file an issue on our repo if you think you need this.

Patching the npm packages

The pattern we use most commonly is to add a postinstall hook to your package.json that patches files after they’ve been fetched from npm.

See /tools/postinstall-patches.js in the Angular repo for an example.

Patching the built-in release

rules_nodejs has a distribution format which is a tgz published to GitHub, and this can make it tricky to make casual changes without forking the project and building your own release artifacts.

Bazel has a handy patching mechanism that lets you easily apply a local patch to the release artifact for built-in rules: the patches attribute to http_archive.

First, make your changes in a clone of the rules_nodejs repo. Export a patch file simply using git diff:

git diff > my.patch

Then copy the patch file somewhere in your repo and point to it from your WORKSPACE file:

    name = "build_bazel_rules_nodejs",
    patch_args = ["-p1"],
    patches = ["//path/to/my.patch"],
    sha256 = "6d4edbf28ff6720aedf5f97f9b9a7679401bf7fca9d14a0fff80f644a99992b4",
    urls = ["https://github.com/bazelbuild/rules_nodejs/releases/download/0.32.2/rules_nodejs-0.32.2.tar.gz"],

Scope of the project

This repository contains an orthogonal set of rules which covers an opinionated toolchain for JavaScript development. When requesting a new rule, describe your use case, why it’s important, and why you can’t do it with the existing rules. This is because we have limited resources to maintain additional rules.

The repository accepts contributions in terms of bug fixes or implementing new features in existing rules. If you’re planning to implement a new rule, please strongly consider opening a feature request first so the project’s maintainers can decide if it belongs to the scope of this project or not.

For rules outside of the scope of the projects we recommend hosting them in your GitHub account or the one of your organization.


Most bazel rules include package management. That is, the WORKSPACE file installs your dependencies as well as the toolchain. In some environments, this is the normal workflow, for example in Java, Gradle and Maven are each both a build tool and a package manager.

In nodejs, there are a variety of package managers and build tools which can interoperate. Also, there is a well-known package installation location (node_modules directory in your project). Command-line and other tools look in this directory to find packages. So we must either download packages twice (risking version skew between them) or point all tools to Bazel’s external directory with NODE_PATH which would be very inconvenient.

Instead, our philosophy is: in the NodeJS ecosystem, Bazel is only a build tool. It is up to the user to install packages into their node_modules directory, though the build tool can verify the contents.

Hermeticity and reproducibility

Bazel generally guarantees builds are correct with respect to their inputs. For example, this means that given the same source tree, you can re-build the same artifacts as an earlier release of your program. In the nodejs rules, Bazel is not the package manager, so some responsibility falls to the developer to avoid builds that use the wrong dependencies. This problem exists with any build system in the JavaScript ecosystem.

Both NPM and Yarn have a lockfile, which ensures that dependencies only change when the lockfile changes. Users are strongly encouraged to use the locking mechanism in their package manager.


  • npm: https://docs.npmjs.com/files/package-lock.json
  • yarn: https://yarnpkg.com/lang/en/docs/yarn-lock/

Note that https://github.com/bazelbuild/rules_nodejs/issues/1 will take the guarantee further: by using the lockfile as an input to Bazel, the nodejs rules can verify the integrity of the dependencies. This would make it impossible for a build to be non-reproducible, so long as you have the same lockfile.