Bazel JavaScript rules

Bazel rules to build and test code that targets a JavaScript runtime, including NodeJS and browsers.

Scope of the project

This repository contains three layers:

  1. A toolchain that fetches a hermetic node, npm, and yarn (independent of what's on the developer's machine), installs dependencies using one of these tools, and generates BUILD files so that Bazel can refer to those dependencies. Also Bazel Providers to allow interop between JS rules.
  2. "Built-in" rules distributed by a .tgz archive, which give the basic ability to run Node.js programs
  3. Custom rules that are distributed on npm that more tightly integrate particular JS tooling options with Bazel.

There are also numerous examples

We would like to avoid adding more custom rules for specific npm packages.

If you would like to write a rule outside the scope of the projects we recommend hosting them in your GitHub account or the one of your organization.


Our goal is to make Bazel be a minimal layering on top of existing npm tooling, and to have maximal compatibility with those tools.

This means you won't find a "Webpack vs. Rollup" debate here. You can run whatever tools you like under Bazel. In fact, we recommend running the same tools you're currently using, so that your Bazel migration only changes one thing at a time.

In many cases, there are trade-offs. We try not to make these decisions for you, so instead of paving one "best" way to do things like many JS tooling options, we provide multiple ways. This increases complexity in understanding and using these rules, but also avoids choosing a wrong "winner". For example, you could install the dependencies yourself, or have Bazel manage its own copy of the dependencies, or have Bazel symlink to the ones in the project.

The JS ecosystem is also full of false equivalence arguments. The first question we often get is "What's better, Webpack or Bazel?". This is understandable, since most JS tooling is forced to provide a single turn-key experience with an isolated ecosystem of plugins, and humans love a head-to-head competition. Instead Bazel just orchestrates calling these tools.


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. These are arbitrary choices, you may use whatever are your favorites.

$ 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 conveniently appear together.

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.