Concatjs is a JavaScript bundler, in a trivial sense: the UNIX cat command is a basic implementation:

$ cat one.js two.js > bundle.js

Clearly self-evident is that this bundler is super-fast and simple. A performant implementation adds some in-memory caching, and for developer ergonomics you add a simple IIFE wrapper around each file so that the Chrome DevTools shows the files in the tree as if they had been independently loaded.

However at its core, concatjs requires a big tradeoff of a migration cost to buy-in, to get this incredible performance. The path of the JavaScript files is lost in the bundling process, so they must contain their module ID internally.

Named AMD/UMD modules and goog.module are the two JS module formats that are compatible with concatjs. Most packages do not ship with this format, so in order to use concatjs tooling, you have to shim your code and dependencies. See the Compatibility section below.

This is at the core of how Google does JavaScript development. So Bazel rules that originated in Google’s codebase have affordances for concatjs. For example ts_library produces named AMD modules in its “devmode” output, and karma_web_test expects to bundle inputs using concatjs.


To make it easier to produce a UMD version of a third-party npm package, we automatically generate a target that uses Browserify to build one, using the main entry from the package’s package.json. In most cases this will make the package loadable under concatjs. This target has a __umd suffix. For example, if your library is at @npm//foo then the UMD target is @npm//foo:foo__umd.

An example where this fixes a users issue: https://github.com/bazelbuild/rules_nodejs/issues/2317#issuecomment-735921318

In some cases, the generated UMD bundle is not sufficient, and in others it fails to build because it requires some special Browserify configuration. You can always write your own shim that grabs a symbol from a package you use, and exposes it in an AMD/require.js-compatible way. For example, even though RxJS ships with a UMD bundle, it contains multiple entry points and uses anonymous modules, not named modules. So our Angular/concatjs example has a rxjs_shims.js file that exposes some RxJS operators, then at https://github.com/bazelbuild/rules_nodejs/blob/2.3.1/examples/angular/src/BUILD.bazel#L65-L71 this is combined in a filegroup with the rxjs.umd.js file. Now we use this filegroup target when depending on RxJS in a concatjs_* rule.

Ultimately by using concatjs, you’re signing up for at least a superficial understanding of these shims and may need to update them when you change your dependencies.

Serving JS in development mode under Bazel

There are two choices for development mode:

  1. Use the concatjs_devserver rule to bring up our simple, fast development server. This is intentionally very simple, to help you get started quickly. However, since there are many development servers available, we do not want to mirror their features in yet another server we maintain.
  2. Teach your real frontend server to serve files from Bazel’s output directory. This is not yet documented. Choose this option if you have an existing server used in development mode, or if your requirements exceed what the concatjs_devserver supports. Be careful that your development round-trip stays fast (should be under two seconds).

To use concatjs_devserver, you simply load the rule, and call it with deps that point to your ts_library target(s):

load("@npm//@bazel/concatjs:index.bzl", "concatjs_devserver")
load("@npm//@bazel/typescript:index.bzl", "ts_library")

    name = "app",
    srcs = ["app.ts"],

    name = "devserver",
    # We'll collect all the devmode JS sources from these TypeScript libraries
    deps = [":app"],
    # This is the path we'll request from the browser, see index.html
    serving_path = "/bundle.js",
    # The devserver can serve our static files too
    static_files = ["index.html"],

The index.html should be the same one you use for production, and it should load the JavaScript bundle from the path indicated in serving_path.

If you don’t have an index.html file, a simple one will be generated by the concatjs_devserver.

See examples/app in this repository for a working example. To run the devserver, we recommend you use ibazel:

$ ibazel run examples/app:devserver

ibazel will keep the devserver program running, and provides a LiveReload server so the browser refreshes the application automatically when each build finishes.

Testing with Karma

The karma_web_test rule runs karma tests with Bazel.

It depends on rules_webtesting, so you need to add this to your WORKSPACE if you use the web testing rules in @bazel/concatjs:

# Fetch transitive Bazel dependencies of karma_web_test
    name = "io_bazel_rules_webtesting",
    sha256 = "9bb461d5ef08e850025480bab185fd269242d4e533bca75bfb748001ceb343c3",
    urls = ["https://github.com/bazelbuild/rules_webtesting/releases/download/0.3.3/rules_webtesting.tar.gz"],

# Set up web testing, choose browsers we can test on
load("@io_bazel_rules_webtesting//web:repositories.bzl", "web_test_repositories")


load("@io_bazel_rules_webtesting//web/versioned:browsers-0.3.2.bzl", "browser_repositories")

    chromium = True,
    firefox = True,

Installing with user-managed dependencies

If you didn’t use the yarn_install or npm_install rule to create an npm workspace, you’ll have to declare a rule in your root BUILD.bazel file to execute karma:

# Create a karma rule to use in karma_web_test_suite karma
# attribute when using user-managed dependencies
    name = "karma/karma",
    entry_point = "//:node_modules/karma/bin/karma",
    # Point bazel to your node_modules to find the entry point
    data = ["//:node_modules"],



concatjs_devserver(name, additional_root_paths, bootstrap, deps, devserver, devserver_host,
                   entry_module, port, scripts, serving_path, static_files)

concatjs_devserver is a simple development server intended for a quick “getting started” experience.

Additional documentation here



(Name, mandatory): A unique name for this target.


(List of strings): Additional root paths to serve static_files from. Paths should include the workspace name such as ["__main__/resources"]

Defaults to []


(List of labels): Scripts to include in the JS bundle before the module loader (require.js)

Defaults to []


(List of labels): Targets that produce JavaScript, such as ts_library

Defaults to []


(Label): Go based devserver executable.

        With cross-platform RBE for OSX & Windows ctx.executable.devserver will be linux as --cpu and
        --host_cpu must be overridden to k8. However, we still want to be able to run the devserver on the host
        machine so we need to include the host devserver binary, which is ctx.executable.devserver_host, in the
        runfiles. For non-RBE and for RBE with a linux host, ctx.executable.devserver & ctx.executable.devserver_host
        will be the same binary.

        Defaults to precompiled go binary setup by @bazel/typescript npm package

Defaults to @npm//@bazel/concatjs/devserver:devserver


(Label): Go based devserver executable for the host platform. Defaults to precompiled go binary setup by @bazel/typescript npm package

Defaults to @npm//@bazel/concatjs/devserver:devserver_darwin_amd64


(String): The entry_module should be the AMD module name of the entry module such as "__main__/src/index". concatjs_devserver concats the following snippet after the bundle to load the application: require(["entry_module"]);

Defaults to ""


(Integer): The port that the devserver will listen on.

Defaults to 5432


(List of labels): User scripts to include in the JS bundle before the application sources

Defaults to []


(String): The path you can request from the client HTML which serves the JavaScript bundle. If you don’t specify one, the JavaScript can be loaded at /_/ts_scripts.js

Defaults to "/_/ts_scripts.js"


(List of labels): Arbitrary files which to be served, such as index.html. They are served relative to the package where this rule is declared.

Defaults to []



karma_web_test(srcs, deps, data, configuration_env_vars, bootstrap, runtime_deps, static_files,
               config_file, tags, peer_deps, kwargs)

Runs unit tests in a browser with Karma.

When executed under bazel test, this uses a headless browser for speed. This is also because bazel test allows multiple targets to be tested together, and we don’t want to open a Chrome window on your machine for each one. Also, under bazel test the test will execute and immediately terminate.

Running under ibazel test gives you a “watch mode” for your tests. The rule is optimized for this case - the test runner server will stay running and just re-serve the up-to-date JavaScript source bundle.

To debug a single test target, run it with bazel run instead. This will open a browser window on your computer. Also you can use any other browser by opening the URL printed when the test starts up. The test will remain running until you cancel the bazel run command.

This rule will use your system Chrome by default. In the default case, your environment must specify CHROME_BIN so that the rule will know which Chrome binary to run. Other browsers and customLaunchers may be set using the a base Karma configuration specified in the config_file attribute.

By default we open a headless Chrome. To use a real Chrome browser window, you can pass --define DISPLAY=true to Bazel, along with configuration_env_vars = ["DISPLAY"] on karma_web_test.



A list of JavaScript test files

Defaults to []


Other targets which produce JavaScript such as ts_library

Defaults to []


Runtime dependencies

Defaults to []


Pass these configuration environment variables to the resulting binary. Chooses a subset of the configuration environment variables (taken from ctx.var), which also includes anything specified via the –define flag. Note, this can lead to different outputs produced by this rule.

Defaults to []


JavaScript files to include before the module loader (require.js). For example, you can include Reflect,js for TypeScript decorator metadata reflection, or UMD bundles for third-party libraries.

Defaults to []


Dependencies which should be loaded after the module loader but before the srcs and deps. These should be a list of targets which produce JavaScript such as ts_library. The files will be loaded in the same order they are declared by that rule.

Defaults to []


Arbitrary files which are available to be served on request. Files are served at: /base/<WORKSPACE_NAME>/<path-to-file>, e.g. /base/npm_bazel_typescript/examples/testing/static_script.js

Defaults to []


User supplied Karma configuration file. Bazel will override certain attributes of this configuration file. Attributes that are overridden will be outputted to the test log.

Defaults to None


Standard Bazel tags, this macro adds tags for ibazel support

Defaults to []


list of peer npm deps required by karma_web_test

Defaults to ["@npm//karma", "@npm//karma-chrome-launcher", "@npm//karma-firefox-launcher", "@npm//karma-jasmine", "@npm//karma-requirejs", "@npm//karma-sourcemap-loader", "@npm//requirejs"]


Passed through to karma_web_test



karma_web_test_suite(name, browsers, web_test_data, wrapped_test_tags, kwargs)

Defines a test_suite of web_test targets that wrap a karma_web_test target.

This macro accepts all parameters in karma_web_test and adds additional parameters for the suite. See karma_web_test docs for all karma_web_test.

The wrapping macro is web_test_suite which comes from rules_websting: https://github.com/bazelbuild/rules_webtesting/blob/master/web/web.bzl.



The base name of the test


A sequence of labels specifying the browsers to use.

Defaults to None


Data dependencies for the wrapper web_test targets.

Defaults to []


A list of test tag strings to use for the wrapped karma_web_test target.

Defaults to ["manual", "noci"]


Arguments for the wrapped karma_web_test target.