# NodeJS rules for Bazel


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**This is an alpha-quality release. Breaking changes are likely.**

The nodejs rules integrate NodeJS development toolchain and runtime with Bazel.

This toolchain can be used to build applications that target a browser runtime,
so this repo can be thought of as "JavaScript rules for Bazel" as well.

## Installation

First, install a current bazel distribution, following the [bazel instructions].

Next, create a `WORKSPACE` file in your project root (or edit the existing one)
containing:

```python
git_repository(
    name = "build_bazel_rules_nodejs",
    remote = "https://github.com/bazelbuild/rules_nodejs.git",
    tag = "0.3.1", # check for the latest tag when you install
)

load("@build_bazel_rules_nodejs//:defs.bzl", "node_repositories")

# NOTE: this rule installs nodejs, npm, and yarn, but does NOT install
# your npm dependencies into your node_modules folder.
# You must still run the package manager to do this.
node_repositories(package_json = ["//:package.json"])
```

### Using self-managed dependencies

If you'd like to have Bazel use the `node_modules` directory you are managing,
then next you will create a `BUILD.bazel` file in your project root containing:

```python
package(default_visibility = ["//visibility:public"])

# NOTE: this may move to node_modules/BUILD in a later release
filegroup(name = "node_modules", srcs = glob(["node_modules/**/*"]))
```

We recommend using the version of the package management tools installed by
Bazel to ensure everything is compatible.

To use the Yarn package manager, which we recommend for its built-in
verification command, you can run:

```sh
$ bazel run @yarn//:yarn
```

If you use npm instead, run:

```sh
$ bazel run @nodejs//:npm install
```

[bazel instructions]: https://docs.bazel.build/versions/master/install.html

### Using auto-managed dependencies

To have Bazel manage its own copy of `node_modules`, which is useful to avoid
juggling multiple toolchains, you can add the following to your `WORKSPACE`
file:

```python
load("@build_bazel_rules_nodejs//:defs.bzl", "npm_install")

npm_install(
    name = "foo",
    # This can also take package.json
    package_json = "//:package-lock.json",
)
```

You can then reference this version of `node_modules` in your `BUILD` rules via:

```python
load("@build_bazel_rules_nodejs//:defs.bzl", "nodejs_binary")

nodejs_binary(
    name = "bar",
    # Ordinarily this defaults to //:node_modules
    node_modules = "@foo//:node_modules",
    ...
)
```

With this approach, Bazel is responsible for making sure that `node_modules` is
up to date with `package[-lock].json`.  This means Bazel will set it up when the
repo is first cloned, and rebuild it whenever it changes.

For Bazel to provide the strongest guarantees about reproducibility and the
fidelity of your build, it is recommended that you let Bazel take responsibility
for this.

However, this approach manages a second copy of `node_modules`, so if you are
juggling Bazel and other tooling, or sensitive to the additional network traffic
this might incur, consider self-managing.

## Usage

The `nodejs_binary` rule allows you to run an application by giving the entry point.
The entry point can come from an external dependency installed by the package manager,
or it can be a `.js` file from a package built by Bazel.

`nodejs_test` is the same as nodejs_binary, but instead of calling it with `bazel run`,
you call it with `bazel test`. The test passes if the program exits with a zero exit code.

The `jasmine_node_test` rule allows you to write a test that executes in NodeJS.

### Running a program from npm

If you have installed the [rollup] package, you could write this rule:

```python
load("@build_bazel_rules_nodejs//:defs.bzl", "nodejs_binary")

nodejs_binary(
    name = "rollup",
    entry_point = "rollup/bin/rollup",
)
```

and run it with

```sh
$ bazel run :rollup -- --help
```

See the `examples/rollup` directory in this repository.

[rollup]: https://www.npmjs.com/package/rollup

### Running a program from local sources

We can reference a path in the local workspace to run a program we write.

```python
load("@build_bazel_rules_nodejs//:defs.bzl", "nodejs_binary")

nodejs_binary(
    name = "example",
    data = [
        "@//:node_modules",
        "main.js",
    ],
    entry_point = "workspace_name/main.js",
    args = ["--node_options=--expose-gc"],
)
```

This example illustrates how to pass arguments to nodejs (as opposed to passing arguments to the program).

The `data` attribute is optional, by default it includes the `node_modules` directory. To include your own
sources, include a file or target that produces JavaScript.

See the `examples/program` directory in this repository.

### Testing

The `jasmine_node_test` rule can be used to run unit tests in NodeJS, using the Jasmine framework.
Targets declared with this rule can be run with `bazel test`.

Attributes:

The `srcs` of a `jasmine_node_test` should include the test `.js` files.
The `deps` should include the production `.js` sources, or other rules which produce `.js` files, such as TypeScript.

The `examples/program/index.spec.js` file illustrates this. Another usage is in https://github.com/angular/tsickle/blob/master/test/BUILD

### Bundling/optimizing

A `rollup_bundle` rule produces three bundle files:

1. ES5 syntax, minified by uglify. This is the default output of the rule, meaning this file will be provided when this rule appears in the `deps[]` of another rule.

```sh
$ bazel build internal/e2e/rollup:bundle
```

2. ES5 syntax, un-minified.

```sh
$ bazel build internal/e2e/rollup:bundle.js
```

3. ES2015 syntax, un-minified.

```
$ bazel build internal/e2e/rollup:bundle.es6.js
```

Attributes:

`srcs` are `.js` files to be included in the bundle

`deps` are other rules which produce `.js` files, such as `ts_library`

`entry_point` is the main file of the application that will be executed. Only
sources reachable from the import graph of this file will be included in the
bundle.

> Note: we expect other bundling rules will follow later, such as Closure compiler and Webpack.

# Design

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 reponsibility 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.

References:

- 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.