neutrino-dev/docs/creating-presets.md

# Creating Neutrino Presets

Creating Neutrino presets can solve a number of use cases in a manner which captures all the necessary configuration
and dependencies necessary to accomplish those use cases:

- You find yourself needing to make the same modifications to all the projects you work on.
- Your team wants to adopt a managed set of linting rules across projects.
- You want to add functionality to the build pipeline that has not yet been encapsulated by an existing preset.
- You want to support building for more platforms than existing presets support.

## Getting Started

Neutrino presets are Node.js modules or packages that export a function which accepts a Neutrino instance. You can use
this instance to modify the configuration, provide your own, expose custom options for your preset, listen for build
events, and execute functionality.

At a bare minimum, let's start with our package boilerplate for an empty Neutrino preset:

```js
module.exports = neutrino => {
  // ...
};
```

If you are using Babel or Neutrino to build your preset (so meta) with ES modules:

```js
export default neutrino => {
  // ...
};
```

## Configuring

The Neutrino instance provided to your custom configurator has a `config` property that is an instance of
[webpack-chain](https://github.com/mozilla-rpweb/webpack-chain). We won't go in-depth of all the configuration
possibilities here, but encourage you to check out the documentation for webpack-chain for instruction on your
particular use case.

This `neutrino.config` is an accumulation of all configuration set up to this moment. Every Neutrino preset interacts
with and makes changes through this config, which is all available to your preset.

## Events

Neutrino exposes events for various stages of the build process your preset can hook into **if necessary**.

- `prestart`: Triggered prior to creating a development bundle or launching a dev server.
- `start`: Triggered after the development bundle has finished or the dev server has been stopped.
- `prebuild`: Triggered prior to creating a production build.
- `build`: Triggered after the production build has completed.
- `pretest`: Triggered prior to invoking any test runners.
- `test`: Triggered when test runners can start, or after they have all completed.

_Example: Log to the console when a build finishes._

```js
module.exports = neutrino => {
  neutrino.on('build', () => console.log('whew!'));
};
```

## Including and merging other presets

If your preset depends on other Neutrino presets, or you are creating a preset that is a combination of multiple
presets, you can install them as dependencies and simply call them from your preset, providing them with your Neutrino
instance. When users install your preset, they will bring along your dependencies defined with your package without
needing to also include your extended presets in their own commands.

_Example: Define a Neutrino preset which combines Node.js and Mocha presets._

```js
const node = require('neutrino-preset-node');
const mocha = require('neutrino-preset-mocha');

module.exports = neutrino => {
  node(neutrino);
  mocha(neutrino);
  
  // neutrino.config now contains the accumulation of configuration from
  // the Node.js and Mocha presets
};
```

## Sample Preset: JavaScript Standard Style

Let's create a preset from scratch which allows users to augment their project with
[JavaScript Standard Style](http://standardjs.com/). For this sample preset we are using
[Yarn](https://yarnpkg.com) for managing dependencies, but you may use the npm client if you desire.

```bash
# Create a new directory for your project and change into it:
mkdir neutrino-preset-standard-style && cd neutrino-preset-standard-style

# Initialize your project with a package.json:
yarn init --yes

# Install the dependencies needed by our preset
yarn add standard-loader standard

# Create the main file to the preset, e.g. index.js
touch index.js
```

Let's edit this `index.js` file to add our preset:

```js
const path = require('path');

module.exports = neutrino => {
  neutrino.config
    .rule('lint')
      .pre()
      .test(/\.jsx?$/)
      .include(path.join(process.cwd(), 'src'))
      .loader('standard', require.resolve('standard-loader'), {
        snazzy: false
      });
};
```

## Custom Data

If you want to expose custom options for your preset that are not appropriate to be stored in the Neutrino config,
there is a `neutrino.custom` object namespace you can attach to. This way you can document to others how they can
go about affecting how your preset works.

_Example:_

```js
module.exports = neutrino => {
  neutrino.custom.standardStyle = {
    quiet: false,
    logLevel: 'warn'
  };
  
  // ...
};
```

## Working with paths

When working with paths, remember that your preset will be running in the context of a project. You should take care
to define application paths by referencing the current working directory with `process.cwd()`. For example if you
wanted to work with the project's "src" directory, you would merge the path via `path.join(process.cwd(), 'src')`

## Loader and Babel modules

Because of package conflicts or unknown layout of a project's `node_modules` directory, it is usually safer to
define loaders, Babel plugins, and Babel presets to Webpack absolutely than by name. In our sample preset above, while
we could have passed the loader as just `'standard-loader'`, it is safer to resolve its location relative to our preset
than having Webpack et al to attempt to load it later from a different, potentially incorrect location. Instead we
passed `require.resolve('standard-loader')`.

As a rule of thumb, if your preset is the one using `require`, you are safe to require by name. If you are passing the
name of the module off to be required by Webpack or Babel, instead pass the path to the module via `require.resolve`.

## Publishing

When your preset is ready to be used by others, feel free to publish and distribute! By putting your preset on npm,
GitHub, or another location, you can share the hard work put into abstracting away configuration and save everyone
in the community time and effort. As long as Neutrino can require your preset, it puts no restrictions on where
you want to host it.