# Neutrino ESLint Middleware

`@neutrinojs/eslint` is Neutrino middleware for linting source code using ESLint
and eslint-loader.

[![NPM version][npm-image]][npm-url] [![NPM downloads][npm-downloads]][npm-url]

## Requirements

- Node.js 10+
- Yarn v1.2.1+, or npm v5.4+
- Neutrino 9 and one of the Neutrino build presets
- webpack 4
- ESLint 6 or 7

## Installation

`@neutrinojs/eslint` can be installed via the Yarn or npm clients. Inside your
project, make sure `@neutrinojs/eslint` and `eslint` are development
dependencies. You will also be using another Neutrino preset for building your
application source code.

#### Yarn

```bash
❯ yarn add --dev @neutrinojs/eslint eslint
```

#### npm

```bash
❯ npm install --save-dev @neutrinojs/eslint eslint
```

## Usage

`@neutrinojs/eslint` can be consumed from the Neutrino API, middleware, or
presets. Require this package and plug it into Neutrino:

```js
const eslint = require('@neutrinojs/eslint');

// Usage shows default values
neutrino.use(
  eslint({
    // Uses extensions from neutrino.options.extensions
    test: neutrino.regexFromExtensions(),
    include: [neutrino.options.source, neutrino.options.tests],
    exclude: [],
    eslint: {
      // For supported options, see:
      // https://github.com/webpack-contrib/eslint-loader#options
      // https://eslint.org/docs/developer-guide/nodejs-api#cliengine
      cache: true,
      // Downgrade errors to warnings when in development, to reduce the noise in
      // the webpack-dev-server overlay (which defaults to showing errors only),
      // and to also ensure hot reloading isn't prevented.
      emitWarning: process.env.NODE_ENV === 'development',
      // Make errors fatal for 'production' and 'test'.
      // However note that even when `false` webpack still fails the build:
      // https://github.com/webpack-contrib/eslint-loader/issues/257
      failOnError: process.env.NODE_ENV !== 'development',
      cwd: neutrino.options.root,
      useEslintrc: false,
      // Can be the name of a built-in ESLint formatter
      // or the module/path of an external one.
      formatter: 'codeframe',
      // The options under `baseConfig` correspond to those
      // that can be used in an `.eslintrc.*` file.
      baseConfig: {
        env: {
          es6: true,
        },
        extends: [],
        globals: {
          process: true,
        },
        overrides: [],
        parser: require.resolve('babel-eslint'),
        parserOptions: {
          ecmaVersion: 2018,
          sourceType: 'module',
        },
        plugins: ['babel'],
        root: true,
        settings: {},
      },
    },
  }),
);
```

- `test`: Test which files should be linted.
- `include`: An array of paths to include in linting. Maps to webpack's
  [`Rule.include`](https://webpack.js.org/configuration/module/#rule-include)
- `exclude`: An array of paths to exclude from linting. Maps to webpack's
  [`Rule.exclude`](https://webpack.js.org/configuration/module/#rule-exclude)
- `eslint`: An object containing
  [eslint-loader options](https://github.com/webpack-contrib/eslint-loader#options),
  which includes options passed to ESLint's
  [CLIEngine](https://eslint.org/docs/developer-guide/nodejs-api#cliengine).

## Exposing generated lint configuration via `.eslintrc.js`

`@neutrinojs/eslint`, provides an `.eslintrc()` output handler for generating
the ESLint configuration in a format suitable for use in an `.eslintrc.js` file.
This allows the ESLint CLI to be used outside of building the project, and for
IDEs and text editors to provide linting hints/fixes.

Create a `.eslintrc.js` file in the root of the project, containing:

```js
// .eslintrc.js
const neutrino = require('neutrino');

module.exports = neutrino().eslintrc();
```

This `.eslintrc.js` configuration will be automatically used when running the
ESLint CLI. For convenience a `lint` script alias can be added to your
`package.json`, allowing linting to be run via `yarn lint` or `npm run lint`:

```json
{
  "scripts": {
    "lint": "eslint --cache --format codeframe --ext mjs,jsx,js src"
  }
}
```

Projects may face a problem when their editor or IDE lints all files and
highlights errors that were normally excluded from source, i.e. Neutrino's
`include` and `exclude` options. This is because the ESLint CLI does not have a
way to specify included and excluded files from the `.eslintrc.js`
configuration. Instead you will need to create an
[.eslintignore](https://eslint.org/docs/user-guide/configuring#ignoring-files-and-directories)
file that controls which files should be excluded from linting.

## Using your own `.eslintrc.*`

If instead you would prefer to use your own non-generated `.eslintrc.*` file,
set `useEslintrc` to `true`. This will cause `@neutrinojs/eslint` to only set
the loader-specific configuration defaults, and leave all other linting
configuration to be managed by the standalone `.eslintrc.*` file.

For example:

```js
neutrino.use(
  eslint({
    eslint: {
      // Use own `.eslintrc.*` file for configuration instead.
      useEslintrc: true,
    },
  }),
);
```

```js
// .eslintrc.js (or any of the other filetypes supported by ESLint)
module.exports = {
  // Configure as you would normally when not using Neutrino:
  // https://eslint.org/docs/user-guide/configuring
  root: true,
  extends: 'eslint-config-airbnb',
  parser: 'babel-eslint',
  settings: {
    // ...
  },
  rules: {
    // ...
  },
};
```

## Customization

`@neutrinojs/eslint` creates some conventions to make overriding the
configuration easier once you are ready to make changes.

### Rules

The following is a list of rules and their identifiers which can be overridden:

| Name   | Description                                                                                                                     | NODE_ENV |
| ------ | ------------------------------------------------------------------------------------------------------------------------------- | -------- |
| `lint` | By default, lints JS and JSX files from the `src` and `test` directories using ESLint. Contains a single loader named `eslint`. | all      |

## Contributing

This middleware is part of the
[neutrino](https://github.com/neutrinojs/neutrino) repository, a monorepo
containing all resources for developing Neutrino and its core presets and
middleware. Follow the
[contributing guide](https://neutrinojs.org/contributing/) for details.

[npm-image]: https://img.shields.io/npm/v/@neutrinojs/eslint.svg
[npm-downloads]: https://img.shields.io/npm/dt/@neutrinojs/eslint.svg
[npm-url]: https://www.npmjs.com/package/@neutrinojs/eslint