# Configuring ESLint 

> **Pluggable JavaScript linter**

The [`@berun/fluent-eslint`](https://www.npmjs.com/package/@berun/fluent-eslint) package provides a high level fluent interface for configuring and using [`ESLint`](https://eslint.org).

## Installation

[`@berun/fluent-eslint`](https://www.npmjs.com/package/@berun/fluent-eslint) is a utility interface that is automatically
included by commonly configured BeRun runners such as [`@berun/runner-eslint`](https://www.npmjs.com/package/@berun/runner-eslint).   If you 
need to install it directly, follow these directions:

```bash
$ npm install --save-dev @berun/fluent-eslint
```

### 1. Embed within `berun.config.ts`

```js
// berun.config.ts

export default {
  use: [
     // ...
    '@berun/fluent-eslint',
    // ...
  ]
}
```

### *or* 2. Use from within other BeRun middleware

```js
const eslint = require('@berun/fluent-eslint');

function(berun) {
    berun.use(eslint);
}
```

## ESLint shorthand methods

```js
berun.eslint
   .fix(fix)
  .cache(cache)
  .formatter(formatter)
  .eslintPath(path)
  .emitError(error)
  .emitWarning(warning)
  .quiet(quiet)
  .failOnWarning(fail)
  .failOnError(fail)
  .outputReport(output)
  .root(root)
  .env(environment)
  .ignore(ignore)
  .allowInlineConfig(allowInlineConfig)
  .baseConfig(baseConfig)
  .cacheFile(cacheFile)
  .cacheLocation(cacheLocation)
  .configFile(configFile)
  .cwd(cwd)
  .envs(envs)
  .extensions(extensions)
  .globals(globals)
  .ignorePath(ignorePath)
  .ignorePattern(ignorePattern)
  .parser(parser)
  .parserOptions(parserOptions)
  .reportUnusedDisableDirectives(reportUnusedDisableDirectives)
  .rulePaths(rulePaths)
  .useEslintrc(useEslintrc)
  .globInputPaths(globInputPaths)
```

Moving to deeper points in the API will change the context of what you
are modifying. You can move back to the higher context by either referencing
the top-level `berun.eslint` again, or by calling `.end()` to move up one level.
All API calls will return the API instance at the current context unless otherwise
specified. This is so you may chain API calls continuously 
if desired (['fluent interface'](https://en.wikipedia.org/wiki/Fluent_interface))

For details on the specific values that are valid for all shorthand and
low-level methods, please refer to their corresponding name in the
[eslint docs](https://eslint.org/docs/user-guide/configuring).

### ESLint plugins

```js
// Backed at berun.eslint.plugins
berun.eslint.plugins(name) : FluentSet

// Example
  berun.eslint
    .plugins.add('import')
    .add('flowtype')
    .add('jsx-a11y')
    .add('react')
    .end()
```

### ESLint rules

```js
// Backed at berun.eslint.fules
berun.eslint.rules : FluentMap

// Example
berun.eslint
   .rules.set('array-callback-return', 'warn')
```


## Conditional configuration

When working with instances of `FluentMap` and `FluentSet`, you can perform
conditional configuration using `when`. You must specify an expression to
`when()` which will be evaluated for truthiness or falsiness. If the expression
is truthy, the first function argument will be invoked with an instance of the
current chained instance. You can optionally provide a second function to be
invoked when the condition is falsy, which is also given the current chained
instance.

```js
// Example: Only add import plugin during production
config
  .when(process.env.NODE_ENV === 'production', berun => {
    berun.eslint
      .plugins.add('import')
      .end()
  });
```

## Inspecting generated configuration

You can inspect the generated ESLint config using `berun.eslint.toConfig()`. 

``` js
expect(berun.eslint.toConfig()).toEqual({
    root: true,

    parser: 'babel-eslint',

    plugins: ['import', 'flowtype', 'jsx-a11y', 'react'],

    env: {
      browser: true,
      commonjs: true,
      es6: true,
      jest: true,
      node: true
    },

    parserOptions: {
      ecmaVersion: 6,
      sourceType: 'module',
      ecmaFeatures: {
        jsx: true,
        generators: true,
        experimentalObjectRestSpread: true
      }
    },

    rules: {
      'array-callback-return': 'warn',
      'default-case': ['warn', { commentPattern: '^no default$' }]
  })
```
