feat: add prefer-flat-map and prefer-flat rules

Author: freaktechnik

.editorconfig

@@ -0,0 +1,19 @@
+# EditorConfig is awesome: http://EditorConfig.org
+
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+charset = utf-8
+indent_style = space
+indent_size = 4
+# C-style doc comments for eclint
+block_comment_start = /*
+block_comment = *
+block_comment_end = */
+
+[{package.json,.travis.yml,package-lock.json,codecov.yml,.all-contributorsrc}]
+indent_style = space
+indent_size = 2

README.md

@@ -17,8 +17,14 @@ Rules for Array functions and methods.
      - [Examples](#examples-2)
    - [`avoid-reverse`](#avoid-reverse)
      - [Examples](#examples-3)
- - [`array-func/recommended` Configuration](#array-funcrecommended-configuration)
-   - [Using the Configuration](#using-the-configuration)
+   - [`prefer-flat-map`](#prefer-flat-map)
+     - [Examples](#examples-4)
+   - [`prefer-flat`](#prefer-flat)
+     - [Examples](#examples-55)
+ - [Configurations](#configurations)
+   - [`array-func/recommended` Configuration](#array-funcrecommended-configuration)
+     - [Using the Configuration](#using-the-configuration)
+   - [`array-func/all` Configuration](#array-funcall-configuration)
  - [License](#license)
 
 ## Installation
@@ -189,7 +195,60 @@ const reverseArray = array.reverse();
 const reverseMap = array.reverse().map((r) => r + 1);
 ```
 
-## `array-func/recommended` Configuration
+### `prefer-flat-map`
+Use `.flatMap()` to flatten an array and map the values instead of using
+`.flat().map()`.
+
+This rule is auto fixable.
+
+#### Examples
+Code that triggers this rule:
+```js
+const flattenedAndMapped = array.flat().map((p) => p);
+```
+
+Code that doesn't trigger this rule:
+```js
+const oneAction = array.flatMap((m) => m);
+
+const flattened = array.flat();
+
+const mapped = array.map((r) => r + 1);
+
+const mappedThenFlattened = array.map((r) => r + 1).flat();
+
+const flatMappedWithExtra = array.flat().reverse().map((r) => r + 1);
+```
+
+### `prefer-flat`
+Use `.flat()` to flatten an array of arrays. This rule currently recognizes two
+patterns and can replace them with a `.flat()` call:
+
+- `[].concat(...array)`
+- `array.reduce((p, n) => p.concat(n), [])`
+
+This rule is auto fixable.
+
+#### Examples
+Code that triggers this rule:
+```js
+const concatFlat = [].concat(...array);
+
+const reduceFlat = array.reduce((p, n) => p.concat(n), []);
+```
+
+Code that doesn't trigger this rule:
+```js
+const flattened = array.flat();
+
+const reverseFlat = array.reduce((p, n) => n.concat(p), []);
+
+const otherReduce = array.reduce((p, n) => n + p, 0);
+```
+
+## Configurations
+
+### `array-func/recommended` Configuration
 The recommended configuration will set your parser ECMA Version to 2015, since that's when the Array functions and methods were added.
 
 Rule | Error level | Fixable
@@ -199,7 +258,7 @@ Rule | Error level | Fixable
 `prefer-array-from` | Error | Yes
 `avoid-reverse` | Error | Yes
 
-### Using the Configuration
+#### Using the Configuration
 To enable this configuration use the `extends` property in your `.eslintrc.json` config file (may look different for other config file styles):
 ```json
 {
@@ -209,5 +268,19 @@ To enable this configuration use the `extends` property in your `.eslintrc.json`
 }
 ```
 
+### `array-func/all` Configuration
+The recommended configuration does not include all rules, since some Array methods
+were added after ES2015. The all configuration enables all rules the plugin
+contains and sets the ECMA version appropriately.
+
+Rule | Error level | Fixable
+---- | ----------- | -------
+`from-map` | Error | Yes
+`no-unnecessary-this-arg` | Error | Sometimes
+`prefer-array-from` | Error | Yes
+`avoid-reverse` | Error | Yes
+`prefer-flat-map` | Error | Yes
+`prefer-flat` | Error | Yes
+
 ## License
 The `array-func` plugin is licensed under the [MIT License](LICENSE).

index.js

@@ -23,6 +23,17 @@ module.exports = {
                 "array-func/prefer-array-from": "error",
                 "array-func/avoid-reverse": "error"
             }
+        },
+        all: {
+            parserOptions: {
+                ecmaVersion: 2018
+            },
+            plugins: [ 'array-func' ],
+            rules: {
+                "array-func/prefer-flat-map": "error",
+                "array-func/prefer-flat": "error"
+            },
+            extends: [ 'plugin:array-func/recommended' ]
         }
     }
 };