Add prefer-array-flat rule (#1126)

docs/rules/prefer-array-flat.md

@@ -0,0 +1,84 @@
+# Prefer `Array#flat()` over legacy techniques to flatten arrays
+
+ES2019 introduced a new method [`Array#flat()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat) that flatten arrays.
+
+This rule is fixable.
+
+## Fail
+
+```js
+const foo = bar.flatMap(x => x);
+```
+
+```js
+const foo = bar.reduce((a, b) => a.concat(b), []);
+```
+
+```js
+const foo = bar.reduce((a, b) => [...a, ...b], []);
+```
+
+```js
+const foo = [].concat(bar);
+```
+
+```js
+const foo = [].concat(...bar);
+```
+
+```js
+const foo = [].concat.apply([], bar);
+```
+
+```js
+const foo = Array.prototype.concat.apply([], bar);
+```
+
+```js
+const foo = _.flatten(bar);
+```
+
+```js
+const foo = lodash.flatten(bar);
+```
+
+```js
+const foo = underscore.flatten(bar);
+```
+
+## Pass
+
+```js
+const foo = bar.flat();
+```
+
+## Options
+
+Type: `object`
+
+### functions
+
+Type: `string[]`
+
+You can also check custom functions that flatten arrays.
+
+Example:
+
+```js
+{
+	'unicorn/prefer-array-flat': [
+		'error',
+		{
+			functions: [
+				'flatArray',
+				'utils.flat'
+			]
+		}
+	]
+}
+```
+
+```js
+// eslint unicorn/prefer-array-flat: ["error", {"functions": ["utils.flat"]}]
+const foo = utils.flat(bar); // Fails
+```

index.js

@@ -84,6 +84,8 @@ module.exports = {
 				'unicorn/prefer-add-event-listener': 'error',
 				'unicorn/prefer-array-find': 'error',
 				// TODO: Enable this by default when targeting Node.js 12.
+				'unicorn/prefer-array-flat': 'off',
+				// TODO: Enable this by default when targeting Node.js 12.
 				'unicorn/prefer-array-flat-map': 'off',
 				'unicorn/prefer-array-index-of': 'error',
 				'unicorn/prefer-array-some': 'error',

readme.md

@@ -76,6 +76,7 @@ Configure it in `package.json`.
 			"unicorn/numeric-separators-style": "off",
 			"unicorn/prefer-add-event-listener": "error",
 			"unicorn/prefer-array-find": "error",
+			"unicorn/prefer-array-flat": "error",
 			"unicorn/prefer-array-flat-map": "error",
 			"unicorn/prefer-array-index-of": "error",
 			"unicorn/prefer-array-some": "error",
@@ -155,6 +156,7 @@ Configure it in `package.json`.
 - [numeric-separators-style](docs/rules/numeric-separators-style.md) - Enforce the style of numeric separators by correctly grouping digits. *(fixable)*
 - [prefer-add-event-listener](docs/rules/prefer-add-event-listener.md) - Prefer `.addEventListener()` and `.removeEventListener()` over `on`-functions. *(partly fixable)*
 - [prefer-array-find](docs/rules/prefer-array-find.md) - Prefer `.find(…)` over the first element from `.filter(…)`. *(partly fixable)*
+- [prefer-array-flat](docs/rules/prefer-array-flat.md) - Prefer `Array#flat()` over legacy techniques to flatten arrays. *(fixable)*
 - [prefer-array-flat-map](docs/rules/prefer-array-flat-map.md) - Prefer `.flatMap(…)` over `.map(…).flat()`. *(fixable)*
 - [prefer-array-index-of](docs/rules/prefer-array-index-of.md) - Prefer `Array#indexOf()` over `Array#findIndex()` when looking for the index of an item. *(partly fixable)*
 - [prefer-array-some](docs/rules/prefer-array-some.md) - Prefer `.some(…)` over `.find(…)`.

rules/no-array-callback-reference.js

@@ -3,7 +3,7 @@ const {isParenthesized} = require('eslint-utils');
 const getDocumentationUrl = require('./utils/get-documentation-url');
 const methodSelector = require('./utils/method-selector');
 const {notFunctionSelector} = require('./utils/not-function');
-const isNodeMatches = require('./utils/is-node-matches');
+const {isNodeMatches} = require('./utils/is-node-matches');
 
 const ERROR_WITH_NAME_MESSAGE_ID = 'error-with-name';
 const ERROR_WITHOUT_NAME_MESSAGE_ID = 'error-without-name';

rules/no-array-for-each.js

@@ -14,7 +14,7 @@ const shouldAddParenthesesToExpressionStatementExpression = require('./utils/sho
 const getParenthesizedTimes = require('./utils/get-parenthesized-times');
 const extendFixRange = require('./utils/extend-fix-range');
 const isFunctionSelfUsedInside = require('./utils/is-function-self-used-inside');
-const isNodeMatches = require('./utils/is-node-matches');
+const {isNodeMatches} = require('./utils/is-node-matches');
 const assertToken = require('./utils/assert-token');
 
 const MESSAGE_ID = 'no-array-for-each';
@@ -24,7 +24,8 @@ const messages = {
 
 const arrayForEachCallSelector = methodSelector({
 	name: 'forEach',
-	includeOptional: true
+	includeOptionalCall: true,
+	includeOptionalMember: true
 });
 
 const continueAbleNodeTypes = new Set([