Add no-magic-array-flat-depth rule (#2335)

configs/recommended.js

@@ -31,6 +31,7 @@ module.exports = {
 	'unicorn/no-invalid-remove-event-listener': 'error',
 	'unicorn/no-keyword-prefix': 'off',
 	'unicorn/no-lonely-if': 'error',
+	'unicorn/no-magic-array-flat-depth': 'error',
 	'no-negated-condition': 'off',
 	'unicorn/no-negated-condition': 'error',
 	'no-nested-ternary': 'off',

docs/rules/no-magic-array-flat-depth.md

@@ -0,0 +1,40 @@
+# Disallow a magic number as the `depth` argument in `Array#flat(…).`
+
+💼 This rule is enabled in the ✅ `recommended` [config](https://github.com/sindresorhus/eslint-plugin-unicorn#preset-configs-eslintconfigjs).
+
+<!-- end auto-generated rule header -->
+<!-- Do not manually modify this header. Run: `npm run fix:eslint-docs` -->
+
+When calling [`Array#flat(depth)`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat), the depth argument should normally be `1` or `Infinity`, otherwise it should be a meaningful variable name or explained with a comment.
+
+## Fail
+
+```js
+const foo = array.flat(2);
+```
+
+```js
+const foo = array.flat(99);
+```
+
+## Pass
+
+```js
+const foo = array.flat();
+```
+
+```js
+const foo = array.flat(Number.POSITIVE_INFINITY);
+```
+
+```js
+const foo = array.flat(Infinity);
+```
+
+```js
+const foo = array.flat(depth);
+```
+
+```js
+const foo = array.flat(/* The depth is always 2 */ 2);
+```

readme.md

@@ -141,6 +141,7 @@ If you don't use the preset, ensure you use the same `env` and `parserOptions` c
 | [no-invalid-remove-event-listener](docs/rules/no-invalid-remove-event-listener.md)               | Prevent calling `EventTarget#removeEventListener()` with the result of an expression.                                                                                                                             | ✅  |    |    |
 | [no-keyword-prefix](docs/rules/no-keyword-prefix.md)                                             | Disallow identifiers starting with `new` or `class`.                                                                                                                                                              |    |    |    |
 | [no-lonely-if](docs/rules/no-lonely-if.md)                                                       | Disallow `if` statements as the only statement in `if` blocks without `else`.                                                                                                                                     | ✅  | 🔧 |    |
+| [no-magic-array-flat-depth](docs/rules/no-magic-array-flat-depth.md)                             | Disallow a magic number as the `depth` argument in `Array#flat(…).`                                                                                                                                               | ✅  |    |    |
 | [no-negated-condition](docs/rules/no-negated-condition.md)                                       | Disallow negated conditions.                                                                                                                                                                                      | ✅  | 🔧 |    |
 | [no-nested-ternary](docs/rules/no-nested-ternary.md)                                             | Disallow nested ternary expressions.                                                                                                                                                                              | ✅  | 🔧 |    |
 | [no-new-array](docs/rules/no-new-array.md)                                                       | Disallow `new Array()`.                                                                                                                                                                                           | ✅  | 🔧 | 💡 |

rules/no-magic-array-flat-depth.js

@@ -0,0 +1,52 @@
+'use strict';
+const {isOpeningParenToken} = require('@eslint-community/eslint-utils');
+const {isMethodCall, isNumberLiteral} = require('./ast/index.js');
+
+const MESSAGE_ID = 'no-magic-array-flat-depth';
+const messages = {
+	[MESSAGE_ID]: 'Magic number as depth is not allowed.',
+};
+
+/** @param {import('eslint').Rule.RuleContext} context */
+const create = context => ({
+	CallExpression(callExpression) {
+		if (!isMethodCall(callExpression, {
+			method: 'flat',
+			argumentsLength: 1,
+			optionalCall: false,
+		})) {
+			return;
+		}
+
+		const [depth] = callExpression.arguments;
+
+		if (!isNumberLiteral(depth) || depth.value === 1) {
+			return;
+		}
+
+		const {sourceCode} = context;
+		const openingParenthesisToken = sourceCode.getTokenAfter(callExpression.callee, isOpeningParenToken);
+		const closingParenthesisToken = sourceCode.getLastToken(callExpression);
+		if (sourceCode.commentsExistBetween(openingParenthesisToken, closingParenthesisToken)) {
+			return;
+		}
+
+		return {
+			node: depth,
+			messageId: MESSAGE_ID,
+		};
+	},
+});
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+	create,
+	meta: {
+		type: 'suggestion',
+		docs: {
+			description: 'Disallow a magic number as the `depth` argument in `Array#flat(…).`',
+			recommended: true,
+		},
+		messages,
+	},
+};

test/no-magic-array-flat-depth.mjs

@@ -0,0 +1,28 @@
+import {getTester} from './utils/test.mjs';
+
+const {test} = getTester(import.meta);
+
+test.snapshot({
+	valid: [
+		'array.flat(1)',
+		'array.flat(1.0)',
+		'array.flat(0x01)',
+		'array.flat(unknown)',
+		'array.flat(Number.POSITIVE_INFINITY)',
+		'array.flat(Infinity)',
+		'array.flat(/* explanation */2)',
+		'array.flat(2/* explanation */)',
+		'array.flat()',
+		'array.flat(2, extraArgument)',
+		'new array.flat(2)',
+		'array.flat?.(2)',
+		'array.notFlat(2)',
+		'flat(2)',
+	],
+	invalid: [
+		'array.flat(2)',
+		'array?.flat(2)',
+		'array.flat(99,)',
+		'array.flat(0b10,)',
+	],
+});

test/snapshots/no-magic-array-flat-depth.mjs.md

@@ -0,0 +1,65 @@
+# Snapshot report for `test/no-magic-array-flat-depth.mjs`
+
+The actual snapshot is saved in `no-magic-array-flat-depth.mjs.snap`.
+
+Generated by [AVA](https://avajs.dev).
+
+## invalid(1): array.flat(2)
+
+> Input
+
+    `␊
+      1 | array.flat(2)␊
+    `
+
+> Error 1/1
+
+    `␊
+    > 1 | array.flat(2)␊
+        |            ^ Magic number as depth is not allowed.␊
+    `
+
+## invalid(2): array?.flat(2)
+
+> Input
+
+    `␊
+      1 | array?.flat(2)␊
+    `
+
+> Error 1/1
+
+    `␊
+    > 1 | array?.flat(2)␊
+        |             ^ Magic number as depth is not allowed.␊
+    `
+
+## invalid(3): array.flat(99,)
+
+> Input
+
+    `␊
+      1 | array.flat(99,)␊
+    `
+
+> Error 1/1
+
+    `␊
+    > 1 | array.flat(99,)␊
+        |            ^^ Magic number as depth is not allowed.␊
+    `
+
+## invalid(4): array.flat(0b10,)
+
+> Input
+
+    `␊
+      1 | array.flat(0b10,)␊
+    `
+
+> Error 1/1
+
+    `␊
+    > 1 | array.flat(0b10,)␊
+        |            ^^^^ Magic number as depth is not allowed.␊
+    `