Add prefer-set-size rule

configs/recommended.js

@@ -101,6 +101,7 @@ module.exports = {
 		'unicorn/prefer-reflect-apply': 'error',
 		'unicorn/prefer-regexp-test': 'error',
 		'unicorn/prefer-set-has': 'error',
+		'unicorn/prefer-set-size': 'error',
 		'unicorn/prefer-spread': 'error',
 		// TODO: Enable this by default when targeting Node.js 16.
 		'unicorn/prefer-string-replace-all': 'off',

docs/rules/prefer-set-size.md

@@ -0,0 +1,26 @@
+# Prefer using `Set#size` instead of `Array#length`
+
+✅ This rule is enabled in the `recommended` [config](https://github.com/sindresorhus/eslint-plugin-unicorn#preset-configs).
+
+🔧 This rule is automatically fixable by the [`--fix` CLI option](https://eslint.org/docs/latest/user-guide/command-line-interface#--fix).
+
+<!-- end auto-generated rule header -->
+<!-- Do not manually modify this header. Run: `npm run fix:eslint-docs` -->
+
+Prefer using `Set#size` directly instead of first converting it to an array and then using its `.length` property.
+
+## Fail
+
+```js
+function isUnique(array) {
+	return [...new Set(array)].length === array.length;
+}
+```
+
+## Pass
+
+```js
+function isUnique(array) {
+	return new Set(array).size === array.length;
+}
+```

readme.md

@@ -139,6 +139,7 @@ Use a [preset config](#preset-configs) or configure each rules in `package.json`
 | [prefer-reflect-apply](docs/rules/prefer-reflect-apply.md)                                       | Prefer `Reflect.apply()` over `Function#apply()`.                                                                                                                                                                 | ✅  | 🔧 |    |
 | [prefer-regexp-test](docs/rules/prefer-regexp-test.md)                                           | Prefer `RegExp#test()` over `String#match()` and `RegExp#exec()`.                                                                                                                                                 | ✅  | 🔧 |    |
 | [prefer-set-has](docs/rules/prefer-set-has.md)                                                   | Prefer `Set#has()` over `Array#includes()` when checking for existence or non-existence.                                                                                                                          | ✅  | 🔧 | 💡 |
+| [prefer-set-size](docs/rules/prefer-set-size.md)                                                 | Prefer using `Set#size` instead of `Array#length`.                                                                                                                                                                | ✅  | 🔧 |    |
 | [prefer-spread](docs/rules/prefer-spread.md)                                                     | Prefer the spread operator over `Array.from(…)`, `Array#concat(…)`, `Array#slice()` and `String#split('')`.                                                                                                       | ✅  | 🔧 | 💡 |
 | [prefer-string-replace-all](docs/rules/prefer-string-replace-all.md)                             | Prefer `String#replaceAll()` over regex searches with the global flag.                                                                                                                                            |    | 🔧 |    |
 | [prefer-string-slice](docs/rules/prefer-string-slice.md)                                         | Prefer `String#slice()` over `String#substr()` and `String#substring()`.                                                                                                                                          | ✅  | 🔧 |    |

rules/prefer-set-size.js

@@ -0,0 +1,101 @@
+'use strict';
+const {findVariable} = require('eslint-utils');
+const {memberExpressionSelector} = require('./selectors/index.js');
+const {fixSpaceAroundKeyword} = require('./fix/index.js');
+
+const MESSAGE_ID = 'prefer-set-size';
+const messages = {
+	[MESSAGE_ID]: 'Prefer using `Set#size` instead of `Array#length`.',
+};
+
+const lengthAccessSelector = [
+	memberExpressionSelector('length'),
+	'[object.type="ArrayExpression"]',
+	'[object.elements.length=1]',
+	'[object.elements.0.type="SpreadElement"]',
+].join('');
+
+const isNewSet = node =>
+	node?.type === 'NewExpression'
+	&& node.callee.type === 'Identifier'
+	&& node.callee.name === 'Set';
+
+function isSet(node, scope) {
+	if (isNewSet(node)) {
+		return true;
+	}
+
+	if (node.type !== 'Identifier') {
+		return false;
+	}
+
+	const variable = findVariable(scope, node);
+
+	if (!variable || variable.defs.length !== 1) {
+		return false;
+	}
+
+	const [definition] = variable.defs;
+
+	if (definition.type !== 'Variable' || definition.kind !== 'const') {
+		return false;
+	}
+
+	const declarator = definition.node;
+	return declarator.type === 'VariableDeclarator'
+		&& declarator.id.type === 'Identifier'
+		&& isNewSet(definition.node.init);
+}
+
+// `[...set].length` -> `set.size`
+function fix(sourceCode, lengthAccessNodes) {
+	const {
+		object: arrayExpression,
+		property,
+	} = lengthAccessNodes;
+	const set = arrayExpression.elements[0].argument;
+
+	if (sourceCode.getCommentsInside(arrayExpression).length > sourceCode.getCommentsInside(set).length) {
+		return;
+	}
+
+	/** @param {import('eslint').Rule.RuleFixer} fixer */
+	return function * (fixer) {
+		yield fixer.replaceText(property, 'size');
+		yield fixer.replaceText(arrayExpression, sourceCode.getText(set));
+		yield * fixSpaceAroundKeyword(fixer, lengthAccessNodes, sourceCode);
+	};
+}
+
+/** @param {import('eslint').Rule.RuleContext} context */
+const create = context => {
+	const sourceCode = context.getSourceCode();
+
+	return {
+		[lengthAccessSelector](node) {
+			const maybeSet = node.object.elements[0].argument;
+			if (!isSet(maybeSet, context.getScope())) {
+				return;
+			}
+
+			return {
+				node: node.property,
+				messageId: MESSAGE_ID,
+				fix: fix(sourceCode, node),
+			};
+		},
+	};
+};
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+	create,
+	meta: {
+		type: 'suggestion',
+		docs: {
+			description: 'Prefer using `Set#size` instead of `Array#length`.',
+		},
+		fixable: 'code',
+		messages,
+	},
+};

test/prefer-set-size.mjs

@@ -0,0 +1,46 @@
+import outdent from 'outdent';
+import {getTester} from './utils/test.mjs';
+
+const {test} = getTester(import.meta);
+
+test.snapshot({
+	valid: [
+		'new Set(foo).size',
+		'for (const foo of bar) console.log([...foo].length)',
+		'[...new Set(array), foo].length',
+		'[foo, ...new Set(array), ].length',
+		'[...new Set(array)].notLength',
+		'[...new Set(array)]?.length',
+		'[...new Set(array)][length]',
+		'[...new Set(array)]["length"]',
+		'[...new NotSet(array)].length',
+		'[...Set(array)].length',
+		'const foo = new NotSet([]);[...foo].length;',
+		'let foo = new Set([]);[...foo].length;',
+		'const {foo} = new Set([]);[...foo].length;',
+		'const [foo] = new Set([]);[...foo].length;',
+		'[...foo].length',
+		'var foo = new Set(); var foo = new Set(); [...foo].length',
+	],
+	invalid: [
+		'[...new Set(array)].length',
+		outdent`
+			const foo = new Set([]);
+			console.log([...foo].length);
+		`,
+		outdent`
+			function isUnique(array) {
+				return[...new Set(array)].length === array.length
+			}
+		`,
+		'[...new Set(array),].length',
+		'[...(( new Set(array) ))].length',
+		'(( [...new Set(array)] )).length',
+		outdent`
+			foo
+			;[...new Set(array)].length
+		`,
+		'[/* comment */...new Set(array)].length',
+		'[...new /* comment */ Set(array)].length',
+	],
+});

test/snapshots/prefer-set-size.mjs.md

@@ -0,0 +1,155 @@
+# Snapshot report for `test/prefer-set-size.mjs`
+
+The actual snapshot is saved in `prefer-set-size.mjs.snap`.
+
+Generated by [AVA](https://avajs.dev).
+
+## Invalid #1
+      1 | [...new Set(array)].length
+
+> Output
+
+    `␊
+      1 | new Set(array).size␊
+    `
+
+> Error 1/1
+
+    `␊
+    > 1 | [...new Set(array)].length␊
+        |                     ^^^^^^ Prefer using \`Set#size\` instead of \`Array#length\`.␊
+    `
+
+## Invalid #2
+      1 | const foo = new Set([]);
+      2 | console.log([...foo].length);
+
+> Output
+
+    `␊
+      1 | const foo = new Set([]);␊
+      2 | console.log(foo.size);␊
+    `
+
+> Error 1/1
+
+    `␊
+      1 | const foo = new Set([]);␊
+    > 2 | console.log([...foo].length);␊
+        |                      ^^^^^^ Prefer using \`Set#size\` instead of \`Array#length\`.␊
+    `
+
+## Invalid #3
+      1 | function isUnique(array) {
+      2 | 	return[...new Set(array)].length === array.length
+      3 | }
+
+> Output
+
+    `␊
+      1 | function isUnique(array) {␊
+      2 | 	return new Set(array).size === array.length␊
+      3 | }␊
+    `
+
+> Error 1/1
+
+    `␊
+      1 | function isUnique(array) {␊
+    > 2 | 	return[...new Set(array)].length === array.length␊
+        | 	                          ^^^^^^ Prefer using \`Set#size\` instead of \`Array#length\`.␊
+      3 | }␊
+    `
+
+## Invalid #4
+      1 | [...new Set(array),].length
+
+> Output
+
+    `␊
+      1 | new Set(array).size␊
+    `
+
+> Error 1/1
+
+    `␊
+    > 1 | [...new Set(array),].length␊
+        |                      ^^^^^^ Prefer using \`Set#size\` instead of \`Array#length\`.␊
+    `
+
+## Invalid #5
+      1 | [...(( new Set(array) ))].length
+
+> Output
+
+    `␊
+      1 | new Set(array).size␊
+    `
+
+> Error 1/1
+
+    `␊
+    > 1 | [...(( new Set(array) ))].length␊
+        |                           ^^^^^^ Prefer using \`Set#size\` instead of \`Array#length\`.␊
+    `
+
+## Invalid #6
+      1 | (( [...new Set(array)] )).length
+
+> Output
+
+    `␊
+      1 | (( new Set(array) )).size␊
+    `
+
+> Error 1/1
+
+    `␊
+    > 1 | (( [...new Set(array)] )).length␊
+        |                           ^^^^^^ Prefer using \`Set#size\` instead of \`Array#length\`.␊
+    `
+
+## Invalid #7
+      1 | foo
+      2 | ;[...new Set(array)].length
+
+> Output
+
+    `␊
+      1 | foo␊
+      2 | ;new Set(array).size␊
+    `
+
+> Error 1/1
+
+    `␊
+      1 | foo␊
+    > 2 | ;[...new Set(array)].length␊
+        |                      ^^^^^^ Prefer using \`Set#size\` instead of \`Array#length\`.␊
+    `
+
+## Invalid #8
+      1 | [/* comment */...new Set(array)].length
+
+> Error 1/1
+
+    `␊
+    > 1 | [/* comment */...new Set(array)].length␊
+        |                                  ^^^^^^ Prefer using \`Set#size\` instead of \`Array#length\`.␊
+    `
+
+## Invalid #9
+      1 | [...new /* comment */ Set(array)].length
+
+> Output
+
+    `␊
+      1 | new /* comment */ Set(array).size␊
+    `
+
+> Error 1/1
+
+    `␊
+    > 1 | [...new /* comment */ Set(array)].length␊
+        |                                   ^^^^^^ Prefer using \`Set#size\` instead of \`Array#length\`.␊
+    `