Add no-typeof-undefined rule

configs/recommended.js

@@ -52,6 +52,7 @@ module.exports = {
 		'unicorn/no-static-only-class': 'error',
 		'unicorn/no-thenable': 'error',
 		'unicorn/no-this-assignment': 'error',
+		'unicorn/no-typeof-undefined': 'error',
 		'unicorn/no-unnecessary-await': 'error',
 		'unicorn/no-unreadable-array-destructuring': 'error',
 		'unicorn/no-unreadable-iife': 'error',

docs/rules/no-typeof-undefined.md

@@ -0,0 +1,49 @@
+# Enforce compare with `undefined` directly
+
+✅ 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` -->
+
+Enforce compare value with `undefined` directly instead of compare `typeof value` with `'undefined'`.
+
+## Fail
+
+```js
+if (typeof foo === 'undefined') {}
+```
+
+```js
+if (typeof foo !== 'undefined') {}
+```
+
+## Pass
+
+```js
+if (foo === undefined) {}
+```
+
+```js
+if (foo !== undefined) {}
+```
+
+## Options
+
+### checkGlobalVariables
+
+Type: `boolean`\
+Default: `true`
+
+Set it to `false` to ignore variables not defined in file.
+
+```js
+// eslint unicorn/no-typeof-undefined: ["error", {"checkGlobalVariables": false}]
+if (typeof undefinedVariable === 'undefined') {} // Passes
+```
+
+```js
+// eslint unicorn/no-typeof-undefined: ["error", {"checkGlobalVariables": false}]
+if (typeof Array === 'undefined') {}  // Passes
+```

readme.md

@@ -91,6 +91,7 @@ Use a [preset config](#preset-configs) or configure each rules in `package.json`
 | [no-static-only-class](docs/rules/no-static-only-class.md)                                       | Disallow classes that only have static members.                                                                                                                                                                   | ✅  | 🔧 |    |
 | [no-thenable](docs/rules/no-thenable.md)                                                         | Disallow `then` property.                                                                                                                                                                                         | ✅  |    |    |
 | [no-this-assignment](docs/rules/no-this-assignment.md)                                           | Disallow assigning `this` to a variable.                                                                                                                                                                          | ✅  |    |    |
+| [no-typeof-undefined](docs/rules/no-typeof-undefined.md)                                         | Enforce compare with `undefined` directly.                                                                                                                                                                        | ✅  | 🔧 |    |
 | [no-unnecessary-await](docs/rules/no-unnecessary-await.md)                                       | Disallow awaiting non-promise values.                                                                                                                                                                             | ✅  | 🔧 |    |
 | [no-unreadable-array-destructuring](docs/rules/no-unreadable-array-destructuring.md)             | Disallow unreadable array destructuring.                                                                                                                                                                          | ✅  | 🔧 |    |
 | [no-unreadable-iife](docs/rules/no-unreadable-iife.md)                                           | Disallow unreadable IIFEs.                                                                                                                                                                                        | ✅  |    |    |

rules/no-static-only-class.js

@@ -49,7 +49,7 @@ function isStaticMember(node) {
 	if (
 		isDeclare
 		|| isReadonly
-		|| typeof accessibility !== 'undefined'
+		|| accessibility !== undefined
 		|| (Array.isArray(decorators) && decorators.length > 0)
 		// TODO: Remove this when we drop support for `@typescript-eslint/parser` v4
 		|| key.type === 'TSPrivateIdentifier'

rules/no-typeof-undefined.js

@@ -0,0 +1,125 @@
+'use strict';
+const isShadowed = require('./utils/is-shadowed.js');
+const {matches} = require('./selectors/index.js');
+const {
+	addParenthesizesToReturnOrThrowExpression,
+} = require('./fix/index.js');
+const {removeSpacesAfter} = require('./fix/index.js');
+const isOnSameLine = require('./utils/is-on-same-line.js');
+const needsSemicolon = require('./utils/needs-semicolon.js');
+const {
+	isParenthesized,
+} = require('./utils/parentheses.js');
+
+const MESSAGE_ID_ERROR = 'no-typeof-undefined';
+const messages = {
+	[MESSAGE_ID_ERROR]: 'Compare with `undefined` directly instead of `typeof` check.',
+};
+
+const selector = [
+	'BinaryExpression',
+	matches(['===', '!==', '==', '!='].map(operator => `[operator="${operator}"]`)),
+	'[left.type="UnaryExpression"]',
+	'[left.operator="typeof"]',
+	'[left.prefix]',
+	'[right.type="Literal"]',
+].join('');
+
+/** @param {import('eslint').Rule.RuleContext} context */
+const create = context => {
+	const {
+		checkGlobalVariables,
+	} = {
+		checkGlobalVariables: true,
+		...context.options[0],
+	};
+
+	return {
+		[selector](binaryExpression) {
+			const {left: typeofNode, right: undefinedString} = binaryExpression;
+			if (undefinedString.value !== 'undefined') {
+				return;
+			}
+
+			const valueNode = typeofNode.argument;
+
+			if (
+				valueNode.type === 'Identifier'
+				&& !checkGlobalVariables
+				&& !isShadowed(context.getScope(), valueNode)
+			) {
+				return;
+			}
+
+			const sourceCode = context.getSourceCode();
+			const [typeofToken, secondToken] = sourceCode.getFirstTokens(typeofNode, 2);
+
+			return {
+				node: binaryExpression,
+				loc: typeofToken.loc,
+				messageId: MESSAGE_ID_ERROR,
+				* fix(fixer) {
+					// Change `==`/`!=` to `===`/`!==`
+					const {operator} = binaryExpression;
+					if (operator === '==' || operator === '!=') {
+						const operatorToken = sourceCode.getTokenAfter(
+							typeofNode,
+							token => token.type === 'Punctuator' && token.value === operator,
+						);
+
+						yield fixer.insertTextAfter(operatorToken, '=');
+					}
+
+					yield fixer.replaceText(undefinedString, 'undefined');
+
+					yield fixer.remove(typeofToken);
+					yield removeSpacesAfter(typeofToken, sourceCode, fixer);
+
+					const {parent} = binaryExpression;
+					if (
+						(parent.type === 'ReturnStatement' || parent.type === 'ThrowStatement')
+						&& parent.argument === binaryExpression
+						&& !isOnSameLine(typeofToken, secondToken)
+						&& !isParenthesized(binaryExpression, sourceCode)
+						&& !isParenthesized(typeofNode, sourceCode)
+					) {
+						yield * addParenthesizesToReturnOrThrowExpression(fixer, parent, sourceCode);
+						return;
+					}
+
+					const tokenBefore = sourceCode.getTokenBefore(binaryExpression);
+					if (needsSemicolon(tokenBefore, sourceCode, secondToken.value)) {
+						yield fixer.insertTextBefore(binaryExpression, ';');
+					}
+				},
+			};
+		},
+	};
+};
+
+const schema = [
+	{
+		type: 'object',
+		additionalProperties: false,
+		properties: {
+			checkGlobalVariables: {
+				type: 'boolean',
+				default: false,
+			},
+		},
+	},
+];
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+	create,
+	meta: {
+		type: 'suggestion',
+		docs: {
+			description: 'Enforce compare with `undefined` directly.',
+		},
+		fixable: 'code',
+		schema,
+		messages,
+	},
+};

rules/utils/is-same-reference.js

@@ -145,7 +145,7 @@ function isSameReference(left, right) {
 			const nameA = getStaticPropertyName(left);
 
 			// `x.y = x["y"]`
-			if (typeof nameA !== 'undefined') {
+			if (nameA !== undefined) {
 				return (
 					isSameReference(left.object, right.object)
 					&& nameA === getStaticPropertyName(right)

test/no-typeof-undefined.mjs

@@ -0,0 +1,85 @@
+import outdent from 'outdent';
+import {getTester} from './utils/test.mjs';
+
+const {test} = getTester(import.meta);
+
+test.snapshot({
+	valid: [
+		'typeof a.b',
+		'typeof a.b > "undefined"',
+		'a.b === "undefined"',
+		'void a.b === "undefined"',
+		'+a.b === "undefined"',
+		'++a.b === "undefined"',
+		'a.b++ === "undefined"',
+		'foo === undefined',
+		'typeof a.b === "string"',
+		'typeof a.b === "string"',
+		'"undefined" === typeof a.b',
+		'const UNDEFINED = "undefined"; typeof a.b === UNDEFINED',
+		'typeof a.b === `undefined`',
+	],
+	invalid: [
+		'typeof a.b === "undefined"',
+		'typeof a.b !== "undefined"',
+		'typeof a.b == "undefined"',
+		'typeof a.b != "undefined"',
+		'typeof a.b == \'undefined\'',
+		'typeof undefinedVariableIdentifier == \'undefined\'',
+		// ASI
+		outdent`
+			foo
+			typeof [] === "undefined";
+		`,
+		outdent`
+			function a() {
+				return typeof // comment
+					a.b === 'undefined';
+			}
+		`,
+		outdent`
+			function a() {
+				return (typeof // ReturnStatement argument is parenthesized
+					a.b === 'undefined');
+			}
+		`,
+		outdent`
+			function a() {
+				return (typeof // UnaryExpression is parenthesized
+					a.b) === 'undefined';
+			}
+		`,
+	],
+});
+
+// `checkGlobalVariables: false`
+test.snapshot({
+	valid: [
+		'typeof foo === "undefined"',
+		'foo = 2; typeof foo === "undefined"',
+		'/* globals foo: readonly */ typeof foo === "undefined"',
+		'/* globals globalThis: readonly */ typeof globalThis === "undefined"',
+	].map(code => ({code, options: [{checkGlobalVariables: false}]})),
+	invalid: [
+		'let foo; typeof foo === "undefined"',
+		'const foo = 1; typeof foo === "undefined"',
+		'var foo; typeof foo === "undefined"',
+		'var foo; var foo; typeof foo === "undefined"',
+		'for (const foo of bar) typeof foo === "undefined";',
+		outdent`
+			let foo;
+			function bar() {
+				typeof foo === "undefined";
+			}
+		`,
+		'function foo() {typeof foo === "undefined"}',
+		'function foo(bar) {typeof bar === "undefined"}',
+		'function foo({bar}) {typeof bar === "undefined"}',
+		'function foo([bar]) {typeof bar === "undefined"}',
+		'typeof foo.bar === "undefined"',
+		outdent`
+			import foo from 'foo';
+			typeof foo.bar === "undefined"
+		`,
+	].map(code => ({code, options: [{checkGlobalVariables: false}]})),
+});