Add no-null rule (#636)

Co-authored-by: fisker Cheung <[email protected]>
Co-authored-by: Sindre Sorhus <[email protected]>

Author: 3 people

docs/rules/no-null.md

@@ -0,0 +1,52 @@
+# Disallow the use of the `null` literal.
+
+Disallow the use of the `null` literal, to encourage using `undefined` instead.
+
+## Fail
+
+```js
+let foo = null;
+```
+
+```js
+if (bar == null) {}
+```
+
+## Pass
+
+```js
+let foo;
+```
+
+```js
+const foo = Object.create(null);
+```
+
+```js
+if (foo === null) {}
+```
+
+## Options
+
+Type: `object`
+
+### checkStrictEquality
+
+Type: `boolean`\
+Default: `false`
+
+Strict equality(`===`) and strict inequality(`!==`) is ignored by default.
+
+#### Fail
+
+```js
+// eslint unicorn/no-null: ["error", {"checkStrictEquality": true}]
+if (foo === null) {}
+```
+
+## Why
+
+- [“Intent to stop using `null` in my JS code”](https://github.com/sindresorhus/meta/issues/7).
+- [TypeScript coding guidelines](https://github.com/Microsoft/TypeScript/wiki/Coding-guidelines#null-and-undefined).
+- [ESLint rule proposal](https://github.com/eslint/eslint/issues/6701).
+- [Douglas Crockford](https://www.youtube.com/watch?v=PSGEjv3Tqo0&t=9m21s) on bottom values in JavaScript.

index.js

@@ -38,6 +38,7 @@ module.exports = {
 				'no-nested-ternary': 'off',
 				'unicorn/no-nested-ternary': 'error',
 				'unicorn/no-new-buffer': 'error',
+				'unicorn/no-null': 'error',
 				'unicorn/no-process-exit': 'error',
 				'unicorn/no-unreadable-array-destructuring': 'error',
 				'unicorn/no-unsafe-regex': 'off',

readme.md

@@ -54,6 +54,7 @@ Configure it in `package.json`.
 			"no-nested-ternary": "off",
 			"unicorn/no-nested-ternary": "error",
 			"unicorn/no-new-buffer": "error",
+			"unicorn/no-null": "error",
 			"unicorn/no-process-exit": "error",
 			"unicorn/no-unreadable-array-destructuring": "error",
 			"unicorn/no-unsafe-regex": "off",
@@ -110,6 +111,7 @@ Configure it in `package.json`.
 - [no-keyword-prefix](docs/rules/no-keyword-prefix.md) - Disallow identifiers starting with `new` or `class`.
 - [no-nested-ternary](docs/rules/no-nested-ternary.md) - Disallow nested ternary expressions. *(partly fixable)*
 - [no-new-buffer](docs/rules/no-new-buffer.md) - Enforce the use of `Buffer.from()` and `Buffer.alloc()` instead of the deprecated `new Buffer()`. *(fixable)*
+- [no-null](docs/rules/no-null.md) - Disallow the use of the `null` literal.
 - [no-process-exit](docs/rules/no-process-exit.md) - Disallow `process.exit()`.
 - [no-unreadable-array-destructuring](docs/rules/no-unreadable-array-destructuring.md) - Disallow unreadable array destructuring.
 - [no-unsafe-regex](docs/rules/no-unsafe-regex.md) - Disallow unsafe regular expressions.

rules/expiring-todo-comments.js

@@ -281,7 +281,6 @@ const create = context => {
 		if (dates.length > 1) {
 			uses++;
 			context.report({
-				node: null,
 				loc: comment.loc,
 				messageId: MESSAGE_ID_AVOID_MULTIPLE_DATES,
 				data: {
@@ -296,7 +295,6 @@ const create = context => {
 			const shouldIgnore = options.ignoreDatesOnPullRequests && ci.isPR;
 			if (!shouldIgnore && reachedDate(date)) {
 				context.report({
-					node: null,
 					loc: comment.loc,
 					messageId: MESSAGE_ID_EXPIRED_TODO,
 					data: {
@@ -310,7 +308,6 @@ const create = context => {
 		if (packageVersions.length > 1) {
 			uses++;
 			context.report({
-				node: null,
 				loc: comment.loc,
 				messageId: MESSAGE_ID_AVOID_MULTIPLE_PACKAGE_VERSIONS,
 				data: {
@@ -330,7 +327,6 @@ const create = context => {
 			const compare = semverComparisonForOperator(condition);
 			if (packageVersion && compare(packageVersion, decidedPackageVersion)) {
 				context.report({
-					node: null,
 					loc: comment.loc,
 					messageId: MESSAGE_ID_REACHED_PACKAGE_VERSION,
 					data: {
@@ -357,7 +353,6 @@ const create = context => {
 
 				if (trigger) {
 					context.report({
-						node: null,
 						loc: comment.loc,
 						messageId,
 						data: {
@@ -382,7 +377,6 @@ const create = context => {
 
 			if (compare(targetPackageVersion, todoVersion)) {
 				context.report({
-					node: null,
 					loc: comment.loc,
 					messageId: MESSAGE_ID_VERSION_MATCHES,
 					data: {
@@ -414,7 +408,6 @@ const create = context => {
 
 			if (compare(targetPackageEngineVersion, todoEngine)) {
 				context.report({
-					node: null,
 					loc: comment.loc,
 					messageId: MESSAGE_ID_ENGINE_MATCHES,
 					data: {
@@ -439,7 +432,6 @@ const create = context => {
 				if (parseArgument(testString).type !== 'unknowns') {
 					uses++;
 					context.report({
-						node: null,
 						loc: comment.loc,
 						messageId: MESSAGE_ID_MISSING_AT_SYMBOL,
 						data: {
@@ -457,7 +449,6 @@ const create = context => {
 			if (parseArgument(withoutWhitespaces).type !== 'unknowns') {
 				uses++;
 				context.report({
-					node: null,
 					loc: comment.loc,
 					messageId: MESSAGE_ID_REMOVE_WHITESPACES,
 					data: {

rules/no-null.js

@@ -0,0 +1,108 @@
+'use strict';
+const getDocumentationUrl = require('./utils/get-documentation-url');
+const methodSelector = require('./utils/method-selector');
+
+const ERROR_MESSAGE_ID = 'error';
+const SUGGESTION_REPLACE_MESSAGE_ID = 'replace';
+const SUGGESTION_REMOVE_MESSAGE_ID = 'remove';
+
+const objectCreateSelector = methodSelector({
+	object: 'Object',
+	name: 'create',
+	length: 1
+});
+
+const selector = [
+	`:not(${objectCreateSelector})`,
+	'>',
+	'Literal',
+	'[raw="null"]'
+].join('');
+
+const isLooseEqual = node => node.type === 'BinaryExpression' && ['==', '!='].includes(node.operator);
+const isStrictEqual = node => node.type === 'BinaryExpression' && ['===', '!=='].includes(node.operator);
+
+const create = context => {
+	const {checkStrictEquality} = {
+		checkStrictEquality: false,
+		...context.options[0]
+	};
+
+	return {
+		[selector]: node => {
+			const problem = {
+				node,
+				messageId: ERROR_MESSAGE_ID
+			};
+
+			/* istanbul ignore next */
+			const {parent = {}} = node;
+
+			if (!checkStrictEquality && isStrictEqual(parent)) {
+				return;
+			}
+
+			const fix = fixer => fixer.replaceText(node, 'undefined');
+			const replaceSuggestion = {
+				messageId: SUGGESTION_REPLACE_MESSAGE_ID,
+				fix
+			};
+
+			if (isLooseEqual(parent)) {
+				problem.fix = fix;
+			} else if (parent.type === 'ReturnStatement' && parent.argument === node) {
+				problem.suggest = [
+					{
+						messageId: SUGGESTION_REMOVE_MESSAGE_ID,
+						fix: fixer => fixer.remove(node)
+					},
+					replaceSuggestion
+				];
+			} else if (parent.type === 'VariableDeclarator' && parent.init === node && parent.parent.kind !== 'const') {
+				problem.suggest = [
+					{
+						messageId: SUGGESTION_REMOVE_MESSAGE_ID,
+						fix: fixer => fixer.removeRange([parent.id.range[1], node.range[1]])
+					},
+					replaceSuggestion
+				];
+			} else {
+				problem.suggest = [
+					replaceSuggestion
+				];
+			}
+
+			context.report(problem);
+		}
+	};
+};
+
+const schema = [
+	{
+		type: 'object',
+		properties: {
+			checkStrictEquality: {
+				type: 'boolean',
+				default: false
+			}
+		},
+		additionalProperties: false
+	}
+];
+
+module.exports = {
+	create,
+	meta: {
+		type: 'suggestion',
+		docs: {
+			url: getDocumentationUrl(__filename)
+		},
+		messages: {
+			[ERROR_MESSAGE_ID]: 'Use `undefined` instead of `null`.',
+			[SUGGESTION_REPLACE_MESSAGE_ID]: 'Replace `null` with `undefined`.',
+			[SUGGESTION_REMOVE_MESSAGE_ID]: 'Remove `null`.'
+		},
+		schema,
+		fixable: 'code'
+	}
+};

rules/no-unused-properties.js

@@ -139,7 +139,7 @@ const create = context => {
 							return {identifier: parent};
 						}
 
-						return null;
+						return undefined;
 					}
 
 					if (parent.type === 'MemberExpression') {
@@ -152,7 +152,7 @@ const create = context => {
 							return {identifier: parent};
 						}
 
-						return null;
+						return undefined;
 					}
 
 					if (
@@ -163,7 +163,7 @@ const create = context => {
 							return {identifier: parent};
 						}
 
-						return null;
+						return undefined;
 					}
 
 					if (
@@ -174,7 +174,7 @@ const create = context => {
 							return {identifier: parent};
 						}
 
-						return null;
+						return undefined;
 					}
 
 					return reference;

rules/prefer-add-event-listener.js

@@ -54,7 +54,7 @@ const create = context => {
 	let isDisabled;
 
 	const nodeReturnsSomething = new WeakMap();
-	let codePathInfo = null;
+	let codePathInfo;
 
 	return {
 		onCodePathStart(codePath, node) {

rules/prefer-reflect-apply.js

@@ -4,6 +4,7 @@ const getDocumentationUrl = require('./utils/get-documentation-url');
 const isLiteralValue = require('./utils/is-literal-value');
 
 const isApplySignature = (argument1, argument2) => (
+	// Please remove this file from `test/lint/lint.js` when fixing `null` issue
 	(isLiteralValue(argument1, null) ||
 		argument1.type === 'ThisExpression') &&
 	(argument2.type === 'ArrayExpression' ||

test/integration/test.js

@@ -148,7 +148,7 @@ list.run()
 					const {line} = error2.eslintMessage;
 
 					console.error(chalk.gray(`${project.repository}/tree/master/${path.relative(destination, file.filePath)}#L${line}`));
-					console.error(chalk.gray(JSON.stringify(error2.eslintMessage, null, 2)));
+					console.error(chalk.gray(JSON.stringify(error2.eslintMessage, undefined, 2)));
 				}
 			}
 		} else {

test/lint/lint.js

@@ -12,7 +12,11 @@ const unicornRules = new Map(Object.entries(unicorn.rules));
 const cli = new CLIEngine({
 	baseConfig: recommended,
 	useEslintrc: false,
-	fix
+	fix,
+	ignorePattern: [
+		// This can't disable by `eslint-disable` before `xo` update
+		'rules/prefer-reflect-apply.js'
+	]
 });
 
 cli.addPlugin('eslint-plugin-unicorn', unicorn);