feat(eslint-plugin): add new rule to check for null before typeof object

Author: bryceosterhaus

projects/eslint-plugin/rules/general/docs/rules/no-typeof-object.md

@@ -0,0 +1,21 @@
+# Check for `null` before typeof === 'object' (no-typeof-object)
+
+`typeof null === 'object'` will evaluate to true, which can easily lead to issues in code. It is safest to explicitly check against null before this expression.
+
+## Rule Details
+
+Examples of **incorrect** code for this rule:
+
+```js
+typeof someVal === 'object';
+```
+
+Examples of **correct** code for this rule:
+
+```js
+someVal !== null && typeof someVal === 'object';
+```
+
+## See also
+
+-   https://github.com/liferay/liferay-frontend-projects/issues/19

projects/eslint-plugin/rules/general/lib/rules/no-typeof-object.js

@@ -0,0 +1,85 @@
+/**
+ * SPDX-FileCopyrightText: © 2021 Liferay, Inc. <https://liferay.com>
+ * SPDX-License-Identifier: MIT
+ */
+
+const message =
+	'checking "typeof x === \'object\'" can be falsy if null. Use "x !== null && typeof x === \'object\'"';
+
+module.exports = {
+	create(context) {
+		return {
+			BinaryExpression(node) {
+				if (
+					node.left &&
+					node.left.type === 'UnaryExpression' &&
+					node.left.operator === 'typeof' &&
+					node.right &&
+					node.right.value === 'object'
+				) {
+					const variableName = node.left.argument.name;
+
+					const logicalLeft =
+						node.parent &&
+						node.parent.type === 'LogicalExpression' &&
+						node.parent.left;
+
+					let hasNullCheck = false;
+
+					if (logicalLeft) {
+						let leftSibling;
+
+						if (logicalLeft.type === 'BinaryExpression') {
+							leftSibling = logicalLeft;
+						}
+
+						if (logicalLeft.type === 'LogicalExpression') {
+							leftSibling =
+								logicalLeft.right.type === 'BinaryExpression' &&
+								logicalLeft.right;
+						}
+
+						hasNullCheck =
+							leftSibling &&
+							leftSibling.left &&
+							leftSibling.left.name === variableName &&
+							leftSibling.operator === '!==' &&
+							leftSibling.right.value === null;
+
+						if (logicalLeft.type === 'UnaryExpression') {
+							hasNullCheck =
+								logicalLeft.operator === '!' &&
+								logicalLeft.argument.name === variableName;
+						}
+					}
+
+					if (!hasNullCheck) {
+						context.report({
+							fix: (fixer) => {
+								return fixer.insertTextBefore(
+									node,
+									`${variableName} !== null && `
+								);
+							},
+							message,
+							node,
+						});
+					}
+				}
+			},
+		};
+	},
+
+	meta: {
+		docs: {
+			category: 'Best Practices',
+			description: message,
+			recommended: false,
+			url:
+				'https://github.com/liferay/liferay-frontend-projects/issues/19',
+		},
+		fixable: true,
+		schema: [],
+		type: 'problem',
+	},
+};

projects/eslint-plugin/rules/general/tests/lib/rules/no-typeof-object.js

@@ -0,0 +1,55 @@
+/**
+ * SPDX-FileCopyrightText: © 2017 Liferay, Inc. <https://liferay.com>
+ * SPDX-License-Identifier: MIT
+ */
+
+const MultiTester = require('../../../../../scripts/MultiTester');
+const rule = require('../../../lib/rules/no-typeof-object');
+
+const parserOptions = {
+	parserOptions: {
+		ecmaVersion: 6,
+		sourceType: 'module',
+	},
+};
+
+const ruleTester = new MultiTester(parserOptions);
+
+ruleTester.run('no-typeof-object', rule, {
+	invalid: [
+		{
+			code: `
+				typeof x === 'object';
+
+				someVal && typeof x === 'object';
+			`,
+			errors: [
+				{
+					message:
+						'checking "typeof x === \'object\'" can be falsy if null. Use "x !== null && typeof x === \'object\'"',
+					type: 'BinaryExpression',
+				},
+				{
+					message:
+						'checking "typeof x === \'object\'" can be falsy if null. Use "x !== null && typeof x === \'object\'"',
+					type: 'BinaryExpression',
+				},
+			],
+			output: `
+				x !== null && typeof x === 'object';
+
+				someVal && x !== null && typeof x === 'object';
+			`,
+		},
+	],
+
+	valid: [
+		{
+			code: `
+				!x && typeof x === 'object';
+				
+				x !== null && typeof x === 'object';
+			`,
+		},
+	],
+});