feat(eslint-plugin): add new rule to disallow 'use strict' in es modules

Author: bryceosterhaus

projects/eslint-plugin/configs/general.js

@@ -107,6 +107,7 @@ const config = {
 		'@liferay/no-it-should': 'error',
 		'@liferay/no-length-jsx-expression': 'error',
 		'@liferay/no-require-and-call': 'error',
+		'@liferay/no-use-strict-in-module': 'error',
 		'@liferay/padded-test-blocks': 'error',
 		'@liferay/ref-name-suffix': 'error',
 		'@liferay/sort-import-destructures': 'error',

projects/eslint-plugin/rules/general/docs/rules/no-use-strict-in-module.md

@@ -0,0 +1,40 @@
+# Disallow 'use strict' in es modules (no-use-strict-in-module)
+
+You never need `'use strict'` in an ES module ([spec](http://www.ecma-international.org/ecma-262/6.0/#sec-strict-mode-code)). We can infer if file is an esmodule if it uses export/import syntax.
+
+## Rule Details
+
+Examples of **incorrect** code for this rule:
+
+```js
+'use strict';
+
+export default () => {};
+```
+
+```js
+'use strict';
+
+import foo from 'bar';
+```
+
+Examples of **correct** code for this rule:
+
+```js
+import foo from 'bar';
+//...
+```
+
+```js
+//...
+export default foo;
+```
+
+```js
+//...
+export foo;
+```
+
+## Further Reading
+
+-   [Initial motivation for this rule](https://github.com/liferay/liferay-frontend-projects/issues/20).

projects/eslint-plugin/rules/general/index.js

@@ -21,6 +21,7 @@ module.exports = {
 	'no-it-should': require('./lib/rules/no-it-should'),
 	'no-length-jsx-expression': require('./lib/rules/no-length-jsx-expression'),
 	'no-require-and-call': require('./lib/rules/no-require-and-call'),
+	'no-use-strict-in-module': require('./lib/rules/no-use-strict-in-module'),
 	'padded-test-blocks': require('./lib/rules/padded-test-blocks'),
 	'ref-name-suffix': require('./lib/rules/ref-name-suffix'),
 	'sort-class-names': require('./lib/rules/sort-class-names'),

projects/eslint-plugin/rules/general/lib/rules/no-use-strict-in-module.js

@@ -0,0 +1,57 @@
+/**
+ * SPDX-FileCopyrightText: © 2021 Liferay, Inc. <https://liferay.com>
+ * SPDX-License-Identifier: MIT
+ */
+
+const message = `'use strict' is unnecessary inside of modules`;
+
+module.exports = {
+	create(context) {
+		const useStrictExpressions = [];
+		let esModule;
+
+		const checkEsModule = () => {
+			if (!esModule) {
+				esModule = true;
+			}
+		};
+
+		return {
+			'ExportDefaultDeclaration': checkEsModule,
+			'ExportNamedDeclaration': checkEsModule,
+			'ExpressionStatement'(node) {
+				if (
+					node.expression.type === 'Literal' &&
+					node.expression.value === 'use strict'
+				) {
+					useStrictExpressions.push(node.expression);
+				}
+			},
+			'ImportDeclaration': checkEsModule,
+			'ImportNamespaceSpecifier': checkEsModule,
+			'Program:exit': () => {
+				if (esModule) {
+					useStrictExpressions.forEach((expression) => {
+						context.report({
+							message,
+							node: expression,
+						});
+					});
+				}
+			},
+		};
+	},
+
+	meta: {
+		docs: {
+			category: 'Best Practices',
+			description: message,
+			recommended: false,
+			url:
+				'https://github.com/liferay/liferay-frontend-projects/issues/20',
+		},
+		fixable: 'code',
+		schema: [],
+		type: 'problem',
+	},
+};

projects/eslint-plugin/rules/general/tests/lib/rules/no-use-strict-in-module.js

@@ -0,0 +1,92 @@
+/**
+ * SPDX-FileCopyrightText: © 2017 Liferay, Inc. <https://liferay.com>
+ * SPDX-License-Identifier: MIT
+ */
+
+const MultiTester = require('../../../../../scripts/MultiTester');
+const rule = require('../../../lib/rules/no-use-strict-in-module');
+
+const parserOptions = {
+	parserOptions: {
+		ecmaVersion: 6,
+		sourceType: 'module',
+	},
+};
+
+const ruleTester = new MultiTester(parserOptions);
+
+ruleTester.run('no-use-strict-in-module', rule, {
+	invalid: [
+		{
+			code: `
+				'use strict'
+
+				import {test} from 'test';
+			 `,
+			errors: [
+				{
+					message: `'use strict' is unnecessary inside of modules`,
+					type: 'Literal',
+				},
+			],
+		},
+		{
+			code: `
+				'use strict'
+
+				function test() {
+					'use strict'
+
+					return 'test';
+				}
+				export default test;
+			 `,
+			errors: [
+				{
+					message: `'use strict' is unnecessary inside of modules`,
+					type: 'Literal',
+				},
+				{
+					message: `'use strict' is unnecessary inside of modules`,
+					type: 'Literal',
+				},
+			],
+		},
+		{
+			code: `
+				'use strict'
+
+				function test() {}
+				export {test};
+			 `,
+			errors: [
+				{
+					message: `'use strict' is unnecessary inside of modules`,
+					type: 'Literal',
+				},
+			],
+		},
+	],
+
+	valid: [
+		{
+			code: `
+				import {test} from 'test';
+			 `,
+		},
+		{
+			code: `
+				'use strict'
+
+				module.export = function test() {}
+			 `,
+		},
+		{
+			code: `
+				'use strict'
+
+				console.log('foo');
+			 `,
+		},
+	],
+});