Add prefer-export-from rule (#1453)

Author: fisker

configs/recommended.js

@@ -63,6 +63,7 @@ module.exports = {
 		'unicorn/prefer-dom-node-dataset': 'error',
 		'unicorn/prefer-dom-node-remove': 'error',
 		'unicorn/prefer-dom-node-text-content': 'error',
+		'unicorn/prefer-export-from': 'error',
 		'unicorn/prefer-includes': 'error',
 		'unicorn/prefer-keyboard-event-key': 'error',
 		'unicorn/prefer-math-trunc': 'error',

docs/rules/prefer-export-from.md

@@ -0,0 +1,61 @@
+# Prefer `export…from` when re-exporting
+
+When re-exporting from a module, it's unnecessary to import and then export. It can be done in a single `export…from` declaration.
+
+This rule is fixable.
+
+## Fail
+
+```js
+import defaultExport from './foo.js';
+export default defaultExport;
+```
+
+```js
+import {named} from './foo.js';
+export {named};
+```
+
+```js
+import * as namespace from './foo.js';
+export {namespace};
+```
+
+```js
+import defaultExport, {named} from './foo.js';
+export default defaultExport;
+export {
+	defaultExport as renamedDefault,
+	named,
+	named as renamedNamed,
+};
+```
+
+## Pass
+
+```js
+export {default} from './foo.js';
+```
+
+```js
+export {named} from './foo.js';
+```
+
+```js
+export * as namespace from './foo.js';
+```
+
+```js
+export {
+	default,
+	default as renamedDefault,
+	named,
+	named as renamedNamed,
+} from './foo.js';
+```
+
+```js
+// There is no substitution
+import * as namespace from './foo.js';
+export default namespace;
+```

readme.md

@@ -95,6 +95,7 @@ Configure it in `package.json`.
 			"unicorn/prefer-dom-node-dataset": "error",
 			"unicorn/prefer-dom-node-remove": "error",
 			"unicorn/prefer-dom-node-text-content": "error",
+			"unicorn/prefer-export-from": "error",
 			"unicorn/prefer-includes": "error",
 			"unicorn/prefer-keyboard-event-key": "error",
 			"unicorn/prefer-math-trunc": "error",
@@ -216,6 +217,7 @@ Each rule has emojis denoting:
 | [prefer-dom-node-dataset](docs/rules/prefer-dom-node-dataset.md) | Prefer using `.dataset` on DOM elements over `.setAttribute(…)`. | ✅ | 🔧 |  |
 | [prefer-dom-node-remove](docs/rules/prefer-dom-node-remove.md) | Prefer `childNode.remove()` over `parentNode.removeChild(childNode)`. | ✅ | 🔧 | 💡 |
 | [prefer-dom-node-text-content](docs/rules/prefer-dom-node-text-content.md) | Prefer `.textContent` over `.innerText`. | ✅ |  | 💡 |
+| [prefer-export-from](docs/rules/prefer-export-from.md) | Prefer `export…from` when re-exporting. | ✅ | 🔧 |  |
 | [prefer-includes](docs/rules/prefer-includes.md) | Prefer `.includes()` over `.indexOf()` and `Array#some()` when checking for existence or non-existence. | ✅ | 🔧 | 💡 |
 | [prefer-keyboard-event-key](docs/rules/prefer-keyboard-event-key.md) | Prefer `KeyboardEvent#key` over `KeyboardEvent#keyCode`. | ✅ | 🔧 |  |
 | [prefer-math-trunc](docs/rules/prefer-math-trunc.md) | Enforce the use of `Math.trunc` instead of bitwise operators. | ✅ | 🔧 | 💡 |

rules/prefer-export-from.js

@@ -0,0 +1,288 @@
+'use strict';
+const {
+	isCommaToken,
+	isOpeningBraceToken,
+	isClosingBraceToken,
+} = require('eslint-utils');
+
+const MESSAGE_ID = 'prefer-export-from';
+const messages = {
+	[MESSAGE_ID]: 'Use `export…from` to re-export `{{exported}}`.',
+};
+
+function * removeSpecifier(node, fixer, sourceCode) {
+	const {parent} = node;
+	const {specifiers} = parent;
+
+	if (specifiers.length === 1) {
+		yield * removeImportOrExport(parent, fixer, sourceCode);
+		return;
+	}
+
+	switch (node.type) {
+		case 'ImportSpecifier': {
+			const hasOtherSpecifiers = specifiers.some(specifier => specifier !== node && specifier.type === node.type);
+			if (!hasOtherSpecifiers) {
+				const closingBraceToken = sourceCode.getTokenAfter(node, isClosingBraceToken);
+
+				// If there are other specifiers, they have to be the default import specifier
+				// And the default import has to write before the named import specifiers
+				// So there must be a comma before
+				const commaToken = sourceCode.getTokenBefore(node, isCommaToken);
+				yield fixer.replaceTextRange([commaToken.range[0], closingBraceToken.range[1]], '');
+				return;
+			}
+			// Fallthrough
+		}
+
+		case 'ExportSpecifier':
+		case 'ImportNamespaceSpecifier':
+		case 'ImportDefaultSpecifier': {
+			yield fixer.remove(node);
+
+			const tokenAfter = sourceCode.getTokenAfter(node);
+			if (isCommaToken(tokenAfter)) {
+				yield fixer.remove(tokenAfter);
+			}
+
+			break;
+		}
+
+		// No default
+	}
+}
+
+function * removeImportOrExport(node, fixer, sourceCode) {
+	switch (node.type) {
+		case 'ImportSpecifier':
+		case 'ExportSpecifier':
+		case 'ImportDefaultSpecifier':
+		case 'ImportNamespaceSpecifier': {
+			yield * removeSpecifier(node, fixer, sourceCode);
+			return;
+		}
+
+		case 'ImportDeclaration':
+		case 'ExportDefaultDeclaration':
+		case 'ExportNamedDeclaration': {
+			yield fixer.remove(node);
+		}
+
+		// No default
+	}
+}
+
+function fix({
+	context,
+	imported,
+	exported,
+	exportDeclarations,
+	program,
+}) {
+	const sourceCode = context.getSourceCode();
+	const sourceNode = imported.declaration.source;
+	const sourceValue = sourceNode.value;
+	const sourceText = sourceCode.getText(sourceNode);
+	const exportDeclaration = exportDeclarations.find(({source}) => source.value === sourceValue);
+
+	/** @param {import('eslint').Rule.RuleFixer} fixer */
+	return function * (fixer) {
+		if (imported.name === '*') {
+			yield fixer.insertTextAfter(
+				program,
+				`\nexport * as ${exported.name} from ${sourceText};`,
+			);
+		} else {
+			const specifier = exported.name === imported.name
+				? exported.name
+				: `${imported.name} as ${exported.name}`;
+
+			if (exportDeclaration) {
+				const lastSpecifier = exportDeclaration.specifiers[exportDeclaration.specifiers.length - 1];
+
+				// `export {} from 'foo';`
+				if (lastSpecifier) {
+					yield fixer.insertTextAfter(lastSpecifier, `, ${specifier}`);
+				} else {
+					const openingBraceToken = sourceCode.getFirstToken(exportDeclaration, isOpeningBraceToken);
+					yield fixer.insertTextAfter(openingBraceToken, specifier);
+				}
+			} else {
+				yield fixer.insertTextAfter(
+					program,
+					`\nexport {${specifier}} from ${sourceText};`,
+				);
+			}
+		}
+
+		if (imported.variable.references.length === 1) {
+			yield * removeImportOrExport(imported.node, fixer, sourceCode);
+		}
+
+		yield * removeImportOrExport(exported.node, fixer, sourceCode);
+	};
+}
+
+function getImportedName(specifier) {
+	switch (specifier.type) {
+		case 'ImportDefaultSpecifier':
+			return 'default';
+
+		case 'ImportSpecifier':
+			return specifier.imported.name;
+
+		case 'ImportNamespaceSpecifier':
+			return '*';
+
+		// No default
+	}
+}
+
+function getExported(identifier, context) {
+	const {parent} = identifier;
+	switch (parent.type) {
+		case 'ExportDefaultDeclaration':
+			return {
+				node: parent,
+				name: 'default',
+			};
+
+		case 'ExportSpecifier':
+			return {
+				node: parent,
+				name: parent.exported.name,
+			};
+
+		case 'VariableDeclarator': {
+			if (
+				parent.init === identifier
+				&& parent.id.type === 'Identifier'
+				&& parent.parent.type === 'VariableDeclaration'
+				&& parent.parent.kind === 'const'
+				&& parent.parent.declarations.length === 1
+				&& parent.parent.declarations[0] === parent
+				&& parent.parent.parent.type === 'ExportNamedDeclaration'
+				&& isVariableUnused(parent, context)
+			) {
+				return {
+					node: parent.parent.parent,
+					name: parent.id.name,
+				};
+			}
+
+			break;
+		}
+
+		// No default
+	}
+}
+
+function isVariableUnused(node, context) {
+	const variables = context.getDeclaredVariables(node);
+
+	/* istanbul ignore next */
+	if (variables.length !== 1) {
+		return false;
+	}
+
+	const [{identifiers, references}] = variables;
+	return identifiers.length === 1
+		&& identifiers[0] === node.id
+		&& references.length === 1
+		&& references[0].identifier === node.id;
+}
+
+function * getProblems({
+	context,
+	variable,
+	program,
+	exportDeclarations,
+}) {
+	const {identifiers, references} = variable;
+
+	if (identifiers.length !== 1 || references.length === 0) {
+		return;
+	}
+
+	const specifier = identifiers[0].parent;
+
+	const imported = {
+		name: getImportedName(specifier),
+		node: specifier,
+		declaration: specifier.parent,
+		variable,
+	};
+
+	for (const {identifier} of references) {
+		const exported = getExported(identifier, context);
+
+		if (!exported) {
+			continue;
+		}
+
+		/*
+		There is no substitution for:
+
+		```js
+		import * as foo from 'foo';
+		export default foo;
+		```
+		*/
+		if (imported.name === '*' && exported.name === 'default') {
+			return;
+		}
+
+		yield {
+			node: exported.node,
+			messageId: MESSAGE_ID,
+			data: {
+				exported: exported.name,
+			},
+			fix: fix({
+				context,
+				imported,
+				exported,
+				exportDeclarations,
+				program,
+			}),
+		};
+	}
+}
+
+/** @param {import('eslint').Rule.RuleContext} context */
+function create(context) {
+	const variables = [];
+	const exportDeclarations = [];
+
+	return {
+		'ImportDeclaration[specifiers.length>0]'(node) {
+			variables.push(...context.getDeclaredVariables(node));
+		},
+		// `ExportAllDeclaration` and `ExportDefaultDeclaration` can't be reused
+		'ExportNamedDeclaration[source.type="Literal"]'(node) {
+			exportDeclarations.push(node);
+		},
+		* 'Program:exit'(program) {
+			for (const variable of variables) {
+				yield * getProblems({
+					context,
+					variable,
+					exportDeclarations,
+					program,
+				});
+			}
+		},
+	};
+}
+
+module.exports = {
+	create,
+	meta: {
+		type: 'suggestion',
+		docs: {
+			description: 'Prefer `export…from` when re-exporting.',
+		},
+		fixable: 'code',
+		messages,
+	},
+};