New: prefer-single-export rule

Author: robertrossmann

Diff for: CHANGELOG.md

@@ -4,6 +4,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 This change log adheres to standards from [Keep a CHANGELOG](http://keepachangelog.com).
 
 ## [Unreleased]
+### Added
+- Add [`prefer-single-export`] rule: style-guide rule to report use of multiple named exports ([#721], thanks [@Alaneor])
+
 ### Changed
 - [`no-extraneous-dependencies`]: use `read-pkg-up` to simplify finding + loading `package.json` ([#680], thanks [@wtgtybhertgeghgtwtg])
 
@@ -376,7 +379,9 @@ for info on changes for earlier releases.
 [`no-webpack-loader-syntax`]: ./docs/rules/no-webpack-loader-syntax.md
 [`no-unassigned-import`]: ./docs/rules/no-unassigned-import.md
 [`unambiguous`]: ./docs/rules/unambiguous.md
+[`prefer-single-export`]: ./docs/rules/prefer-single-export.md
 
+[#721]: https://github.com/benmosher/eslint-plugin-import/pull/721
 [#680]: https://github.com/benmosher/eslint-plugin-import/pull/680
 [#654]: https://github.com/benmosher/eslint-plugin-import/pull/654
 [#639]: https://github.com/benmosher/eslint-plugin-import/pull/639
@@ -561,3 +566,4 @@ for info on changes for earlier releases.
 [@ntdb]: https://github.com/ntdb
 [@jakubsta]: https://github.com/jakubsta
 [@wtgtybhertgeghgtwtg]: https://github.com/wtgtybhertgeghgtwtg
+[@Alaneor]: https://github.com/Alaneor

Diff for: README.md

@@ -76,6 +76,7 @@ This plugin intends to support linting of ES2015+ (ES6+) import/export syntax, a
 * Limit the maximum number of dependencies a module can have ([`max-dependencies`])
 * Forbid unassigned imports ([`no-unassigned-import`])
 * Forbid named default exports ([`no-named-default`])
+* Prefer single named export declaration ([`prefer-single-export`])
 
 [`first`]: ./docs/rules/first.md
 [`no-duplicates`]: ./docs/rules/no-duplicates.md
@@ -87,6 +88,7 @@ This plugin intends to support linting of ES2015+ (ES6+) import/export syntax, a
 [`max-dependencies`]: ./docs/rules/max-dependencies.md
 [`no-unassigned-import`]: ./docs/rules/no-unassigned-import.md
 [`no-named-default`]: ./docs/rules/no-named-default.md
+[`prefer-single-export`]: ./docs/rules/prefer-single-export.md
 
 ## Installation
 

Diff for: docs/rules/prefer-single-export.md

@@ -0,0 +1,54 @@
+# prefer-single-export
+
+Reports when multiple named exports or CommonJS assignments are present in a single file.
+
+**Rationale:** An `export` declaration or `module.exports` assignment can appear anywhere in the code. By requiring a single export declaration, your exports will remain at one place, making it easier to see what named exports a module provides.
+
+## Rule Details
+
+This rule warns whenever a single file contains multiple named exports or assignments to `module.exports` (or `exports`).
+
+### Valid
+
+```js
+export default function test() {}
+// A single named export -> ok
+export const valid = true
+```
+
+```js
+const first = true
+const second = true
+
+// A single named export -> ok
+export {
+  first,
+  second,
+}
+```
+
+```js
+// A single exports assignment -> ok
+module.exports = {
+  first: true,
+  second: true
+}
+```
+
+### Invalid
+
+```js
+// Multiple named exports -> not ok!
+export const first = true
+export const second = true
+```
+
+```js
+// Multiple exports assignments -> not ok!
+exports.first = true
+exports.second = true
+```
+
+## When Not To Use It
+
+If you do not mind having multiple named exports in a single module, you can safely turn this rule off.

Diff for: src/index.js

@@ -9,6 +9,7 @@ export const rules = {
   'extensions': require('./rules/extensions'),
   'no-restricted-paths': require('./rules/no-restricted-paths'),
   'no-internal-modules': require('./rules/no-internal-modules'),
+  'prefer-single-export': require('./rules/prefer-single-export'),
 
   'no-named-default': require('./rules/no-named-default'),
   'no-named-as-default': require('./rules/no-named-as-default'),

Diff for: src/rules/prefer-single-export.js

@@ -0,0 +1,37 @@
+const MULTI_EXPORT = 'Multiple named export declarations'
+const MULTI_MODULE_EXPORTS = 'Multiple CommonJS exports'
+
+const meta = {}
+
+function create(context) {
+  const issues = new Map()
+
+  return {
+    ExportNamedDeclaration(node) {
+      issues.set(node, MULTI_EXPORT)
+    },
+
+    MemberExpression(node) {
+      if (node.object.name === 'module' && node.property.name === 'exports') {
+        issues.set(node, MULTI_MODULE_EXPORTS)
+      }
+
+      if (node.object.name === 'exports') {
+        issues.set(node, MULTI_MODULE_EXPORTS)
+      }
+    },
+
+    'Program:exit': function onExit() {
+      if (issues.size > 1) {
+        for (const [node, message] of issues) {
+          context.report({ node, message })
+        }
+      }
+    },
+  }
+}
+
+export default {
+  meta,
+  create,
+}

Diff for: tests/src/rules/prefer-single-export.js

@@ -0,0 +1,54 @@
+import { test } from '../utils'
+import { RuleTester } from 'eslint'
+import rule from 'rules/prefer-single-export'
+
+const ruleTester = new RuleTester()
+
+ruleTester.run('prefer-single-export', rule, {
+  valid: [
+    test({ code: 'export const test = true' }),
+    test({ code: 'export default {}\nexport const test = true' }),
+    test({ code: [
+      'const first = true',
+      'const second = true',
+      'export { first,\nsecond }',
+    ].join('\n') }),
+    test({ code: 'module.exports = {} '}),
+    test({ code: 'module.exports = { test: true,\nanother: false }' }),
+    test({ code: 'exports.test = true' }),
+  ],
+  invalid: [
+    test({
+      code: [
+        'export const test = true',
+        'export const another = true',
+      ].join('\n'),
+      errors: [
+        'Multiple named export declarations',
+        'Multiple named export declarations',
+      ],
+    }),
+    test({
+      code: [
+        'module.exports = {}',
+        'module.exports.test = true',
+        'module.exports.another = true',
+      ].join('\n'),
+      errors: [
+        'Multiple CommonJS exports',
+        'Multiple CommonJS exports',
+        'Multiple CommonJS exports',
+      ],
+    }),
+    test({
+      code: [
+        'module.exports = {}',
+        'module.exports = {}',
+      ].join('\n'),
+      errors: [
+        'Multiple CommonJS exports',
+        'Multiple CommonJS exports',
+      ],
+    }),
+  ],
+})