Add prefer-default-export rule

Author: gavriguy

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
+- [`prefer-default-export`], new rule. ([#308], thanks [@gavriguy])
+
 ### Fixed
 - ignore namespace / ES7 re-exports in [`no-mutable-exports`]. ([#317], fixed by [#322]. thanks [@borisyankov] + [@jfmengels])
 
@@ -204,8 +207,10 @@ for info on changes for earlier releases.
 [`named`]: ./docs/rules/named.md
 [`newline-after-import`]: ./docs/rules/newline-after-import.md
 [`no-mutable-exports`]: ./docs/rules/no-mutable-exports.md
+[`prefer-default-export`]: ./docs/rules/prefer-default-export.md
 
 [#322]: https://github.com/benmosher/eslint-plugin-import/pull/322
+[#308]: https://github.com/benmosher/eslint-plugin-import/pull/308
 [#298]: https://github.com/benmosher/eslint-plugin-import/pull/298
 [#297]: https://github.com/benmosher/eslint-plugin-import/pull/297
 [#296]: https://github.com/benmosher/eslint-plugin-import/pull/296
@@ -281,3 +286,4 @@ for info on changes for earlier releases.
 [@SimenB]: https://github.com/SimenB
 [@josh]: https://github.com/josh
 [@borisyankov]: https://github.com/borisyankov
+[@gavriguy]: https://github.com/gavriguy

Diff for: README.md

@@ -64,6 +64,7 @@ This plugin intends to support linting of ES2015+ (ES6+) import/export syntax, a
 [`extensions`]: ./docs/rules/extensions.md
 [`order`]: ./docs/rules/order.md
 [`newline-after-import`]: ./docs/rules/newline-after-import.md
+[`prefer-default-export`]: ./docs/rules/prefer-default-export.md
 
 
 ## Installation

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

@@ -0,0 +1,51 @@
+# prefer-default-export
+
+When there is only a single export from a module prefer using default export over named export.
+
+## Rule Details
+
+The following patterns are considered warnings:
+
+```javascript
+// bad.js
+
+// There is only a single module export and its a named export.
+export const foo = 'foo';
+
+```
+
+The following patterns are not warnings:
+
+```javascript
+// good1.js
+
+// There is a default export.
+export const foo = 'foo';
+const bar = 'bar';
+export default 'bar';
+```
+
+```javascript
+// good2.js
+
+// There is more thank one named exports in the module.
+export const foo = 'foo';
+export const bar = 'bar';
+```
+
+```javascript
+// good3.js
+
+// There is more thank one named exports in the module
+const foo = 'foo';
+const bar = 'bar';
+export { foo, bar }
+```
+
+```javascript
+// good4.js
+
+// There is a default export.
+const foo = 'foo';
+export { foo as default }
+```

Diff for: src/index.js

@@ -19,6 +19,7 @@ export const rules = {
   'no-nodejs-modules': require('./rules/no-nodejs-modules'),
   'order': require('./rules/order'),
   'newline-after-import': require('./rules/newline-after-import'),
+  'prefer-default-export': require('./rules/prefer-default-export'),
 
   // metadata-based
   'no-deprecated': require('./rules/no-deprecated'),

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

@@ -0,0 +1,31 @@
+'use strict'
+
+module.exports = function(context) {
+  let namedExportCount = 0
+  let specifierExportCount = 0
+  let hasDefaultExport = false
+  let namedExportNode = null
+  return {
+    'ExportSpecifier': function(node) {
+      if (node.exported.name === 'default') {
+        hasDefaultExport = true
+      } else {
+        specifierExportCount++
+        namedExportNode = node
+      }
+    },
+    'ExportNamedDeclaration': function(node) {
+      namedExportCount++
+      namedExportNode = node
+    },
+    'ExportDefaultDeclaration': function() {
+      hasDefaultExport = true
+    },
+
+    'Program:exit': function() {
+      if (namedExportCount === 1 &&  specifierExportCount < 2 && !hasDefaultExport) {
+        context.report(namedExportNode, 'Prefer default export.')
+      }
+    },
+  }
+}

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

@@ -0,0 +1,48 @@
+import { test } from '../utils'
+
+import { RuleTester } from 'eslint'
+
+const ruleTester = new RuleTester()
+    , rule = require('rules/prefer-default-export')
+
+ruleTester.run('prefer-default-export', rule, {
+  valid: [
+    test({
+      code: `
+        export const foo = 'foo';
+        export const bar = 'bar';`,
+      }),
+    test({
+      code: `
+        export const foo = 'foo';
+        export default bar;`,
+      }),
+    test({
+      code: `
+        export { foo, bar }`,
+      }),
+    test({
+      code: `
+        export { foo as default }`,
+      }),
+  ],
+  invalid: [
+    test({
+      code: `
+        export const foo = 'foo';`,
+      errors: [{
+        ruleId: 'ExportNamedDeclaration',
+        message: 'Prefer default export.',
+      }],
+    }),
+    test({
+      code: `
+        const foo = 'foo';
+        export { foo };`,
+      errors: [{
+        ruleId: 'ExportNamedDeclaration',
+        message: 'Prefer default export.',
+      }],
+    }),
+  ],
+})