Add no-mutable-exports

CHANGELOG.md

@@ -6,6 +6,7 @@ This change log adheres to standards from [Keep a CHANGELOG](http://keepachangel
 ## [Unreleased]
 ### Added
 - [`newline-after-import`], new rule. ([#245], thanks [@singles])
+- [`no-mutable-exports`], new rule. ([#290], thanks [@josh])
 
 ## resolvers/webpack/0.2.4 - 2016-04-29
 ### Changed

README.md

@@ -36,6 +36,7 @@ This plugin intends to support linting of ES2015+ (ES6+) import/export syntax, a
 [`no-named-as-default-member`]: ./docs/rules/no-named-as-default-member.md
 [`no-deprecated`]: ./docs/rules/no-deprecated.md
 [`no-extraneous-dependencies`]: ./docs/rules/no-extraneous-dependencies.md
+[`no-mutable-exports`]: ./docs/rules/no-mutable-exports.md
 
 **Module systems:**
 

docs/rules/no-mutable-exports.md

@@ -0,0 +1,23 @@
+# no-mutable-exports
+
+Forbids the use of mutable exports with `var` or `let`.
+
+## Rule Details
+
+Valid:
+
+```js
+export const count = 1
+export function getCount() {}
+```
+
+...whereas here exports will be reported:
+
+```js
+export let count = 2
+export var count = 3
+```
+
+## When Not To Use It
+
+If your environment correctly implements mutable export bindings.

src/index.js

@@ -5,6 +5,7 @@ export const rules = {
   'namespace': require('./rules/namespace'),
   'no-namespace': require('./rules/no-namespace'),
   'export': require('./rules/export'),
+  'no-mutable-exports': require('./rules/no-mutable-exports'),
   'extensions': require('./rules/extensions'),
 
   'no-named-as-default': require('./rules/no-named-as-default'),

src/rules/no-mutable-exports.js

@@ -0,0 +1,45 @@
+module.exports = function (context) {
+  function checkDeclaration(node) {
+    const {kind} = node
+    if (kind === 'var' || kind === 'let') {
+      context.report(node, `Exporting mutable '${kind}' binding, use 'const' instead.`)
+    }
+  }
+
+  function checkDeclarationsInScope({variables}, name) {
+    for (let variable of variables) {
+      if (variable.name === name) {
+        for (let def of variable.defs) {
+          if (def.type === 'Variable') {
+            checkDeclaration(def.parent)
+          }
+        }
+      }
+    }
+  }
+
+  function handleExportDefault(node) {
+    const scope = context.getScope()
+
+    if (node.declaration.name) {
+      checkDeclarationsInScope(scope, node.declaration.name)
+    }
+  }
+
+  function handleExportNamed(node) {
+    const scope = context.getScope()
+
+    if (node.declaration)  {
+      checkDeclaration(node.declaration)
+    } else {
+      for (let specifier of node.specifiers) {
+        checkDeclarationsInScope(scope, specifier.local.name)
+      }
+    }
+  }
+
+  return {
+    'ExportDefaultDeclaration': handleExportDefault,
+    'ExportNamedDeclaration': handleExportNamed,
+  }
+}

tests/src/rules/no-mutable-exports.js

@@ -0,0 +1,62 @@
+import {test} from '../utils'
+import {RuleTester} from 'eslint'
+import rule from 'rules/no-mutable-exports'
+
+const ruleTester = new RuleTester()
+
+ruleTester.run('no-mutable-exports', rule, {
+  valid: [
+    test({ code: 'export const count = 1'}),
+    test({ code: 'export function getCount() {}'}),
+    test({ code: 'export class Counter {}'}),
+    test({ code: 'export default count = 1'}),
+    test({ code: 'export default function getCount() {}'}),
+    test({ code: 'export default class Counter {}'}),
+    test({ code: 'const count = 1\nexport { count }'}),
+    test({ code: 'const count = 1\nexport { count as counter }'}),
+    test({ code: 'const count = 1\nexport default count'}),
+    test({ code: 'const count = 1\nexport { count as default }'}),
+    test({ code: 'function getCount() {}\nexport { getCount }'}),
+    test({ code: 'function getCount() {}\nexport { getCount as getCounter }'}),
+    test({ code: 'function getCount() {}\nexport default getCount'}),
+    test({ code: 'function getCount() {}\nexport { getCount as default }'}),
+    test({ code: 'class Counter {}\nexport { Counter }'}),
+    test({ code: 'class Counter {}\nexport { Counter as Count }'}),
+    test({ code: 'class Counter {}\nexport default Counter'}),
+    test({ code: 'class Counter {}\nexport { Counter as default }'}),
+  ],
+  invalid: [
+    test({
+      code: 'export let count = 1',
+      errors: ['Exporting mutable \'let\' binding, use \'const\' instead.'],
+    }),
+    test({
+      code: 'export var count = 1',
+      errors: ['Exporting mutable \'var\' binding, use \'const\' instead.'],
+    }),
+    test({
+      code: 'let count = 1\nexport { count }',
+      errors: ['Exporting mutable \'let\' binding, use \'const\' instead.'],
+    }),
+    test({
+      code: 'var count = 1\nexport { count }',
+      errors: ['Exporting mutable \'var\' binding, use \'const\' instead.'],
+    }),
+    test({
+      code: 'let count = 1\nexport { count as counter }',
+      errors: ['Exporting mutable \'let\' binding, use \'const\' instead.'],
+    }),
+    test({
+      code: 'var count = 1\nexport { count as counter }',
+      errors: ['Exporting mutable \'var\' binding, use \'const\' instead.'],
+    }),
+    test({
+      code: 'let count = 1\nexport default count',
+      errors: ['Exporting mutable \'let\' binding, use \'const\' instead.'],
+    }),
+    test({
+      code: 'var count = 1\nexport default count',
+      errors: ['Exporting mutable \'var\' binding, use \'const\' instead.'],
+    }),
+  ],
+})