Add no-mutable-exports

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 +1,10 @@
+module.exports = function (context) {
+	return {
+		'ExportNamedDeclaration': function (node) {
+      const kind = node.declaration.kind
+      if (kind === 'var' || kind === 'let') {
+        context.report(node, `Exporting mutable '${kind}' binding, use const instead.`)
+      }
+		},
+	}
+}

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

@@ -0,0 +1,23 @@
+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 {}'}),
+  ],
+  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.'],
+    }),
+  ],
+})