[New] jsx-filename-extension add "allow" option with "as-needed"

docs/rules/jsx-filename-extension.md

@@ -20,8 +20,22 @@ function MyComponent() {
 }
 ```
 
+Beware this rule **only** reports JSX syntax, **not** other non-standard syntax such as experimental features or type annotations.
+
 ## Rule Options
 
+### `allow` (default: `"always"`)
+
+When to allow a JSX filename extension. By default all files may have a JSX extension. Set this to `as-needed` to only allow JSX file extensions in files that contain JSX syntax.
+
+```js
+"rules": {
+  "react/jsx-filename-extension": [1, { "allow": "as-needed" }]
+}
+```
+
+### `extensions` (default: `[".jsx"]`)
+
 The set of allowed extensions is configurable. By default '.jsx' is allowed. If you wanted to allow both '.jsx' and '.js', the configuration would be:
 
 ```js

lib/rules/jsx-filename-extension.js

@@ -13,6 +13,7 @@ const docsUrl = require('../util/docsUrl');
 // ------------------------------------------------------------------------------
 
 const DEFAULTS = {
+  allow: 'always',
   extensions: ['.jsx']
 };
 
@@ -32,6 +33,9 @@ module.exports = {
     schema: [{
       type: 'object',
       properties: {
+        allow: {
+          enum: ['always', 'as-needed']
+        },
         extensions: {
           type: 'array',
           items: {
@@ -44,32 +48,23 @@ module.exports = {
   },
 
   create(context) {
-    let invalidExtension;
-    let invalidNode;
+    const filename = context.getFilename();
 
-    function getExtensionsConfig() {
-      return context.options[0] && context.options[0].extensions || DEFAULTS.extensions;
-    }
+    let jsxNode;
 
-    function handleJSX(node) {
-      const filename = context.getFilename();
-      if (filename === '<text>') {
-        return;
-      }
-
-      if (invalidNode) {
-        return;
-      }
+    if (filename === '<text>') {
+      // No need to traverse any nodes.
+      return {};
+    }
 
-      const allowedExtensions = getExtensionsConfig();
-      const isAllowedExtension = allowedExtensions.some((extension) => filename.slice(-extension.length) === extension);
+    const allow = (context.options[0] && context.options[0].allow) || DEFAULTS.allow;
+    const allowedExtensions = (context.options[0] && context.options[0].extensions) || DEFAULTS.extensions;
+    const isAllowedExtension = allowedExtensions.some((extension) => filename.slice(-extension.length) === extension);
 
-      if (isAllowedExtension) {
-        return;
+    function handleJSX(node) {
+      if (!jsxNode) {
+        jsxNode = node;
       }
-
-      invalidNode = node;
-      invalidExtension = path.extname(filename);
     }
 
     // --------------------------------------------------------------------------
@@ -80,15 +75,23 @@ module.exports = {
       JSXElement: handleJSX,
       JSXFragment: handleJSX,
 
-      'Program:exit'() {
-        if (!invalidNode) {
+      'Program:exit'(node) {
+        if (jsxNode) {
+          if (!isAllowedExtension) {
+            context.report({
+              node: jsxNode,
+              message: `JSX not allowed in files with extension '${path.extname(filename)}'`
+            });
+          }
           return;
         }
 
-        context.report({
-          node: invalidNode,
-          message: `JSX not allowed in files with extension '${invalidExtension}'`
-        });
+        if (isAllowedExtension && allow === 'as-needed') {
+          context.report({
+            node,
+            message: `Only files containing JSX may use the extension '${path.extname(filename)}'`
+          });
+        }
       }
     };
   }

tests/lib/rules/jsx-filename-extension.js

@@ -45,6 +45,14 @@ ruleTester.run('jsx-filename-extension', rule, {
     {
       filename: 'MyComponent.jsx',
       code: withJSXElement
+    }, {
+      filename: 'MyComponent.js',
+      code: withoutJSX,
+      options: [{allow: 'as-needed'}]
+    }, {
+      filename: 'MyComponent.jsx',
+      code: withJSXElement,
+      options: [{allow: 'as-needed'}]
     }, {
       filename: 'MyComponent.js',
       options: [{extensions: ['.js', '.jsx']}],
@@ -74,6 +82,16 @@ ruleTester.run('jsx-filename-extension', rule, {
       filename: 'MyComponent.js',
       code: withJSXElement,
       errors: [{message: 'JSX not allowed in files with extension \'.js\''}]
+    }, {
+      filename: 'MyComponent.jsx',
+      code: withoutJSX,
+      options: [{allow: 'as-needed'}],
+      errors: [{message: 'Only files containing JSX may use the extension \'.jsx\''}]
+    }, {
+      filename: 'notAComponent.js',
+      code: withJSXElement,
+      options: [{allow: 'as-needed'}],
+      errors: [{message: 'JSX not allowed in files with extension \'.js\''}]
     }, {
       filename: 'MyComponent.jsx',
       code: withJSXElement,