Merge pull request airbnb#328 from airbnb/es6-trailing-commas

[commas] additional trailing commas good in es6. fixes airbnb#323

Author: goatslacker

README.md

@@ -1470,31 +1470,46 @@
     };
     ```
 
-  - Additional trailing comma: **Nope.** This can cause problems with IE6/7 and IE9 if it's in quirksmode. Also, in some implementations of ES3 would add length to an array if it had an additional trailing comma. This was clarified in ES5 ([source](http://es5.github.io/#D)):
+  - Additional trailing comma: **Yup.**
 
-  > Edition 5 clarifies the fact that a trailing comma at the end of an ArrayInitialiser does not add to the length of the array. This is not a semantic change from Edition 3 but some implementations may have previously misinterpreted this.
+  > Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don't have to worry about the [trailing comma problem](es5/README.md#commas) in legacy browsers.
 
     ```javascript
+    // bad - git diff without trailing comma
+    const hero = {
+         firstName: 'Bob',
+    -    lastName: 'Parr'
+    +    lastName: 'Parr',
+    +    heroName: 'Mr. Incredible'
+    }
+
+    // good - git diff with trailing comma
+    const hero = {
+         firstName: 'Bob',
+         lastName: 'Parr',
+    +    heroName: 'Mr. Incredible',
+    }
+
     // bad
     const hero = {
       firstName: 'Kevin',
-      lastName: 'Flynn',
+      lastName: 'Flynn'
     };
 
     const heroes = [
       'Batman',
-      'Superman',
+      'Superman'
     ];
 
     // good
     const hero = {
       firstName: 'Kevin',
-      lastName: 'Flynn'
+      lastName: 'Flynn',
     };
 
     const heroes = [
       'Batman',
-      'Superman'
+      'Superman',
     ];
     ```