Document for-in restricted usage

cmalard · web-flow · commit 7fb8b037147b · 2016-07-06T17:00:42.000+02:00
airbnb#851 (comment)
diff --git a/README.md b/README.md
@@ -1215,10 +1215,12 @@ Other Style Guides
 ## Iterators and Generators
 
   <a name="iterators--nope"></a><a name="11.1"></a>
-  - [11.1](#iterators--nope) Don't use iterators. Prefer JavaScript's higher-order functions like `map()` and `reduce()` instead of loops like `for-of`. eslint: [`no-iterator`](http://eslint.org/docs/rules/no-iterator.html)
+  - [11.1](#iterators--nope) Don't use iterators. Prefer JavaScript's higher-order functions instead of loops like `for-in` or `for-of`. eslint: [`no-iterator`](http://eslint.org/docs/rules/no-iterator.html) [`no-restricted-syntax`](http://eslint.org/docs/rules/no-restricted-syntax)
 
     > Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects.
 
+    > Use `map()` / `every()` / `filter()` / `find()` / `findIndex()` / `reduce()` / `some()` / ... to iterate over arrays, and `Object.keys()` / `Object.values()` / `Object.entries()` to produce arrays so you can iterate over objects.
+
     ```javascript
     const numbers = [1, 2, 3, 4, 5];
 
