javascript-101/loops: add for...in loop

Closes gh-588
Fixes gh-321

Author: arthurvr

page/javascript-101/loops.md

@@ -46,6 +46,18 @@ for (var i = 0, limit = 100; i < limit; i++) {
 }
 ```
 
+## The `for...in` loop
+
++A `for...in` loop iterates over the properties of an object. For each property, statements can be executed.
++
++```
++for ( prop in obj ) {
+	// statements here will be executed for every key in the object
+	console.log( prop + ': ' + obj[ prop ] );
+}
+``
+
+
 ## The `while` loop
 
 A while loop is similar to an `if` statement, except that its body will keep executing until the condition evaluates to a falsy value.