diff --git a/src/shared/md/2010-02-15-undefined-is-mutable.md b/src/shared/md/2010-02-15-undefined-is-mutable.md
index 19baacf..dc853b7 100644
--- a/src/shared/md/2010-02-15-undefined-is-mutable.md
+++ b/src/shared/md/2010-02-15-undefined-is-mutable.md
@@ -14,4 +14,15 @@ Due to the mutability of undefined, it is generally a better idea to check for u
     typeof a.b == 'undefined'; // always true
 ```
 
-– [@mathias](http://mathiasbynens.be/)
\ No newline at end of file
+– [@mathias](http://mathiasbynens.be/)
+
+That is no longer the case in [ECMAScript 5](http://ecma-international.org/ecma-262/5.1/#sec-15.1.1.3)
+
+In ECMAScript 3, a [trick you can use](http://www.2ality.com/2011/05/void-operator.html) is comparing to `void 0` because the **void** expression always evaluates to **undefined**:
+
+```
+    var a = {};
+    a.b === void 0; // true
+```
+
+– [@specious](https://github.com/specious)
\ No newline at end of file