This is something trivial, which I've forgotten.
There are possibly duplicates - I searched a little, found similar, but couldn't find as concise.

```
   String.prototype.test = function(){this.bar = this.length + 2;}

   var str = "foo";
   str.test();

   console.log(str);                         // foo
   console.log(str.bar);                     // undefined

```

Pretty sure it has to do with `this` being trapped in the closure.