I have just completed to develop a detailed guide for learning Java Script (in Hebrew). You can find it online available for free personal and academic usage at http://www.zindell.com/israeliJSBook/israeliJSBookWebText1.htm. Make sure you set the correct encoding (Hebrew Windows).
The Visitor Design Pattern
The visitor design pattern allows us to add operations to objects that already exist without modifying their classes and without extending them.