The YAHOO global object lives at the top of the namespace tree for all YUI components. It also includes some utility methods for type checking, user agent detection, and module extension.
YAHOO.langtype checking methods to deal with unpredictable data
YAHOO.env.uacan tell you with some accuracy which browser your page is being viewed in.
YAHOO.lang.extend: Leverage class inheritance in an object-oriented architecture.
YAHOO.lang.augmentPrototo help modularize class behaviors.
augmentObjectto extend static classes, object literals, or class instances.
YAHOO.lang.merge: Merge several objects, creating a unique set.