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.lang type checking methods to deal with unpredictable data
YAHOO.env.ua can 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.augmentProto to help modularize class behaviors.
augmentObject to extend static classes, object literals, or class instances.
YAHOO.lang.merge: Merge several objects, creating a unique set.