ExtJS Class Handling is good to handle, but you have to pay it with the UI stack, loading all the unnecessary stuff (wich is very memory intensive, and slows down the browser load, I have trouble with firebug an ExtJS, it’s consuming by 20 time pageload more and more memory).
Is there a way for only using the Class Handling System from ExtJS, as is the case jsClass?
And on demand loading the UI-Components.
Check out Ext Core. It’s a stripped down version of ExtJS 3. Unfortunately there’s no version based on ExtJS 4 and I’m not sure they will make it… The upside is that Ext Core is completely free.