There are some core YUI methods that I think were really good. The one that comes to mind the most is Y.extend(), which is different from _.extend() in that it

There are some core YUI methods that I think were really good. The one that comes to mind the most is Y.extend(), which is different from _.extend() in that it actually subclasses a class for you, and not just "mixes" two objects. I know some people have some kind of hate or disgust for JavaScript's emulation of classical inheritance in this manner, but I liked it a lot!