.extend and .prototype

Javascript's inheritance is prototype based, so you extend the prototypes of objects such as Date, Math, and even your own custom ones.

Date.prototype.lol = function() { alert('hi'); }; ( new Date ).lol() // alert message

In the snippet above, I define a method for all Date objects ( already existing ones and all new ones ).

extend is usually a high level function that copies the prototype of a new subclass that you want to extend from the base class.

So you can do something like:

extend( Fighter, Human )

And the Fighter constructor/object will inherit the prototype of Human, so if you define methods such as live and die on Human then Fighter will also inherit those.