gistfile1.js

// For Y.Foo

// Class extensions that will be used in the base class
function ExtA() {}
...
Y.FooExtA = ExtA;

// Base Class definition
function FooBase() {}
...
Y.FooBase = Y.Base.create('foo', Y.Base, [FooBase, Y.FooExtA]);
Y.Foo = Y.Base.create('foo', Y.FooBase, []);

// Optional class extensions
function ExtB() {}
...
Y.FooExtB = ExtB;
Y.Foo = Y.Base.mix(Y.Foo, [Y.FooExtB]);