type Point = { x:int, y:int } // this isn’t a class, just a description of a possible type
class MyClass { public var x:int; public var y:int; }
function acceptPoint( aPoint like Point ){ /* do something */ }
var aPointLikeThing:MyClass = new MyClass;
acceptPoint( aPointLikeThing );

In this case there is a difference between that which is a true subclass (e.g. instanceof which is an Ecma script 4 operator as well) and that which is just ‘like’ a particular type description. This gets pretty cool with unions of types, where you can specify a type identifier as being anyone of a number of types, e.g:

type AnyNumber = (byte,int,uint,double,decimal,Number)

check out: http://www.ecmascript.org/es4/spec/overview.pdf

This is the most interesting addition to a language I have seen in quite some time and I am truly excited about it. In an optionally typed language (like es4) it provides compiler checking and a high level of polymorphism without requiring Interfaces and sub-classing.

To see this added to Python is a real pleasure since it is my newly adopted second language.