Subclassing With JavaScript

I am trying to create a Rectangle class by subclassing a Polygon class through replacing the Rectangle.prototype object. I tried to put everything inside the Rectangle constructor for better encapsulation but it seems that this does not work in both IE7 & FF. Just wondering why it doesn't.

<script> // Rectangle.prototype=new Polygon(4); <--- have to be here!!why? r = new Rectangle(4,3); alert(r.sides); function Rectangle(w,h) { this.width= w; this.height=h ; Rectangle.prototype=new Polygon(4); // <-- not working inside constructor!!? } function Polygon(s) { this.sides=s; } </script>

Changing the prototype in the constructor is too late. The object has already been constructed from the prototype by the time the constructor is called.