​How do I create a method that returns a new constructor instead of an object?

How to extend a javascript constructor?I want to be able to extend a constructor and use its methods. The problem is when I do the following property values are being shared.
 

var Message = new Resource('message');



var Conversation = new Resource('conversation);



Conversation.limit = 5;



Message.limit = 8;



Conversation.getData() // pulls 8 items instead

How do I create a method that returns a new constructor instead of an object?