OOP - Extending A Class And Overriding A Method?

I have this class, (just the signature)

javascript Code:

var parent_child = function(po, ps, co, cs){
var url = 'http://xyz.com/abc/';
var pars = [], childs = {}, tmp;
this.p_added = function(urlFrag){

[Code]....

What i want to do is to create a new class with the above one as the super class, pass in one more variable to the constructor and override the p_added function. I thought of doing something like xpc = new parent_child(); and override the function with xpc.prototype.p_added = function(){}

how to pass in another variable to the constructor and make it private to the new class.

I believe there is a nice and concise way to achieve what i want to, but i can't find it. And the fact that OOP in javascript can be implemented in several different ways doesn't help my cause.