How do I use constructor of a remote Page to create an Object in my UserScript

[minigweek]

Hi Folks,

The page on which my userscript will run has a namespace, the namespace defines a constructor function. I would like to create an object using the same constructor and use methods of the object in my userscript. So far I have been unsuccessful. Here's what I am trying to do.

The Page has the following native javascript block :

var namespace={ constructor : function(){
   this.sum = function(value1,value2){
   alert(value1+value2);
    }
  }
}

being used like:

var pageObject=new namespace.constructor();
pageObject.sum(1,2);

In My Userscript its my intention to create an object just like pageObject and call sum from that with my own parameters.

I have tried doing the following :

var greaseNameSpace = unsafeWindow.namespace;
var greaseObject = new greaseNameSpace.constructor();
greaseObject.sum(1,2);

No Luck, appears though greaseNameSpace exist, and even greaseNameSpace.constructor is a valid function , using new greaseNameSpace.constructor() yields undefined.

also tried following :

var greaseObject =new unsafeWindow.namespace.constructor();

again greaseObject remains undefined.

I found one thread here GreaseMonkey: How can I create an object of a class which defined in the remote page?

But it uses eval , and I wonder if thats the right way ?

Any and all help would be much appreciated :) thanks!!