Extend Javascript object

Why the following coding does not work?

FireFox report me that :
Manager.superclass.qk is undefined.

In fact, is it any easier way to extend javascript?
thank you

<html>
	<head>
	</head>
	<body>
		
 
		<script language=javascript>
			
			function extend(subclass, superclass)
			{
			  function Dummy(){}
			  Dummy.prototype = superclass.prototype;
			  subclass.prototype = new Dummy();
			  
			  subclass.superclass = superclass;
			  
			  for(var property in superclass)
			   {
        			subclass[property] = superclass[property];
    			}
			}
			
			function Person(first, last)
			{
			    this.first = first;
			    this.last = last;
			    this.toString = function()
			    {
    				return this.first + ' ' + this.last;
				};
				this.qk=function(id)
				{
					return alert(id);
				}
			}
			function Manager(first, last, id, department) 
			{
			    Manager.superclass.call(first, last);
			    Manager.superclass.qk.call(id);
			    this.department = department;
			    
			    this.printDepartment=function()
			    {
				    this.qk(this.department);
			    }
			}
			extend(Manager,Person);
        	var m=new Manager("rona","baggio","id","department");
        	m.printDepartment();
		</script>	
 
	</body>
</html>

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The problem is more delicate than you think :). Its just because you want to call function of non existing object. If you give a function like you do:

this.qk = function () ...

Then to fire this function you need to use "new" statement on whole object.

Extending function itself is working fine.

This is first version that will work:

function Manager(first, last, id, department)
{
var x= new Manager.superclass(first, last);
x.qk(id);
this.department = department;

this.printDepartment=function()
{
x.qk(this.department);
}
}

Other version that moves out .qk parameter with .prototype should look like this.

Both of mine solutions work...

                        function extend(subclass, superclass)
                        {
                          function Dummy(){}
                          Dummy.prototype = superclass.prototype;
                          subclass.prototype = new Dummy();
                          
                          subclass.superclass = superclass;
                          
                          for(var property in superclass)
                           {
                                subclass[property] = superclass[property];
                        }
                        }
                        
                        function Person(first, last)
                        {
                            this.first = first;
                            this.last = last;
                            this.toString = function()
                            {
                                return this.first + ' ' + this.last;
                                };
                        }
                         Person.prototype.qk=function(id)
                                {
                                        return alert(id);
                                }
                        
                        function Manager(first, last, id, department) 
                        {
                            Manager.superclass.call(first, last);
                            this.qk(id);
                            this.department = department;
                            
                            this.printDepartment=function()
                            {
                                    this.qk(this.department);
                            }
                        }
                        extend(Manager,Person);
                var m=new Manager("rona","baggio","id","department");
                m.printDepartment();

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<html> <head> </head> <body> <script language=javascript> Manager.prototype=new Person(); function Person(first, last) { this.first = first; this.last = last; this.toString = function() { return this.first + ' ' + this.last; }; this.qk=function(id) { return alert(id); } } function Manager(first, last, id, department) { this.department = department; this.qk(id); this.printDepartment=function() { this.qk(this.department); } } var m=new Manager("rona","baggio","id","department"); m.printDepartment(); </script> </body> </html>

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Extend Javascript object

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