JS prototype

1.<script type="text/javascript">  
2.function employee(name,jobtitle,born)  
3.{  
4.this.name=name;  
5.this.jobtitle=jobtitle ;  
6.this.born=born;  
7.}  
8.var fred=new employee("Fred Flintstone","Caveman",1970);  
9.employee.prototype.salary=null;  
10.fred.salary=20000;  
11.document.write(fred.salary);  
12.</script>