override method in javascript

Question

My problem here that i don't understand why this override do not work here is the source

window.onload=function()
{
   function Person(first,last)
   {
      this.First=first;
      this.Last = last;
      this.Full=function()
      {
         return this.First+" "+this.Last;
      };
   }

   Person.prototype.GetFullName=function()
   {
      return this.First + " " + this.Last;
   } ;



   function Employee(first,last,position)
   {
      Person.call(this,first,last);
      this.Position=position;
   }
   /* Employee.prototype = new Person();
   var t = new Employee("Mohamed","Zaki","Web Developer");

   alert(t.GetFullName());
    */
   Employee.prototype.GetFullName=function()
   {
      var x = Person.prototype.GetFullName.call(this);
      return x + " , " + this.Position ; 
   }
   var e = new Employee("Mohamed","Zaki","Web Developer");
   alert(e.GetFullName());
   }

When I run this code (after adding a final }), it alerts Mohamed Zaki , Web Developer. What were you expecting? — Ken Browning
– Ken Browning, Commented Apr 13, 2012 at 4:02
i know the code is working but i ask about the code in comment,, why it doesn't work ? — Tarek Saied
– Tarek Saied, Commented Apr 13, 2012 at 4:10
because even though you set this.Position=position, nothing ever reads it. were you expecting it to be in GetFullName? maybe you intended to include more code in the comment? — Ken Browning
– Ken Browning, Commented Apr 13, 2012 at 4:16
i didn't ask about Position ...i just ask why the code in the comment doesn't work ? — Tarek Saied
– Tarek Saied, Commented Apr 13, 2012 at 4:25
if we uncomment this code it won't work (the message doesn't appear) — Tarek Saied
– Tarek Saied, Commented Apr 13, 2012 at 4:33

Bill · Accepted Answer · 2012-04-13 04:40:19Z

1

If I understand your question, the code that you've commented out doesn't work because it is executed before GetFullName is overridden.

/* 
   **** this code is executed before GetFullName is overridden and will use
   **** original function 
   Employee.prototype = new Person();
   var t = new Employee("Mohamed","Zaki","Web Developer");

   alert(t.GetFullName());
    */

   Employee.prototype.GetFullName=function()
   {
      var x = Person.prototype.GetFullName.call(this);
      return x + " , " + this.Position ; 
   }

   /**** This code is executed after GetFullName is overridden uses the new version */
   var e = new Employee("Mohamed","Zaki","Web Developer");
   alert(e.GetFullName());
   }

answered Apr 13, 2012 at 4:40

Bill

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Comments

RobG · Accepted Answer · 2012-04-13 04:43:34Z

First, remove the window.onload wrapper because it's not doing anything useful. Explanation below:

  function Person(first,last) { 
    this.First = first; 
    this.Last = last; 
    this.Full = function() { 
      return this.First + " " + this.Last; 
    }; 
  } 

  Person.prototype.GetFullName = function() {  
    return this.First + " " + this.Last; 
  } ; 

  function Employee(first,last,position) { 
    Person.call(this,first,last); 
    this.Position = position; 
  } 
  Employee.prototype = new Person(); 

  var t = new Employee("Mohamed","Zaki","Web Developer"); 

  // Here getFullName is called before the new method is added
  // to Person.prototype so you only get first and last name
  alert(t.GetFullName()); 

  // Here is where the new method is added
  Employee.prototype.GetFullName=function() { 
    var x = Person.prototype.GetFullName.call(this); 
    return x + " , " + this.Position ; 
  }

  var e = new Employee("Mohamed","Zaki","Web Developer"); 

  // so here you get first, last and position
  alert(e.GetFullName()); 

  // And existing instances get the new method too
  alert(t.GetFullName());

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