this in javascript

 <script>
 function Foo(i, j){
 var i=i;
 var j=j;
 var a=3;
 alert(i);
 alert(this.i);
 }   
 var foo = new Foo(1,2);   
  </script>
 

 

The result is "1" and "undefined",so we can conclude that the  the "var i"  is not a property belong to Object  foo. It is a variable can be used in this function(notice:treat it as a function) ,but is not a property the class(I conside it as a java class).

 

 

<script>
 function Foo(i, j){
 var i=i;
 var j=j;
 var a=3;
 alert(i);
 alert(this.i);
 }   
 var foo = new Foo(1,2);   
  </script>

 

 In this way ,we declare a property named "i" of class Foo. and we can use  the property of Object foo:

<script>
alert(foo.i);
</script>