函数定义的有关问题

函数定义的问题
function   show(){this.a   =   1;};  
var   show2   =   new   show();
show   =   function   (){this.a   =   2;};  
show.prototype.a   =   3;
var   show3   =   new   show();

alert(show2.a);//1
alert(show3.a);//2

delete(show2.a);
delete(show3.a);

alert(show2.a);//u
alert(show3.a);//3

为什么alert(show2.a);这里是udf

[解决办法]
因为被delete(show2.a); 删了a 了