对于为什么要使用prototype来实现继承，我就不说了，网上很多
下面主要是我对于prototype的一些见解：
// The constructor function initializes those properties that
// will be different for each instance.
function Rectangle(w, h) {
this.width = w;
this.height = h;
}
// The prototype object holds methods and other properties that
// should be shared by each instance.
Rectangle.prototype.area = function( ) { return this.width * this.height; }
var r = new Rectangle(2, 3);
r.hasOwnProperty("width"); // true: width is a direct property of r
r.hasOwnProperty("area"); // false: area is an inherited property of r
"area" in r; // true: "area" is a property of r
>>> function pp(){}
>>> pp.prototype.p="33"
"33"
>>> var t=new pp()
>>> tt.p
tt is not defined
[Break on this error] undefined
javascript: with ... (line 1)
>>> t.p
"33"
>>> t.p="44"
"44"
>>> t.p
"44"
>>> var tt=new pp()
>>> tt.p
"33"
>>> pp.prototype.constructor
pp()
>>> tt.constructor
pp()
>>> tt.prototype.constructor
tt.prototype has no properties
[Break on this error] undefined
javascript: with ... (line 1)
>>> tt.prototype.oo="22"
tt.prototype has no properties
[Break on this error] undefined
javascript: with ... (line 1)
>>> "p" in tt
true
>>> tt.hasOwnProperty
hasOwnProperty()
>>> tt.hasOwnProperty("p")
false
>>> tt.tt=0
0
>>> tt.hasOwnProperty("tt")
true
//从我的firebug调试中你能看出来吗？嘿嘿