第2段代码为什么会出错？

Question

                function Odemo(){}
                var a=new Odemo();                
                Odemo.prototype.sayName=function(name)
                {
                 return this.name=name;
                }

                alert(a.sayName('asd')); //显示asd                function Odemo(){}
                var a=new Odemo();
                Odemo.prototype={
                 name:'asd',
                  sayName:function(){
                   return this.name;
                  }
                }
                alert(a.sayName()); //为什么会报错                function Odemo(){}
                Odemo.prototype={
                 name:'asd',
                  sayName:function(){
                   return this.name;
                  }
                }
                
                var a=new Odemo();
                alert(a.sayName()); //显示asd只不过是代码所编写的位置不同而已，
为什么第2段代码会出错呢？第一段代码不也是在var a=new Odemo();的下面编写的么？

Answer 1

这个貌似跟js中的对象的prototype有关系  坐等高人解答

Answer 2

Odemo.prototype={
                    name:'asd',
                  sayName:function(){
                      return this.name;
                  }
                }
这种方式会切断以前生成的对象之间的联系，只与后面创建的对象有影响，因为
{
                    name:'asd',
                  sayName:function(){
                      return this.name;
                  }
                }是一个新的对象，Odemo.prototype的引用已经改变，而之前创建的对象中保持的还是之前的Odemo.prototype的引用