JavaScript 值无法保存, 请教高人指点!

代码太长.又写了短的代码来说明一下.

JScript code:
xu = function(element){
this.eid = document.getElementById(element);
this.eid.onfocus = this.show;
}
xu.prototype.eid = null;
xu.prototype.show = function(){
alert(this.eid.id);
}


HTML code:
<input type="text" name="User_Name" id="User_Name" value="" size="32" />
<input type="password" name="User_Password" id="User_Password" value="" size="32" />
alert(xu.prototype.eid);
var a1 = new xu("User_Name");
var a2 = new xu("User_Password");
a1.show();
a2.show();
alert(xu.prototype.eid);

运行时都是正常的.手工执行"a1.show();"时对话框中能显示各自的ID值. 但是在绑定的事件里却显示this.eid 为 "undefined".

网页错误详细信息

消息: 'this.eid.id' 为空或不是对象
行: 11
字符: 2
代码: 0
URI: http://localhost/fapm/xuScript/Test.js

---

xu = function(element){
    this.eid = document.getElementById(element);
    this.eid.onfocus = this.show;
}
//你将xu.prototype