ajax javascript对象问题 向老鸟求救

function ajax()
    {
        var xmlhttp=createXMLHttp();
        this.url_str='';//php后台文件
        this.div_id='';//加载的图层id
        this.str='';//传送的数据
        var that=this;
        this.cc=function()
            {
                xmlhttp.onreadystatechange=function ()
                    {
                        if(xmlhttp.readyState==4)
                            {
                                document.getElementById(that.div_id).innerHTML=xmlhttp.responseText;
                                document.getElementById("newDrag").style.display="block";
                                bu_re_change();//注意这边 bu_re_change（）放在这里有效 标号1
                            }       
                    }
                xmlhttp.open('POST',this.url_str,true);
                xmlhttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded"); 
                xmlhttp.send(this.str);
            }
    }function character_ajax(user_id,type)
{
css_change('character');
var aa=new ajax();
aa.url_str='character.php';
aa.div_id="newBody";
aa.str="user_id="+user_id
aa.cc();
}
ajax.prototype.ccc=function()
{
bu_re_change(); //bu_re_change（）放在这里没有效果呢 标号2
}
bu_re_change();实现的是按钮变换和隐藏效果 
为什么标号1能实现 而标号2不能呢 如果我一定要放在标号2位置 标号1去掉 那要怎么做 求救了