AJax验证问题,请高手帮帮忙~~

这有问题么？为啥不能调用checkAjax页面里的方法呢？ 
<script type="text/javascript">
        
        function checkUsername(){
           var xmlHttp = null;
           var username = document.getElementById("userName");
           var msgname = document.getElementById("msgName");
           if(username.value == ""){
                msgname.style.color = "#cc0000";
                msgname.innerText = "用户名为必添项!";
                username.focus();
           }else if(username.value.match(/^[a-zA-Z0-9]+$/) && username.value.length < 5 || username.value.length >16){
                msgname.style.color = "#cc0000";
                msgname.innerText = "不符合规范!";
                username.focus();
                username.value = "";
           }else{
                msgname.innerText = "";
                try{
                    xmlHttp = new ActiveXObject("Msxml12.XMLHTTP");
                }catch(e){
                   try{
                        xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
                   }catch(e){
                        try{
                            xmlHttp = new XMLHttpRequest();
                        }catch(e){}
                   }
               }
           }
           xmlHttp.open("get","/checkAjax.aspx?username="+username.value,true);
           xmlHttp.onreadystatechange = function(){
                msgname.innerText = "正在检查用户名..."
                if(xmlHttp.readyState == 4){
                    if(xmlHttp.status == 200){
                        msgname.innerText = xmlHttp.responseText;
                    }
                }
                
                xmlHttp.send(null);
           }
        }