ajax做无刷新登录功能，弄了两天了，请细心的朋友一定要过来看看，50分相送。该如何解决

ajax做无刷新登录功能，弄了两天了，请细心的朋友一定要过来看看，50分相送。
1:jsp页面。
<   form   action= "glxx.jsp "   method= "post "   name= "mainForm "   id= "mainForm ">
<input   type= "image "   name= "imageField "   src= "../img/lz34.gif "    
onclick= "return   SelectLoginUser() "/>
<   /   form>

2：js
<%@   page   language= "java "   contentType= "text/html;   charset=utf-8 "
        pageEncoding= "utf-8 "%>
<script>
function   queryStringName()
{
var   name   =   document.getElementById( "name ").value;
return   name;
}
function   queryStringMobile()
{
var   Mobile   =   document.getElementById( "Mobile ").value;
return   Mobile;
}
function   SelectLoginUser()
{
if   (queryStringName()!= " ")   {
if   (queryStringMobile()!= " ")   {
        alert(SelectUser());//提示是undefined类型，所以用户名密码输错了，也提交了。
        return   SelectUser();       //去调用
 
}else
{
document.getElementById( "returninf ").value= "密码不能为空 ";
        return   false;
}
}else
{
document.getElementById( "returninf ").value= "用户名不能为空 ";
        return   false;
}
}
</script>

<script>
function   SelectUser()
{
var   xmlHttp;
var   aa=false;
if(window.XMLHttpRequest)
{
xmlHttp=new   XMLHttpRequest()
}
else   if(window.ActiveXObject)
{
xmlHttp=new   ActiveXObject( "Microsoft.XMLHTTP ")
}
xmlHttp.onreadystatechange=function()
{
if((xmlHttp.readystate==4)&&(xmlHttp.status==200))
{
var   xmlDoc=xmlHttp.responseXML;
                                document.getElementById( "returninf ").value=xmlDoc.getElementsByTagName( "info ")[0].text;

if   (xmlDoc.getElementsByTagName( "info ")[0].text= "OK ")   ／／如果数据中用户名密码输入正确，就返回OK
{
    document.getElementById( "returninf ").value=xmlDoc.getElementsByTagName( "info ")[0].text;
aa=true;
return   aa;
}
else
{
document.getElementById( "returninf ").value= "aabb ";
                          　　return   aa;
}
  }else
{
    //return   aa;
//alert( "3 "+aa);
}
}
//alert( "1 "+aa);
xmlHttp.open( "POST ", "SelectLoginUser?name= "+escape(queryStringName()).replace(/\+/g,   "%2B ").replace(/%/g,   "%25 ")+ "&mobile= "+escape(queryStringMobile()).replace(/\+/g,   "%2B ").replace(/%/g,   "%25 "),true);
xmlHttp.setRequestHeader( "CONTENT-TYPE ", "application/x-www-form-urlencoded;text/xml ");    
        xmlHttp.send();
//alert( "2 "+aa);
}
</script>