Ajax小應用函數

function Ajax(data){   createXMLHttpRequest();  if(data==document.getElementById("username"))  {    xmlhttp.open("GET","user_ck.php?username="+data.value,true);           document.getElementById('username_notice').innerHTML = process_request;//顯示狀態  }  else if(data==document.getElementById("veruserid"))  {    xmlhttp.open("GET","user_ck.php?veruserid="+data.value,true);           document.getElementById("veruserid_notice").innerHTML = process_request;//顯示狀態  }   else if (data==document.getElementById("verify"))  {xmlhttp.open("POST","veruser_check.php",true);xmlhttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');    document.getElementById("verify_notice").innerHTML = process_request;//顯示狀態   }//"checkcode="+checkcode.value+"&username="+    xmlhttp.onreadystatechange=function(){  //alert("zhong");if (4==xmlhttp.readyState){   if (200==xmlhttp.status){   var responseText = xmlhttp.responseText;responseText= responseText.replace(/(^\s*)|(\s*$)/g,"");if (responseText=="true" ){      ck_user(data,"true");}else{     ck_user(data,"false");}}else{alert("發生錯誤!");}}} if(data==document.getElementById("veruserid")||data==document.getElementById("username"))        {             xmlhttp.setRequestHeader("If-Modified-Since","0");   xmlhttp.send(null); } else if (data==document.getElementById("verify")) { xmlhttp.send("veruserid="+document.getElementById("veruserid").value+"&veruserpwd="+document.getElementById("verpassword").value);}}