setTimeOut问题

function chat()
{
    chase();
}
chase（）为：
function chase()
{
  window.setTimeout("chase()",200);
  alert("h");
  createXMLHttpRequest();
  xmlhttp.onreadystatechange=callback;
  var url="data.asp?id=1"
  xmlhttp.open("GET",url,true);
  xmlhttp.setRequestHeader("Content-Type","application/x-www-form-encoded");
  xmlhttp.send(null);

}
function callback()
{
  if(xmlhttp.readyState==4)
  {
     if(xmlhttp.status==200)
     {
        var text=unescape(xmlhttp.responseText);
        alert(text);
        var da=$("table");
        var td=da.append("<tr><td>"+ text +"</td></tr>");
      }
   }
}