var xmlHttp = createXmlHttpRequestObject();

// creates an XMLHttpRequest instance
function createXmlHttpRequestObject() {
  // will store the reference to the XMLHttpRequest object
  var xmlHttp;
  // this should work for all browsers except IE6 and older
  try {
    // try to create XMLHttpRequest object
    xmlHttp = new XMLHttpRequest();
  }
  catch(e) {
    // assume IE6 or older
    var XmlHttpVersions = new Array("MSXML2.XMLHTTP.6.0",
                                    "MSXML2.XMLHTTP.5.0",
                                    "MSXML2.XMLHTTP.4.0",
                                    "MSXML2.XMLHTTP.3.0",
                                    "MSXML2.XMLHTTP",
                                    "Microsoft.XMLHTTP");
    // try every id until one works
    for (var i=0; i<XmlHttpVersions.length && !xmlHttp; i++) {
      try { 
        // try to create XMLHttpRequest object
        xmlHttp = new ActiveXObject(XmlHttpVersions[i]);
      } 
      catch (e) {} // ignore potential error
    }
  }
  // return the created object or display an error message
  if (!xmlHttp) {
    displayError("Error creating the XMLHttpRequest object.");
  } else  {
    return xmlHttp;
  }
}

// function that displays an error message
function displayError($message) {
  // ignore errors if showErrors is false
  if (showErrors) {
    // turn error displaying Off
    showErrors = false;
    // display error message
 
    alert("Error encountered: \n" + $message);
  }
}

// function that handles the HTTP response
function handleRequestStateChange() {
  // when readyState is 4, we read the server response
  if (xmlHttp.readyState == 4) {
    // continue only if HTTP status is "OK"
    if (xmlHttp.status == 200) {
      try {
        // read the response from the server
        readResponse();
      }
      catch(e) {
        // display error message
        displayError(e.toString());
      }
    } else {
      // display error message
      displayError(xmlHttp.statusText);
    }
  }
}

// read server's response 
function readResponse() {
  // retrieve the server's response 
  var response = xmlHttp.responseText;
  // server error?
//  if (response.indexOf("ERRNO") >= 0 || response.indexOf("error:") >= 0 || response.length == 0) {
//  	throw(response.length == 0 ? "Server error." : response);
//  }
  // get response in XML format (assume the response is valid XML)
//  responseXml = xmlHttp.responseXML;
  processResponse(response);
}