// stores the reference to the XMLHttpRequest object
var xmlHttp = createXmlHttpRequestObject();

// retrieves the XMLHttpRequest object
function createXmlHttpRequestObject()
{
  // try to grab an xmlHttp
  try
  {
    return window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject("Microsoft.XMLHttp");
  }
  catch (e) 
  {
    // display an error message
    alert("Sorry, your browser is not AJAX-enabled.");
  }
}