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

// retrieves the XMLHttpRequest object
function createXmlHttpRequestObject() 
{  
  // will store the reference to the XMLHttpRequest object
  var xmlHttp;
  // if running Internet Explorer
  if(window.ActiveXObject)
  {
    try
    {
      xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    catch (e) 
    {
      xmlHttp = false;
    }
  }
  // if running Mozilla or other browsers
  else
  {
    try 
    {
      xmlHttp = new XMLHttpRequest();
    }
    catch (e) 
    {
      xmlHttp = false;
    }
  }
  // return the created object or display an error message
  if (!xmlHttp)


    alert("Error creating the XMLHttpRequest object.");
  else 
    return xmlHttp;
}

// make asynchronous HTTP request using the XMLHttpRequest object 
function onStateChange()
{
  document.getElementById("div_city").innerHTML = "<img src='images/loading.gif' border='0' alt='loading'>";
  // proceed only if the xmlHttp object isn't busy
  if (xmlHttp.readyState == 4 || xmlHttp.readyState == 0)
  {
    // retrieve the name typed by the user on the form
    state = encodeURIComponent(document.getElementById("state").value);
    // execute the quickstart.php page from the server
     xmlHttp.open("GET", "cities.php?state=" + state, true);  
    // define the method to handle server responses
    xmlHttp.onreadystatechange = LoadCities;
    // make the server request
    xmlHttp.send(null);
  }
  else
    // if the connection is busy, try again after one second  
    setTimeout('1000');
}

// executed automatically when a message is received from the server
function LoadCities()
{
  // move forward only if the transaction has completed
   if (xmlHttp.readyState == 4) 
  {
    // status of 200 indicates the transaction completed successfully
    if (xmlHttp.status == 200) 
    {
      // update the client display using the data received from the server
	  document.getElementById("div_city").innerHTML = xmlHttp.responseText ;
	  
	  document.getElementById("div_zip").innerHTML = '<select name="zip_code" id="zip_code"><option value="0">Select Zip Code</option></select>';
	  
      // restart sequence
      setTimeout('1000');
    } 
    // a HTTP status different than 200 signals an error
    else 
    {
      alert("here was a problem accessing the server: " + xmlHttp.statusText);
    }
  }
}

function onCityChange()
{
  document.getElementById("div_zip").innerHTML = "<img src='images/loading.gif' border='0' alt='loading'>";
  // proceed only if the xmlHttp object isn't busy
  if (xmlHttp.readyState == 4 || xmlHttp.readyState == 0)
  {
    // retrieve the name typed by the user on the form
    city = encodeURIComponent(document.getElementById("city").value);
    // execute the quickstart.php page from the server
     xmlHttp.open("GET", "zipcode.php?city=" + city, true);  
    // define the method to handle server responses
    xmlHttp.onreadystatechange = LoadZipCodes;
    // make the server request
    xmlHttp.send(null);
  }
  else
    // if the connection is busy, try again after one second  
    setTimeout('1000');
}

// executed automatically when a message is received from the server
function LoadZipCodes()
{
  // move forward only if the transaction has completed
   if (xmlHttp.readyState == 4) 
  {
    // status of 200 indicates the transaction completed successfully
    if (xmlHttp.status == 200) 
    {
      // update the client display using the data received from the server
	  document.getElementById("div_zip").innerHTML = xmlHttp.responseText ;
      // restart sequence
      setTimeout('1000');
    } 
    // a HTTP status different than 200 signals an error
    else 
    {
      alert("here was a problem accessing the server: " + xmlHttp.statusText);
    }
  }
}