Dropdown Box Using AJAX

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What is AJAX?

AJAX, an acronym for Asynchronous JavaScript and XML, is a web development technique for creating interactive web applications. The intent is to make web pages feel more responsive by exchanging small amounts of data with the server behind the scenes, so that the entire web page does not have to be reloaded each time the user makes a change. This is meant to increase the web page's interactivity, speed, and usability.

Using the XmlHttp Object

An XmlHttp object can be created like this:

<script language="javascript">
  //Global XMLHTTP Request object

var XmlHttp;

//Creating and setting the instance of appropriate XMLHTTP Request object to 

//a "XmlHttp" variable  

function CreateXmlHttp()
{
 //Creating object of XMLHTTP in IE

 try
 {
  XmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
 }
 catch(e)
 {
  try
  {
   XmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
  } 
  catch(oc)
  {
   XmlHttp = null;
  }
 }
 //Creating object of XMLHTTP in Mozilla and Safari 

 if(!XmlHttp && typeof XMLHttpRequest != "undefined") 
 {
  XmlHttp = new XMLHttpRequest();
 }
}
</script>

Send the request to the server side page

We can send a request to the server page to retrieve the data from the server through the XmlHttp object.

var requestUrl = "WebForm2.aspx" + "?SelectedCountry=" + (selectedCountry);
 CreateXmlHttp();
 
 // If browser supports XMLHTTPRequest object

 if(XmlHttp)
 {
  //Setting the event handler for the response

  XmlHttp.onreadystatechange = HandleResponse;
  
  //Initializes the request object with GET (METHOD of posting), 

  //Request URL and sets the request as asynchronous.

  XmlHttp.open("GET", requestUrl,  true);
  
  //Sends the request to server

  XmlHttp.send(null);  
 }

The above code explains how to send a request to webpage2.aspx with the request object collection. This collection contains the selected country itself.

Process the client request

When the request is recieved on the server side, the server page executes the code and gives the response through the XmlHttp object:

Dim xList As XmlNodeList
Dim xNode As XmlNode
Dim Str As String
xmlDoc.Load(Server.MapPath("CountriesAndStates.xml"))

Dim Country As String = Request.QueryString("SelectedCountry")
Dim query As String = ("/countries/country[@name='" & country & "']")
xlist = xmlDoc.SelectNodes(query)

Response.Clear()
 
For Each xNode In xList
     If xNode.HasChildNodes = True Then
          For Each xN As XmlNode In xNode.ChildNodes
               Str &= xN.InnerText & "-"
          Next
     End If
Next

Response.Clear()
Response.ContentType = "text/xml"
Response.Write(str)
Response.End()

Using the XmlHttp object to retrieve the data

Recieving the response from the server page can be done like this:

The valid list of XMLHttpRequest ready states is listed in the following table:

The list of status codes for the HTTP status is given below:

1xx Informational

Request received, continuing process.

• 100: Continue
• 101: Switching Protocols

2xx Success

The action was successfully received, understood, and accepted.

• 200: OK
• 201: Created
• 202: Accepted
• 203: Non-Authoritative Information
• 204: No Content
• 205: Reset Content
• 206: Partial Content
• 207: Multi-Status

3xx Redirection

The client must take additional action to complete the request.

• 300: Multiple Choices
• 301: Moved Permanently. This and all future requests should be directed to another URI.
• 302: Found this the most popular redirect code, but also an example of industrial practice contradicting the standard. HTTP/1.0 specification (RFC 1945) required the client to perform a temporary redirect (the original describing phrase was "Moved Temporarily"), but popular browsers implemented it as a 303 See Other. Therefore, HTTP/1.1 added status codes 303 and 307 to disambiguate between the two behaviors. However, majority of web applications and frameworks still use the 302 status code as if it were the 303.
• 303: See Other (since HTTP/1.1). The response to the request can be found under another URI using a GET method.
• 304: Not Modified
• 305: Use Proxy (since HTTP/1.1). Many HTTP clients (such as Mozilla and Internet Explorer) don't correctly handle responses with this status code.
• 306 is no longer used, but reserved. Was used for 'Switch Proxy'.
• 307: Temporary Redirect (since HTTP/1.1). In this occasion, the request should be repeated with another URI, but future requests can still be directed to the original URI. In contrast to 303, the original POST request must be repeated with another POST request.

4xx Client Error

The request contains bad syntax or cannot be fulfilled.

• 400: Bad Request
• 401: Unauthorized. Similar to 403/Forbidden, but specifically for use when authentication is possible but has failed or not yet been provided. See basic authentication scheme and digest access authentication.
• 402: Payment Required. The original intention was that this code might be used as part of some form of digital cash/micropayment scheme, but that has not happened, and this code has never been used.
• 403: Forbidden
• 404: Not Found
• 405: Method Not Allowed
• 406: Not Acceptable
• 407: Proxy Authentication Required
• 408: Request Timeout
• 409: Conflict
• 410: Gone
• 411: Length Required
• 412: Precondition Failed
• 413: Request Entity Too Large
• 414: Request-URI Too Long
• 415: Unsupported Media Type
• 416: Requested Range Not Satisfiable
• 417: Expectation Failed
• 449: Retry With A Microsoft extension: The request should be retried after doing the appropriate action.

5xx Server Error

The server failed to fulfill an apparently valid request.

• 500: Internal Server Error
• 501: Not Implemented
• 502: Bad Gateway
• 503: Service Unavailable
• 504: Gateway Timeout
• 505: HTTP Version Not Supported
• 509: Bandwidth Limit Exceeded. This status code, while used by many servers, is not an official HTTP status code.

if(XmlHttp.readyState == 4)
 {
  // To make sure valid response is received from the server, 

  // 200 means response received is OK

  if(XmlHttp.status == 200)
  {  
   ClearAndSetStateListItems(XmlHttp.responseText);
  }
  else
  {
   alert("There was a problem retrieving data from the server." );
  }
 }

There are two types of XmlHttp responses for rectrieving the response.

1. responseText
2. responseXML

Show the states in the dropdownbox

The responseText is split using the split function and the response is assigned to an array.

Then, the array values are added in the dropdown list.

var stateList = document.getElementById("stateList");
 //Clears the state combo box contents.

 for (var count = stateList.options.length-1; count >-1; count--)
 {
  stateList.options[count] = null;
 }

 var stateNodes = countryNode.split("-");
 //window.alert(stateNodes) 

 var textValue; 
 var optionItem;
 //Add new states list to the state combo box.

 for (var count = 0; count < stateNodes.length; count++)
 {
     textValue = (stateNodes[count]);
  optionItem = new Option( textValue, textValue,  false, false);
  //window.alert(textValue); 

  //stateList.appendChild(textValue); 

  stateList.options[stateList.length] = optionItem;
 }

This is my first article in The Code Project.