All the available browsers can not support AJAX. Here is the list of major browsers which support AJAX.

• Mozilla Firefox 1.0 and above

• Netscape version 7.1 and above

• Apple Safari 1.2 and above.

• Microsoft Internet Exporer 5 and above

• Konqueror

• Opera 7.6 and above

So now when you write your application then you would have to take care of the browsers who do not support AJAX.

NOTE: When we are saying that browser does not support AJAX it simply means that browser does not support creation of Javascript object XMLHttpRequest object.

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Writing Browser Specific Code

Simple way of making your source code compatible to a browser is to use try...catch blocks in your javascript.

    
     Name: 
      
     
Time: 
     
    

In the above Javascript code, we try three times to make our XMLHttpRequest object. Our first attempt:

• ajaxRequest = new XMLHttpRequest();

is for the Opera 8.0+, Firefox and Safari browsers. If that fails we try two more times to make the correct object for an Internet Explorer browser with:

• ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
• ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");>

If that doesn't work, then they are using a very outdated browser that doesn't support XMLHttpRequest, which also means it doesn't support Ajax.

Most likely though, our variable ajaxRequest will now be set to whatever XMLHttpRequest standard the browser uses and we can start sending data to the server.

Next section will give you step by step explaination of AJAX work flow.

This section will give you clear picture of the exact steps of AJAX operation.

Steps of AJAX Operation

1. A client event occurs

2. An XMLHttpRequest object is created

3. The XMLHttpRequest object is configured

4. The XMLHttpRequest object makes an asynchronous request to the Webserver.

5. Webserver returns the result containing XML document.

6. The XMLHttpRequest object calls the callback() function and processes the result.

7. The HTML DOM is updated

Lets take these steps one by one

1. A client event occurs

• A JavaScript function is called as the result of an event

• Example: validateUserId() JavaScript function is mapped as a event handler to a onkeyup event on input form field whose id is set to "userid"

• <input type="text" size="20" id="userid" name="id" οnkeyup="validateUserId();">

2. The XMLHttpRequest object is created

var ajaxRequest;  // The variable that makes Ajax possible!function ajaxFunction(){ try{   // Opera 8.0+, Firefox, Safari   ajaxRequest = new XMLHttpRequest(); }catch (e){   // Internet Explorer Browsers   try{      ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");   }catch (e) {      try{         ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");      }catch (e){         // Something went wrong         alert("Your browser broke!");         return false;      }   } }}

 

3. The XMLHttpRequest object is Configured

In this step we will write a function which will be triggered by the client event and a callback function processRequest() will be registered

function validateUserId() {   ajaxFunction();   // Here processRequest() is the callback function.   ajaxRequest.onreadystatechange = processRequest;   if (!target) target = document.getElementById("userid");   var url = "validate?id=" + escape(target.value);   ajaxRequest.open("GET", url, true);   ajaxRequest.send(null);}

 

4. Making Asynchornous Request to the Webserver

Source code is available in the above piece of code. Code written in blue color is responsible to make a request to the web server. This is all being done using XMLHttpRequest object ajaxRequest

function validateUserId() {   ajaxFunction();   // Here processRequest() is the callback function.   ajaxRequest.onreadystatechange = processRequest;   if (!target) target = document.getElementById("userid");   var url = "validate?id=" + escape(target.value);   ajaxRequest.open("GET", url, true);   ajaxRequest.send(null);}

Assume if you enter mohammad in userid box then in the above request URL is set to validate?id=mohammad

5. Webserver returns the result containing XML document

You can implement your server side script in any language. But logic should be as follows

• Get a request from the client

• Parse the input from the client

• Do required processing.

• Send the output to the client.

If we assume that you are going to write a servlet then here is the piece of code

public void doGet(HttpServletRequest request,                     HttpServletResponse response)                        throws IOException, ServletException {   String targetId = request.getParameter("id");   if ((targetId != null) &&        !accounts.containsKey(targetId.trim()))    {      response.setContentType("text/xml");      response.setHeader("Cache-Control", "no-cache");      response.getWriter().write("true");   }    else    {      response.setContentType("text/xml");      response.setHeader("Cache-Control", "no-cache");      response.getWriter().write("false");   }}

 

6. Callback function processRequest() is called

The XMLHttpRequest object was configured to call the processRequest() function when there is a state change to the readyState of the XMLHttpRequest object. Now this function will recieve the result from the server and will do required processing. As in the following example it sets a variable message on true or false based on retruned value from the Webserver.

function processRequest() {   if (req.readyState == 4) {      if (req.status == 200) {         var message = ...;...}

 

7. The HTML DOM is updated

This is the final step and in this step your HTML page will be updated. It happens in the following way

• • <LI

JavaScript technology gets a reference to any element in a page using DOM API

• The recommended way to gain a reference to an element is to call.

document.getElementById("userIdMessage"), // where "userIdMessage" is the ID attribute // of an element appearing in the HTML document

• JavaScript technology may now be used to modify the element's attributes; modify the element's style properties; or add, remove, or modify child elements. Here is the example

thats it...if you understood above mentioned seven steps then you are almost done with AJAX. In next chapter we will see XMLHttpRequest object in more detail.

The XMLHttpRequest object is the key to AJAX. It has been available ever since Internet Explorer 5.5 was released in July 2000, but not fully discovered before people started to talk about AJAX and Web 2.0 in 2005.

XMLHttpRequest (XHR) is an API that can be used by JavaScript, JScript, VBScript and other web browser scripting languages to transfer and manipulate XML data to and from a web server using HTTP, establishing an independent connection channel between a web page's Client-Side and Server-Side.

The data returned from XMLHttpRequest calls will often be provided by back-end databases. Besides XML, XMLHttpRequest can be used to fetch data in other formats, e.g. JSON or even plain text.

You already have seen couple of examples on how to create a XMLHttpRequest object.

Below is listed some of the methods and properties you have to become familiar with.

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XMLHttpRequest Methods

• abort()

  Cancels the current request.

• getAllResponseHeaders()

  Returns the complete set of HTTP headers as a string.

• getResponseHeader( headerName )

  Returns the value of the specified HTTP header.

• open( method, URL )

  open( method, URL, async )
  open( method, URL, async, userName )
  open( method, URL, async, userName, password )
  Specifies the method, URL, and other optional attributes of a request.
  The method parameter can have a value of "GET", "POST", or "HEAD". Other HTTP methods, such as "PUT" and "DELETE" (primarily used in REST applications), may be possible
  The "async" parameter specifies whether the request should be handled asynchronously or not . "true" means that script processing carries on after the send() method, without waiting for a response, and "false" means that the script waits for a response before continuing script processing.

• send( content )

  Sends the request.

• setRequestHeader( label, value )

  Adds a label/value pair to the HTTP header to be sent.

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XMLHttpRequest Properties

• • onreadystatechange

    An event handler for an event that fires at every state change.

  • readyState

    The readyState property defines the current state of the XMLHttpRequest object.

    Here are the possible values for the readyState propery:

    State	Description
    0	The request is not initialized
    1	The request has been set up
    2	The request has been sent
    3	The request is in process
    4	The request is completed

    readyState=0 after you have created the XMLHttpRequest object, but before you have called the open() method.

    readyState=1 after you have called the open() method, but before you have called send().

    readyState=2 after you have called send().

    readyState=3 after the browser has established a communication with the server, but before the server has completed the response.

    readyState=4 after the request has been completed, and the response data have been completely received from the server.

     

  • responseText

    Returns the response as a string.

  • responseXML

    Returns the response as XML. This property returns an XML document object, which can be examined and parsed using W3C DOM node tree methods and properties.

  • status

    Returns the status as a number (e.g. 404 for "Not Found" and 200 for "OK").

  • statusText

    Returns the status as a string (e.g. "Not Found" or "OK").

To clearly illustrate how easy it is to access information from a database using Ajax, we are going to build MySQL queries on the fly and display the results on "ajax.html". But before we proceed, lets do ground work. Create a table using the following command.

NOTE: We are asuing you have sufficient privilege to perform following MySQL operations

CREATE TABLE `ajax_example` (  `name` varchar(50) NOT NULL,  `age` int(11) NOT NULL,  `sex` varchar(1) NOT NULL,  `wpm` int(11) NOT NULL,  PRIMARY KEY  (`name`))

Now dump the following data into this table using the following SQL statements

INSERT INTO `ajax_example` VALUES ('Jerry', 120, 'm', 20);INSERT INTO `ajax_example` VALUES ('Regis', 75, 'm', 44);INSERT INTO `ajax_example` VALUES ('Frank', 45, 'm', 87);INSERT INTO `ajax_example` VALUES ('Jill', 22, 'f', 72);INSERT INTO `ajax_example` VALUES ('Tracy', 27, 'f', 0);INSERT INTO `ajax_example` VALUES ('Julie', 35, 'f', 90);

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Client Side HTML file

Now lets have our client side HTML file which is ajax.html and it will have following code

        
         Max Age: 
          
         
Max WPM: 
         
         
Sex: 
         mf
         
        
        
         Your result will display here
        

NOTE: The way of passing variables in the Query is according to HTTP standard and the have formA

URL?variable1=value1;&variable2=value2;

Now the above code will give you a screen as given below

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NOTE: This is dummy screen and would not work

Max Age: 

Max WPM:

Sex: mf

Your result will display here

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Server Side PHP file

So now your client side script is ready. Now we have to write our server side script which will fetch age, wpm and sex from the database and will send it back to the client. Put the following code into "ajax-example.php" file

        ";$display_string .= "";$display_string .= "Name";$display_string .= "Age";$display_string .= "Sex";$display_string .= "WPM";$display_string .= "";// Insert a new row in the table for each person returnedwhile($row = mysql_fetch_array($qry_result)){	$display_string .= "";	$display_string .= "$row[name]";	$display_string .= "$row[age]";	$display_string .= "$row[sex]";	$display_string .= "$row[wpm]";	$display_string .= "";	}echo "Query: " . $query . "
";$display_string .= "";echo $display_string;?>

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Now try by entering a valid value (For example 120) in Max Age or any other box and then click Query MySQL button.

Max Age: 

Max WPM:

Sex: mf

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If you have successfully completed this lesson then you know how to use MySQL, PHP, HTML, and Javascript in tandem to write Ajax applications.