ajax introduction

1. AJAX
• Asynchronous JavaScript And XML
• AJAX is based on JavaScript and HTTP requests.

2. HISTORY
• 1995 – Java Applets
• 1996 – MS IE5 IFrame
• 1999/2000 – MS IE5 XMLHTTP ActiveX Control
• By 2004 – Other browsers - XMLHttpRequest object
• 2005 – Google made it popular through google suggest,
gmail and other apps
• 2006 – W3C released the first draft specification

3. Technologies
• HTML and CSS for marking up and styling information.
• The DOM accessed with JavaScript.
• The XMLHttpRequest (XHR) object is usually used for
exchanging data asynchronously between browser and
server.
• Common data exchange formats include XML, pre-
formatted HTML, plain text, and JSON.

4. Advantages
• Page reload avoided to display same common content
every time.
• The use of asynchronous requests allows the client's
Web browser UI to be more interactive and to respond
quickly to inputs.
• Reduce connections to the server.
• State can be maintained throughout a web site.

5. Disadvantages
• Browser's "back" button issue.
• Difficult for a user to bookmark a particular state of the
application.
• Web crawlers do not execute JavaScript code, making it
difficult to index public pages.
• Browser that do not support Ajax or JavaScript, or simply
have JavaScript disabled.
• The Same origin policy prevents some Ajax techniques
from being used across domains.
• Testing for malicious attacks and debugging difficult.

6. XMLHttpRequest (XHR)
• The open method
– (type, url, async, user, password)
• The setRequestHeader method
– Content-Type, User-Agent, etc
• The send method
– Parameter content to be sent
• The onreadystatechange event listener
– readyState (1 for open, 2 for send, 3 for loading, 4 for complete)
• The HTTP response
– responseXML, responseText

7. Browser Object Model

8. Ajax Libraries
• JQuery
• ExtJs
• Dojo
• Prototype
• GWT