Ajax

AJAX = ASYNCHRONOUS JAVASCRIPT AND XML

1.  AJAX IS THE ART OF EXCHANGING DATA WITH A SERVER, AND UPDATING PARTS OF A WEB PAGE - WITHOUT RELOADING THE WHOLE PAGE.
2. AJAX IS PARTIAL PAGE RENDERING.
3.   AJAX IS ABOUT UPDATING THE PARTS OF THE WEBPAGE,WITHOUT RELOADING THE WHOLE PAGE.

AJAX IS BASED ON INTERNET STANDARDS, AND USES A COMBINATION OF:

• XMLHTTPREQUEST OBJECT (TO EXCHANGE DATA ASYNCHRONOUSLY WITH A SERVER)
• JAVASCRIPT/DOM (TO DISPLAY/INTERACT WITH THE INFORMATION)
• CSS (TO STYLE THE DATA)
• XML (OFTEN USED AS THE FORMAT FOR TRANSFERRING DATA)
•      AJAX APPLICATIONS ARE BROWSER- AND PLATFORM-INDEPENDENT!

THE XMLHTTPREQUEST OBJECT

All modern browsers support the XMLHttpRequest object (IE5 and IE6 use an ActiveXObject).
The XMLHttpRequest object is used to exchange data with a server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.

Send a Request To a Server

To send a request to a server, we use the open() and send() methods of the XMLHttpRequest object:

xmlhttp.open("GET","ajax_info.txt",true);
xmlhttp.send();

Method	Description
open(method,url,async)	Specifies the type of request, the URL, and if the request should be handled asynchronously or not. method: the type of request: GET or POST url: the location of the file on the server async: true (asynchronous) or false (synchronous)
send(string)	Sends the request off to the server. string: Only used for POST requests

GET OR POST?

GET is simpler and faster than POST, and can be used in most cases.

However, always use POST requests when:

• A cached file is not an option (update a file or database on the server)
• Sending a large amount of data to the server (POST has no size limitations)
• Sending user input (which can contain unknown characters), POST is more robust and secure than GET

THE URL - A FILE ON A SERVER

The url parameter of the open() method, is an address to a file on a server:

xmlhttp.open("GET","ajax_test.asp",true);

The file can be any kind of file, like .txt and .xml, or server scripting files like .asp and .php (which can perform actions on the server before sending the response back).

Asynchronous - True or False?

AJAX stands for Asynchronous JavaScript and XML, and for the XMLHttpRequest object to behave as AJAX, the async parameter of the open() method has to be set to true:

xmlhttp.open("GET","ajax_test.asp",true);

Sending asynchronous requests is a huge improvement for web developers. Many of the tasks performed on the server are very time consuming. Before AJAX, this operation could cause the application to hang or stop.

With AJAX, the JavaScript does not have to wait for the server response, but can instead:

• execute other scripts while waiting for server response
• deal with the response when the response ready

 

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Async=true

When using async=true, specify a function to execute when the response is ready in the onreadystatechange event:

When a request to a server is sent, we want to perform some actions based on the response.

The onreadystatechange event is triggered every time the readyState changes.

The readyState property holds the status of the XMLHttpRequest.

Three important properties of the XMLHttpRequest object:

Property	Description
onreadystatechange	Stores a function (or the name of a function) to be called automatically each time the readyState property changes
readyState	Holds the status of the XMLHttpRequest. Changes from 0 to 4: 0: request not initialized 1: server connection established 2: request received 3: processing request 4: request finished and response is ready
status	200: "OK" 404: Page not found

In the onreadystatechange event, we specify what will happen when the server response is ready to be processed.

When readyState is 4 and status is 200, the response is ready:

Example

xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
    }
  }