Ajax (Asynchronous Javascript + XML)

Brief history

Ajax is only a name given to a set of tools that were previously existing.
The main part is XMLHttpRequest, a server-side object usable in JavaScript, that was implemented into Internet Explorer since the 4.0 version. In Internet Explorer it is an ActiveX object that was first named XMLHTTP some times, before to be generalized on all browser under the name XMLHttpRequest, when the Ajax technology becomes commonly used. The use of XMLHttpRequest in 2005 by Google, in Gmail and GoogleMaps has contributed to the success of this format. But this is the when the name Ajax was itself coined that the technology started to be so popular.

Why use Ajax?

Mainly to build a fast, dynamic website, but also to save resources.
For improving sharing of resources, it is better to use the power of all the client computers rather than just a unique server and network. Ajax allows to perform processing on client computer (in JavaScript) with data taken from the server.
The processing of web page formerly was only server-side, using web services or PHP scripts, before the whole page was sent within the network.
But Ajax can selectively modify a part of a page displayed by the browser, and update it without the need to reload the whole document with all images, menus, etc...
For example, fields of forms, choices of user, may be processed and the result displayed immediately into the same page.

What is Ajax in depth?

Ajax is a set of technologies, supported by a web browser, including these elements:

• HTML and CSS for presenting.
• JavaScript (ECMAScript) for local processing, and DOM (Document Object Model) to access data inside the page or to access elements of XML file read on the server (with the getElementByTagName method for example)...
• The XMLHttpRequest object is used to read or send data on the server asynchronously.

Optionally...

• DOMParser may be used
• PHP or another scripting language may be used on the server.
• XML and XSLT to process the data if returned in XML form.
• SOAP may be used to dialog with the server.

The "asynchronous" word, means that the response of the server while be processed when available, without to wait and to freeze the display of the page.

How does it works?

Ajax uses a programming model with display and events. These events are user actions, they call functions associated to elements of the web page.
Interactivity is achieved with forms and buttons. DOM allows to link elements of the page with actions and also to extract data from XML files provided by the server.
To get data on the server, XMLHttpRequest provides two methods:
- open: create a connection.
- send: send a request to the server.
Data furnished by the server will be found in the attributes of the XMLHttpRequest object:
- responseXml for an XML file or
- responseText for a plain text.

Take note that a new XMLHttpRequest object has to be created for each new file to load.

We have to wait for the data to be available to process it, and in this purpose, the state of availability of data is given by the readyState attribute of XMLHttpRequest.

States of readyState follow (only the last one is really useful):

0: not initialized.
1: connection established.
2: request received.
3: answer in process.
4: finished.

The XMLHttpRequest object

Allows to interact with the servers, thanks to its methods and attributes.

Attributes

readyState	the code successively changes value from 0 to 4 that means for "ready".
status	200 is OK 404 if the page is not found.
responseText	holds loaded data as a string of characters.
responseXml	holds an XML loaded file, DOM's method allows to extract data.
onreadystatechange	property that takes a function as value that is invoked when the readystatechange event is dispatched.

Methods

open(mode, url, boolean)	mode: type of request, GET or POST url: the location of the file, with a path. boolean: true (asynchronous) / false (synchronous). optionally, a login and a password may be added to arguments.
send("string")	null for a GET command.

Building a request, step by step

First step: create an instance

This is just a classical instance of class, but two options must be tried, for browser compatibility.

if (window.XMLHttpRequest) // Object of the current windows { xhr = new XMLHttpRequest(); // Firefox, Safari, ... } else if (window.ActiveXObject) // ActiveX version { xhr = new ActiveXObject("Microsoft.XMLHTTP"); // Internet Explorer }

Or exceptions may be used instead:

try { xhr = new ActiveXObject("Microsoft.XMLHTTP"); // Trying Internet Explorer } catch(e) // Failed { xhr = new XMLHttpRequest(); // Other browsers. }

Second step: wait for the response

The response and further processing are included in a function and the return of the function will be assigned to the onreadystatechange attribute of the object previously created.

xhr.onreadystatechange = function() { // instructions to process the response }; if (xhr.readyState == 4) { // Received, OK } else { // Wait... }

Third step: make the request itself

Two methods of XMLHttpRequest are used:
- open: command GET or POST, URL of the document, true for asynchronous.
- send: with POST only, the data to send to the server.

The request below read a document on the server.

xhr.open('GET', 'http://www.xul.fr/somefile.xml', true); xhr.send(null);