AJAX = Asynchronous JavaScript and XML

AJAX = Asynchronous JavaScript and XML
AJAX is not a new programming language, but a technique for creating better, faster, and more interactive web applications.
With AJAX, your JavaScript can communicate directly with the server, using the JavaScript XMLHttpRequest object. With this object, your JavaScript can trade data with a web server, without reloading the page.
AJAX uses asynchronous data transfer (HTTP requests) between the browser and the web server, allowing web pages to request small bits of information from the server instead of whole pages.
The AJAX technique makes Internet applications smaller, faster and more user-friendly.
The XMLHttpRequest Object
By using the XMLHttpRequest object, a web developer can update a page with data from the server after the page has loaded!
AJAX was made popular in 2005 by Google (with Google Suggest).
a href="http://www.google.com/webhp?complete=1&hl=en" target="_blank">Google Suggest/a> is using the XMLHttpRequest object to create a very dynamic web interface: When you start typing in Google's search box, a JavaScript sends the letters off to a server and the server returns a list of suggestions.
The XMLHttpRequest object is supported in Internet Explorer 5.0+, Safari 1.2, Mozilla 1.0 / Firefox, Opera 8+, and Netscape 7.

Your First AJAX Application
To understand how AJAX works, we will create a small AJAX application.
First, we are going to create a standard HTML form with two text fields: username and time. The username field will be filled in by the user and the time field will be filled in using AJAX.
The HTML file will be named "testAjax.htm", and it looks like this (notice that the HTML form below has no submit button!):
html>body>form name="myForm">Name: input type="text" name="username">Time: input type="text" name="time">/form>/body>/html>
The next chapters will explain the keystones of AJAX.

AJAX - Browser Support
The keystone of AJAX is the XMLHttpRequest object.
Different browsers use different methods to create the XMLHttpRequest object.
Internet Explorer uses an ActiveXObject, while other browsers uses the built-in JavaScript object called XMLHttpRequest.
To create this object, and deal with different browsers, we are going to use a "try and catch" statement. You can read more about the a href="http://www.w3schools.com/js/js_try_catch.asp">try and catch statement/a> in our JavaScript tutorial.
Let's update our "testAjax.htm" file with the JavaScript that creates the XMLHttpRequest object:
function ajaxFunction(){
var xmlHttp;try {
// Firefox, Opera 8.0+, Safari
xmlHttp=new XMLHttpRequest();
}catch (e) {
// Internet Explorer
try {
xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
try {
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
} catch (e) {
alert("Your browser does not support AJAX!");
return false;
}
}
}
}
/script>form name="myForm">Name: input type="text" name="username">Time: input type="text" name="time">/form>/body>/html>
Example explained: First create a variable xmlHttp to hold the XMLHttpRequest object.
Then try to create the object with XMLHttp=new XMLHttpRequest(). This is for the Firefox, Opera, and Safari browsers. If that fails, try xmlHttp=new ActiveXObject("Msxml2.XMLHTTP") which is for Internet Explorer 6.0+, if that also fails, try xmlHttp=new ActiveXObject("Microsoft.XMLHTTP") which is for Internet Explorer 5.5+
If none of the three methods work, the user has a very outdated browser, and he or she will get an alert stating that the browser doesn't support AJAX.
Note: The browser-specific code above is long and quite complex. However, this is the code you can use every time you need to create an XMLHttpRequest object, so you can just copy and paste it whenever you need it. The code above is compatible with all the popular browsers: Internet Explorer, Opera, Firefox, and Safari.
The next chapter shows how to use the XMLHttpRequest object to communicate with the server.