13-03-2012, 11:54 AM
AJAX Tutorial
AJAX = Asynchronous JavaScript and XML.
AJAX is not a new programming language, but a new way to use existing standards.
AJAX is the art of exchanging data with a server, and update parts of a web page - without reloading the whole page.
AJAX is about updating parts of a web page, without reloading the whole page.
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AJAX Introduction
What You Should Already Know
Before you continue you should have a basic understanding of the following:
• HTML / XHTML
• CSS
• JavaScript / DOM
If you want to study these subjects first, find the tutorials on our Home page.
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What is AJAX?
AJAX = Asynchronous JavaScript and XML.
AJAX is a technique for creating fast and dynamic web pages.
AJAX allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.
Classic web pages, (which do not use AJAX) must reload the entire page if the content should change.
Examples of applications using AJAX: Google Maps, Gmail, Youtube, and Facebook tabs.
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AJAX is Based on Internet Standards
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!
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Google Suggest
AJAX was made popular in 2005 by Google, with Google Suggest.
Google Suggest is using AJAX 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.
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Start Using AJAX Today
AJAX is based on existing standards. These standards have been used by developers for several years. Read our next chapters to see how it works!
AJAX Example
To understand how AJAX works, we will create a small AJAX application:
Example
Let AJAX change this text
Change Content
Try it yourself »
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AJAX Example Explained
The AJAX application above contains one div section and one button.
The div section will be used to display information returned from a server. The button calls a function named loadXMLDoc(), if it is clicked:
<html>
<body>
<div id="myDiv"><h2>Let AJAX change this text</h2></div>
<button type="button" onclick="loadXMLDoc()">Change Content</button>
</body>
</html>
Next, add a <script> tag to the page's head section. The script section contains the loadXMLDoc() function:
<head>
<script type="text/javascript">
function loadXMLDoc()
{
.... AJAX script goes here ...
}
</script>
</head>
AJAX - Create an XMLHttpRequest Object
The keystone of AJAX is the XMLHttpRequest object.
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The XMLHttpRequest Object
All modern browsers support the XMLHttpRequest object (IE5 and IE6 uses 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.
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Create an XMLHttpRequest Object
All modern browsers (IE7+, Firefox, Chrome, Safari, and Opera) have a built-in XMLHttpRequest object.
Syntax for creating an XMLHttpRequest object:
xmlhttp=new XMLHttpRequest();
Old versions of Internet Explorer (IE5 and IE6) uses an ActiveX Object:
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
To handle all modern browsers, including IE5 and IE6, check if the browser supports the XMLHttpRequest object. If it does, create an XMLHttpRequest object, if not, create an ActiveXObject:
Example
if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
Try it yourself »
AJAX - Send a Request To a Server
The XMLHttpRequest object is used to exchange data with a server.
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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
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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
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GET Requests
A simple GET request:
Example
xmlhttp.open("GET","demo_get.asp",true);
xmlhttp.send();
Try it yourself »
In the example above, you may get a cached result.
To avoid this, add a unique ID to the URL:
Example
xmlhttp.open("GET","demo_get.asp?t=" + Math.random(),true);
xmlhttp.send();
Try it yourself »
If you want to send information with the GET method, add the information to the URL:
Example
xmlhttp.open("GET","demo_get2.asp?fname=Henry&lname=Ford",true);
xmlhttp.send();
Try it yourself »
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POST Requests
A simple POST request:
Example
xmlhttp.open("POST","demo_post.asp",true);
xmlhttp.send();
Try it yourself »
To POST data like an HTML form, add an HTTP header with setRequestHeader(). Specify the data you want to send in the send() method:
Example
xmlhttp.open("POST","ajax_test.asp",true);
xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
xmlhttp.send("fname=Henry&lname=Ford");
Try it yourself »
Method Description
setRequestHeader(header,value) Adds HTTP headers to the request.
header: specifies the header name
value: specifies the header value
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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).
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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 asynchronously 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:
Example
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
}
}
xmlhttp.open("GET","ajax_info.txt",true);
xmlhttp.send();
Try it yourself »
You will learn more about onreadystatechange in a later chapter.
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Async=false
To use async=false, change the third parameter in the open() method to false:
xmlhttp.open("GET","ajax_info.txt",false);
Using async=false is not recommended, but for a few small requests this can be ok.
Remember that the JavaScript will NOT continue to execute, until the server response is ready. If the server is busy or slow, the application will hang or stop.
Note: When you use async=false, do NOT write an onreadystatechange function - just put the code after the send() statement:
