Ajax

AJAX
Let’s Get Started… By: Dumindu Pahalawatta

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 updating parts of a
web page - without reloading the whole page.

What you should already know
Before you continue you should have a basic understanding of the following:
• HTML / XHTML
• CSS
• JavaScript / DOM

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.

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!
AJAX was made popular in 2005 by Google, with Google Suggest.

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.

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:
variable=new XMLHttpRequest();
Old versions of Internet Explorer (IE5 and IE6) uses an ActiveX Object:
variable=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
var xmlhttp;
if (window.XMLHttpRequest)
{
// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
}
else
{
// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}

AJAX - Send a Request To a Server
To send a request to a server, we use the open() and send() methods of the
XMLHttpRequest object:

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

POST Requests
A simple POST request:
Example:
To POST data like an HTML form, add an HTTP header with
setRequestHeader(). Specify the data you want to send in the send() method:
xmlhttp.open("POST","demo_post.asp",true);
xmlhttp.send();
xmlhttp.open("POST","ajax_test.asp",true);
xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
xmlhttp.send("fname=Henry&lname=Ford");

GET Requests
A simple GET request:
Example:
If you want to send information with the GET method, add the information
to the URL:
xmlhttp.open("GET","demo_get.asp",true);
xmlhttp.send();
xmlhttp.open("GET","demo_get.asp?t=" + Math.random(),true);
xmlhttp.send();
xmlhttp.open("GET","demo_get2.asp?fname=Henry&lname=Ford",true);
xmlhttp.send();

The url - A File On a Server
The url parameter of the open() method, is an address to a file on a server:
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).
xmlhttp.open("GET","ajax_test.asp",true);

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

Async=false
To use async=false, change the third parameter in the open() method to
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:
xmlhttp.open("GET","ajax_info.txt",false);
xmlhttp.open("GET","ajax_info.txt",false);
xmlhttp.send();
document.getElementById("myDiv").innerHTML=xmlhttp.responseText;

Async=true
When using async=true, specify a function to execute when the response is
ready in the onreadystatechange event:
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
}
}
xmlhttp.open("GET","ajax_info.txt",true);
xmlhttp.send();

Server Response
To get the response from a server, use the responseText or responseXML
property of the XMLHttpRequest object.

The responseText Property
If the response from the server is not XML, use the responseText property.
The responseText property returns the response as a string, and you can use
it accordingly:

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

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:
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
}
}

<html>
<head>
<script>
function showHint(str) {
if (str.length == 0) {
document.getElementById("txtHint").innerHTML = "";
return;
} else {
var xmlhttp = new XMLHttpRequest();
xmlhttp.onreadystatechange = function() {
if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
document.getElementById("txtHint").innerHTML =
xmlhttp.responseText;
}
}
xmlhttp.open("GET", "gethint.php?q=" + str, true);
xmlhttp.send();
}
}
</script>
</head>
<body>
<p><b>Start typing a name in the input field below:</b></p>
<form>
First name: <input type="text" onkeyup="showHint(this.value)">
</form>
<p>Suggestions: <span id="txtHint"></span></p>
</body>
</html>

<?php
// Array with names
$a[] = "Anna";
$a[] = "Brittany";
$a[] = "Cinderella";
$a[] = "Diana";
$a[] = "Eva";
$a[] = "Fiona";
$a[] = "Gunda";
$a[] = "Hege";
$a[] = "Inga";
$a[] = "Johanna";
$a[] = "Kitty";
$a[] = "Linda";
$a[] = "Nina";
$a[] = "Ophelia";
$a[] = "Petunia";
$a[] = "Amanda";
$a[] = "Raquel";
$a[] = "Cindy";
$a[] = "Doris";
$a[] = "Eve";
$a[] = "Evita";
$a[] = "Sunniva";
$a[] = "Tove";
$a[] = "Unni";
$a[] = "Violet";
$a[] = "Liza";
$a[] = "Elizabeth";
$a[] = "Ellen";
$a[] = "Wenche";
$a[] = "Vicky";

// get the q parameter from URL
$q = $_REQUEST["q"];
$hint = "";
// lookup all hints from array if $q is different from ""
if ($q !== "") {
$q = strtolower($q);
$len=strlen($q);
foreach($a as $name) {
if (stristr($q, substr($name, 0, $len))) {
if ($hint === "") {
$hint = $name;
} else {
$hint .= ", $name";
}
}
}
}
// Output "no suggestion" if no hint was found or output correct values
echo $hint === "" ? "no suggestion" : $hint;
?>

Ajax

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