AJAX
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 updating
parts of a web page - without reloading the whole page.
AJAX Introduction
AJAX is
about 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
If you
want to study these subjects first, find the tutorials on our Home page.
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.
How AJAX Works
AJAX
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)
lamp AJAX applications are browser- and
platform-independent!
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.
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
AJAX is
not a new programming language.
AJAX is
a technique for creating fast and dynamic web pages.
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:
<!DOCTYPE
html>
<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>
function
loadXMLDoc()
{
....
AJAX script goes here ...
}
</script>
</head>
AJAX - Create an XMLHttpRequest Object
The
keystone of AJAX is the XMLHttpRequest object.
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 - Create an XMLHttpRequest Object
The
keystone of AJAX is the XMLHttpRequest object.
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:
The
XMLHttpRequest object is used to exchange data with a server.
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:
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
GET Requests
A
simple GET request:
Example
xmlhttp.open("GET","demo_get.asp",true);
xmlhttp.send();
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();
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();
POST Requests
A
simple POST request:
Example
xmlhttp.open("POST","demo_post.asp",true);
xmlhttp.send();
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");
header:
specifies the header name
value:
specifies the header value
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
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();
You
will learn more about onreadystatechange in a later chapter.
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:
Example
xmlhttp.open("GET","ajax_info.txt",false);
xmlhttp.send();
document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
AJAX -
Server Response
Server Response
To get
the response from a server, use the responseText or responseXML property of the
XMLHttpRequest object.
responseText get the response data as a string
responseXML get the response data as XML data
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:
Example
document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
The responseXML Property
If the
response from the server is XML, and you want to parse it as an XML object, use
the responseXML property:
Example
Request
the file cd_catalog.xml and parse the response:
xmlDoc=xmlhttp.responseXML;
txt="";
x=xmlDoc.getElementsByTagName("ARTIST");
for (i=0;i<x.length;i++)
{
txt=txt +
x[i].childNodes[0].nodeValue + "<br>";
}
document.getElementById("myDiv").innerHTML=txt;
AJAX ASP/PHP Example
AJAX is
used to create more interactive applications.
AJAX ASP/PHP Example
The
following example will demonstrate how a web page can communicate with a web
server while a user type characters in an input field
Example
Start
typing a name in the input field below:
First
name:
Suggestions:
Example
Explained - The showHint() Function
When a
user types a character in the input field above, the function
"showHint()" is executed. The function is triggered by the
"onkeyup" event:
function showHint(str)
{
var xmlhttp;
if (str.length==0)
{
document.getElementById("txtHint").innerHTML="";
return;
}
if (window.XMLHttpRequest)
{// code for IE7+,
Firefox, Chrome, Opera, Safari
xmlhttp=new
XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new
ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4
&& xmlhttp.status==200)
{
document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
}
}
xmlhttp.open("GET","gethint.asp?q="+str,true);
xmlhttp.send();
}
Source
code explanation:
If the
input field is empty (str.length==0), the function clears the content of the
txtHint placeholder and exits the function.
If the
input field is not empty, the showHint() function executes the following:
Create
an XMLHttpRequest object
Create
the function to be executed when the server response is ready
Send
the request off to a file on the server
Notice
that a parameter (q) is added to the URL (with the content of the input field)
The
AJAX Server Page - ASP and PHP
The
page on the server called by the JavaScript above is an ASP file called
"gethint.asp".
Below
we have created two versions of the server file, one written in ASP and one in
PHP.
The ASP
File
The
source code in "gethint.asp" checks an array of names, and returns
the corresponding name(s) to the browser:
<%
response.expires=-1
dim a(30)
'Fill up array with names
a(1)="Anna"
a(2)="Brittany"
a(3)="Cinderella"
a(4)="Diana"
a(5)="Eva"
a(6)="Fiona"
a(7)="Gunda"
a(8)="Hege"
a(9)="Inga"
a(10)="Johanna"
a(11)="Kitty"
a(12)="Linda"
a(13)="Nina"
a(14)="Ophelia"
a(15)="Petunia"
a(16)="Amanda"
a(17)="Raquel"
a(18)="Cindy"
a(19)="Doris"
a(20)="Eve"
a(21)="Evita"
a(22)="Sunniva"
a(23)="Tove"
a(24)="Unni"
a(25)="Violet"
a(26)="Liza"
a(27)="Elizabeth"
a(28)="Ellen"
a(29)="Wenche"
a(30)="Vicky"
'get the q parameter from URL
q=ucase(request.querystring("q"))
'lookup all hints from array if length of q>0
if len(q)>0 then
hint=""
for i=1 to 30
if
q=ucase(mid(a(i),1,len(q))) then
if hint=""
then
hint=a(i)
else
hint=hint &
" , " & a(i)
end if
end if
next
end if
'Output "no suggestion" if no hint were found
'or output the correct values
if hint="" then
response.write("no
suggestion")
else
response.write(hint)
end if
%>
The PHP File
Here is
the code above rewritten in PHP:
<?php
// Fill up 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
$q=$_GET["q"];
//lookup all hints from array if length of q>0
if (strlen($q) > 0)
{
$hint="";
for($i=0; $i<count($a);
$i++)
{
if
(strtolower($q)==strtolower(substr($a[$i],0,strlen($q))))
{
if
($hint=="")
{
$hint=$a[$i];
}
else
{
$hint=$hint." ,
".$a[$i];
}
}
}
}
// Set output to "no suggestion" if no hint were found
// or to the correct values
if ($hint == "")
{
$response="no
suggestion";
}
else
{
$response=$hint;
}
//output the response
echo $response;
?>
AJAX Database Example
AJAX can be used for interactive communication with a database.
AJAX Database Example
The
following example will demonstrate how a web page can fetch information from a
database with AJAX:
Example
Customer
info will be listed here...
Example
Explained - The showCustomer() Function
When a
user selects a customer in the dropdown list above, a function called
"showCustomer()" is executed. The function is triggered by the
"onchange" event:
function showCustomer(str)
{
var xmlhttp;
if (str=="")
{
document.getElementById("txtHint").innerHTML="";
return;
}
if (window.XMLHttpRequest)
{// code for IE7+,
Firefox, Chrome, Opera, Safari
xmlhttp=new
XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new
ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4
&& xmlhttp.status==200)
{
document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
}
}
xmlhttp.open("GET","getcustomer.asp?q="+str,true);
xmlhttp.send();
}
The showCustomer() function does the following:
Check
if a customer is selected
Create
an XMLHttpRequest object
Create
the function to be executed when the server response is ready
Send the
request off to a file on the server
Notice
that a parameter (q) is added to the URL (with the content of the dropdown
list)
The
AJAX Server Page
The
page on the server called by the JavaScript above is an ASP file called
"getcustomer.asp".
The
server file could easily be rewritten in PHP, or some other server languages.
Look at a corresponding example in PHP.
The
source code in "getcustomer.asp" runs a query against a database, and
returns the result in an HTML table:
<%
response.expires=-1
sql="SELECT * FROM CUSTOMERS WHERE CUSTOMERID="
sql=sql & "'" &
request.querystring("q") & "'"
set conn=Server.CreateObject("ADODB.Connection")
conn.Provider="Microsoft.Jet.OLEDB.4.0"
conn.Open(Server.Mappath("/db/northwind.mdb"))
set rs=Server.CreateObject("ADODB.recordset")
rs.Open sql,conn
response.write("<table>")
do until rs.EOF
for each x in rs.Fields
response.write("<tr><td><b>" & x.name
& "</b></td>")
response.write("<td>" & x.value &
"</td></tr>")
next
rs.MoveNext
loop
response.write("</table>")
%>
AJAX XML Example
AJAX
can be used for interactive communication with an XML file.
AJAX XML Example
The
following example will demonstrate how a web page can fetch information from an
XML file with AJAX:
Example
<!DOCTYPE html>
<html>
<head>
<script>
function loadXMLDoc(url)
{
var xmlhttp;
var txt,x,xx,i;
if (window.XMLHttpRequest)
{// code for IE7+,
Firefox, Chrome, Opera, Safari
xmlhttp=new
XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new
ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4
&& xmlhttp.status==200)
{
txt="<table
border='1'><tr><th>Title</th><th>Artist</th></tr>";
x=xmlhttp.responseXML.documentElement.getElementsByTagName("CD");
for (i=0;i<x.length;i++)
{
txt=txt +
"<tr>";
xx=x[i].getElementsByTagName("TITLE");
{
try
{
txt=txt +
"<td>" + xx[0].firstChild.nodeValue + "</td>";
}
catch (er)
{
txt=txt +
"<td> </td>";
}
}
xx=x[i].getElementsByTagName("ARTIST");
{
try
{
txt=txt +
"<td>" + xx[0].firstChild.nodeValue + "</td>";
}
catch (er)
{
txt=txt +
"<td> </td>";
}
}
txt=txt +
"</tr>";
}
txt=txt +
"</table>";
document.getElementById('txtCDInfo').innerHTML=txt;
}
}
xmlhttp.open("GET",url,true);
xmlhttp.send();
}</script>
</head>
<body>
<div id="txtCDInfo">
<button onclick="loadXMLDoc('cd_catalog.xml')">Get
CD info</button>
</div>
</body>
</html>
Get CD info
Example
Explained - The stateChange() Function
When a
user clicks on the "Get CD info" button above, the loadXMLDoc()
function is executed.
The
loadXMLDoc() function creates an XMLHttpRequest object, adds the function to be
executed when the server response is ready, and sends the request off to the
server.When the server response is ready, an HTML table is built, nodes
(elements) are extracted from the XML file, and it finally updates the
txtCDInfo placeholder with the HTML table filled with XML data:
function loadXMLDoc(url)
{var xmlhttp;
var txt,xx,x,i;
if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new
ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4
&& xmlhttp.status==200)
{
txt="<table
border='1'><tr><th>Title</th><th>Artist</th></tr>";
x=xmlhttp.responseXML.documentElement.getElementsByTagName("CD");
for
(i=0;i<x.length;i++)
{
txt=txt +
"<tr>";
xx=x[i].getElementsByTagName("TITLE");
{
try
{
txt=txt +
"<td>" + xx[0].firstChild.nodeValue + "</td>";
}
catch (er)
{
txt=txt +
"<td> </td>";
}
}
xx=x[i].getElementsByTagName("ARTIST");
{
try
{
txt=txt +
"<td>" + xx[0].firstChild.nodeValue + "</td>";
}
catch (er)
{
txt=txt +
"<td> </td>";
}
}
txt=txt +
"</tr>";
}
txt=txt +
"</table>";
document.getElementById('txtCDInfo').innerHTML=txt;
}
}
xmlhttp.open("GET",url,true);
xmlhttp.send();
}
The AJAX Server Page
The
page on the server, used in the example above, is an XML file called
"cd_catalog.xml".
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