Ajax Tutorial for Beginners

8/12/2019 Ajax Tutorial for Beginners

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IntroductionHello everyone. This is my first article on The Code Project. I hope you will get some benefit from it. I

have been learning Ajax for the last 5 months and now I want to share my knowledge with you.

Please provide feedback on any mistakes which I have made in this article. Believe me guys, your

harsh words would be received with love and treated with the top most priority.

I have explained Ajax with XML and JSONin part 2, which you can readhere

There are many books and articles out there explaining the 5 Ws (Who, What, Where, When, Why) of

Ajax, but I will explain to you in my own simple way. So what is Ajax? The term AJAX (Asynchronous

JavaScript and XML) has been around for three years created by Jesse James Garrett in 2005. The

technologies that make Ajax work, however, have been around for almost a decade. Ajax makes it

possible to update a page without a refresh. Using Ajax, we can refresh a particular DOM objectwithout refreshing the full page.

Background of AjaxIn Jesse Garretts original article that coined the term, it was AJAX. The X in AJAX really stands

for XMLHttpRequestthough, and not XML. Jesse later conceded that Ajax should be a word and not

an acronym and updated his article to reflect his change in heart. So Ajax is the correct casing. As

its name implies, Ajax relies primarily on two technologies to work: JavaScript and

the XMLHttpRequest. Standardization of the browser DOM (Document Object Model) and DHTML

also play an important part in Ajaxs success, but for the purposes of our discussion we won't

examine these technologies in depth.

How Ajax WorksAt the heart of Ajax is the ability to communicate with a Web server asynchronously without takingaway the users ability to interact with the page. TheXMLHttpRequest is what makes this possible.

Ajax makes it possible to update a page without a refresh. By Ajax, we can refresh a particular DOM

object without refreshing the full page. Let's see now what actually happens when a user submits a

request:
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1. Web browser requests for the content of just the part of the page that it needs.

2. Web server analyzes the received request and builds up an XML message which is then sent back

tothe Web browser.

3. After the Web browser receives the XML message, it parses the message in order to update the

content of that part of the page.

AJAX uses JavaScript language through HTTP protocol to send/receive XML messages

asynchronously to/from Web server. Asynchronously means thatdatais not sent in a sequence.Common steps: The figure below describes the structure of HTML pages and a sequence of actions

in Ajax Webapplication:
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1. The JavaScript function handEvent() will be invoked when an event occurred on the HTML

element.

2. In the handEvent() method, an instance of XMLHttpRequestobject is created.

3. The XMLHttpRequest object organizes an XML message within about the status of the HTML page,

and then sends it to the Web server.4. After sending the request, the XMLHttpRequest object listens to the message from the Web server.

5. The XMLHttpRequest object parses the message returned from the Web server and updates it into

the DOM object.

Let's See How Ajax Works in the CodeLetsstartto put these ideas together in some code examples.

form id="form1" runat="server"

div

input type="text" id="lblTime"

br

input type="button" id="btnGetTime" value="Get Time" onclick="GetTime();"

div

form

1. From the above code, our DOM object is (input type="text" id="lblTime") which we have to

refresh without refreshing the whole page. This will be done when we press the event of a button, i.e.

onclick.

2. In this code, our handEvent() is GetTime() from the above Figure-2 if you take a look at it.

Let's see now how we create an XmlHttpRequest object and make Ajax work for us.

The basic implementation of the XMLHttpRequest in JavaScript looks like this:

varxmlHttpRequest;

functionGetTime()

{

//create XMLHttpRequest object

xmlHttpRequest = (window.XMLHttpRequest) ?

newXMLHttpRequest() : newActiveXObject("Msxml2.XMLHTTP");

//If the browser doesn't support Ajax, exit now

if(xmlHttpRequest == null)

return;

//Initiate the XMLHttpRequest object

xmlHttpRequest.open("GET", "Time.aspx", true);

//Setup the callback function

xmlHttpRequest.onreadystatechange = StateChange;

//Send the Ajax request to the server with the GET data

xmlHttpRequest.send(null);

}functionStateChange()

{

if(xmlHttpRequest.readyState == 4){

document.getElementById( 'lblTime').value = xmlHttpRequest.responseText;
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}

}

1. Now from the above Figure-2 handEvent() i.e. GetTime() creates an XMLHttpRequest object

inside it like this:

//create XMLHttpRequest object

xmlHttpRequest = (window.XMLHttpRequest) ?new XMLHttpRequest() : new ActiveXObject("Msxml2.XMLHTTP");

2. If the browser does not support Ajax, it will return false.

3. Next we open our connection to the server with our newly created XMLHttpRequest object. Here

theTime.aspxpage is the page from which we will get XML message that is stored on the Web server.

//Initiate the XMLHttpRequest object

xmlHttpRequest.open("GET", "Time.aspx", true);

4. You often hear the term callback replace the term postback when you work with Ajax. Thats

because Ajax uses a callback function to catch the servers response when it is done processing

your Ajax request. We establish a reference to that callback function like this. Here StateChangeis a

function where we update or set a new value to our DOM object, i.e "lblTime".//Setup the callback function

xmlHttpRequest.onreadystatechange = StateChange;

Let's have a look at our callback function:

functionStateChange(){

if(xmlHttpRequest.readyState == 4){

document.getElementById( 'lblTime').value = xmlHttpRequest.responseText;

}

}

onreadystatechange will fire multiple times during an Ajax request, so we must evaluate

theXMLHttpRequests readyState property to determine when the server response is completewhich is 4. Now if readyState is 4, we can update the DOM object with the response message

we get from the Web server.

5. As the request method we are sending is "GET" (remember it is case sensitive), there is no need to

send any extra information to the server.

//Send the Ajax request to the server with the GET data

xmlHttpRequest.send(null);

In Time.aspx.cson Page_Load event, write a simple response like this which is our response

message:

Response.Write( DateTime.Now.Hour + ":"+ DateTime.Now.Minute +

":"+ DateTime.Now.Second );

There are three key points in creating an Ajax application,

1. Use XMLHttpRequest object to send XML message to the Web server

2. Create a service that runs on Web server to respond to request and

3. Parse XMLHttpRequest object, then update to DOM object of the HTML page on client-side

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Ajax Tutorial for Beginners