AJAX Introduction

AJAX stands for Asynchronous JavaScript And XML.

What You Should Already Know

Before you continue you should have a basic understanding of the following:

• HTML / XHTML
• JavaScript

If you want to study these subjects first, find the tutorials on our Home page.

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.

  AJAX is a browser technology independent of web server software.

AJAX is Based on Web Standards

AJAX is based on the following web standards:

• JavaScript
• XML
• HTML
• CSS

The web standards used in AJAX are well defined, and supported by all major browsers. AJAX applications are browser and platform independent.

AJAX is About Better Internet Applications

Web applications have many benefits over desktop applications; they can reach a larger audience, they are easier to install and support, and easier to develop.

However, Internet applications are not always as "rich" and user-friendly as traditional desktop applications.

With AJAX, Internet applications can be made richer and more user-friendly.

You Can Start Using AJAX Today

There is nothing new to learn.

AJAX is based on existing standards. These standards have been used by most developers for several years.

AJAX Http Requests

AJAX Uses HTTP Requests

In traditional JavaScript coding, if you want to get any information from a database or a file on the server, or send user information to a server, you will have to make an HTML form and GET or POST data to the server. The user will have to click the "Submit" button to send/get the information, wait for the server to respond, then a new page will load with the results.

Because the server returns a new page each time the user submits input, traditional web applications can run slowly and tend to be less user-friendly.

With AJAX, your JavaScript communicates directly with the server, through the JavaScript XMLHttpRequest object

With an HTTP request, a web page can make a request to, and get a response from a web server - without reloading the page. The user will stay on the same page, and he or she will not notice that scripts request pages, or send data to a server in the background.

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).

Google Suggest 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.

AJAX Example

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!):

					Name: 

					Time: 

					

The next chapters will explain the keystones of AJAX.

AJAX Browser Support

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 try and catch statement 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;

					      }

					    }

					  }

					  }

					

					Name: 

					Time: 

					

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.

AJAX - The XMLHttpRequest Object

AJAX - More About the XMLHttpRequest Object

Before sending data to the server, we have to explain three important properties of the XMLHttpRequest object.

The onreadystatechange Property

After a request to the server, we need a function that can receive the data that is returned by the server.

The onreadystatechange property stores the function that will process the response from a server. The following code defines an empty function and sets the onreadystatechange property at the same time:

xmlHttp.onreadystatechange=function()

					{

					// We are going to write some code here

					}

The readyState Property

The readyState property holds the status of the server's response. Each time the readyState changes, the onreadystatechange function will be executed.

Here are the possible values for the readyState property:

We are going to add an If statement to the onreadystatechange function to test if our response is complete (this means that we can get our data):

xmlHttp.onreadystatechange=function()

					{

					if(xmlHttp.readyState==4)

					  {

					  // Get the data from the server's response

					  }

					}

The responseText Property

The data sent back from the server can be retrieved with the responseText property.

In our code, we will set the value of our "time" input field equal to responseText:

xmlHttp.onreadystatechange=function()

					{

					if(xmlHttp.readyState==4)

					  {

					  document.myForm.time.value=xmlHttp.responseText;

					  }

					}

The next chapter shows how to ask the server for some data!

AJAX - Request a Server

AJAX - Sending a Request to the Server

To send off a request to the server, we use the open() method and the send() method.

The open() method takes three arguments. The first argument defines which method to use when sending the request (GET or POST). The second argument specifies the URL of the server-side script. The third argument specifies that the request should be handled asynchronously. The send() method sends the request off to the server. If we assume that the HTML and ASP file are in the same directory, the code would be:

xmlHttp.open("GET","time.asp",true);

					xmlHttp.send(null);

Now we must decide when the AJAX function should be executed. We will let the function run "behind the scenes" when the user types something in the username text field:

					Name: 
					onkeyup="ajaxFunction();" name="username" />

					Time: 

					

Our updated AJAX-ready "testAjax.htm" file now looks like this:

					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;

					      }

					    }

					  }

					  xmlHttp.onreadystatechange=function()

					    {

					    if(xmlHttp.readyState==4)

					      {

					      document.myForm.time.value=xmlHttp.responseText;

					      }

					    }

					  xmlHttp.open("GET","time.asp",true);

					  xmlHttp.send(null);

					  }

					

					Name: 
					onkeyup="ajaxFunction();" name="username" />

					Time: 

					

The next chapter makes our AJAX application complete with the "time.asp" script.

AJAX - The Server-Side Script

AJAX - The Server-Side ASP Script

Now we are going to create the script that displays the current server time.

The responseText property (explained in the previous chapter) will store the data returned from the server. Here we want to send back the current time. The code in "time.asp" looks like this:

<%

					response.expires=-1

					response.write(time)

					%>

Note: The Expires property sets how long (in minutes) a page will be cached on a browser before it expires. If a user returns to the same page before it expires, the cached version is displayed. Response.Expires=-1 indicates that the page will never be cached.

Run Your AJAX Application

Try the AJAX application by typing some text into the Name text box below, then click inside the Time text box:

The Time text box gets the server's time from "time.asp" file without reloading the page!