Ajax, which consists of HTML, JavaScript™ technology, DHTML, and DOM, is an outstanding approach that helps you transform clunky Web interfaces into interactive Ajax applications. The author, an Ajax expert, demonstrates how these technologies work together -- from an overview to a detailed look -- to make extremely efficient Web development an easy reality. He also unveils the central concepts of Ajax, including the XMLHttpRequest object.

Five years ago, if you didn't know XML, you were the ugly duckling whom nobody talked to. Eighteen months ago, Ruby came into the limelight and programmers who didn't know what was going on with Ruby weren't welcome at the water cooler. Today, if you want to get into the latest technology rage, Ajax is where it's at.

However, Ajax is far more than just a fad; it's a powerful approach to building Web sites and it's not nearly as hard to learn as an entire new language.

Before I dig into what Ajax is, though, let's spend just a few moments understanding what Ajax does. When you write an application today, you have two basic choices:

• Desktop applications
• Web applications

These are both familiar; desktop applications usually come on a CD (or sometimes are downloaded from a Web site) and install completely on your computer. They might use the Internet to download updates, but the code that runs these applications resides on your desktop. Web applications -- and there's no surprise here -- run on a Web server somewhere and you access the application with your Web browser.

More important than where the code for these applications runs, though, is how the applications behave and how you interact with them. Desktop applications are usually pretty fast (they're running on your computer; you're not waiting on an Internet connection), have great user interfaces (usually interacting with your operating system), and are incredibly dynamic. You can click, point, type, pull up menus and sub-menus, and cruise around, with almost no waiting around.

On the other hand, Web applications are usually up-to-the-second current and they provide services you could never get on your desktop (think about Amazon.com and eBay). However, with the power of the Web comes waiting -- waiting for a server to respond, waiting for a screen to refresh, waiting for a request to come back and generate a new page.

Obviously this is a bit of an oversimplification, but you get the basic idea. As you might already be suspecting, Ajax attempts to bridge the gap between the functionality and interactivity of a desktop application and the always-updated Web application. You can use dynamic user interfaces and fancier controls like you'd find on a desktop application, but it's available to you on a Web application.

So what are you waiting for? Start looking at Ajax and how to turn your clunky Web interfaces into responsive Ajax applications.

Old technology, new tricks

When it comes to Ajax, the reality is that it involves a lot of technologies -- to get beyond the basics, you need to drill down into several different technologies (which is why I'll spend the first several articles in this series breaking apart each one of them). The good news is that you might already know a decent bit about many of these technologies -- better yet, most of these individual technologies are easy to learn -- certainly not as difficult as an entire programming language like Java or Ruby.

Here are the basic technologies involved in Ajax applications:

• HTML is used to build Web forms and identify fields for use in the rest of your application. By the way, Ajax is shorthand for Asynchronous JavaScript and XML (and DHTML, and so on). The phrase was coined by Jesse James Garrett of Adaptive Path and is, according to Jesse, not meant to be an acronym.
• JavaScript code is the core code running Ajax applications and it helps facilitate communication with server applications.
• DHTML, or Dynamic HTML, helps you update your forms dynamically. You'll use div, span, and other dynamic HTML elements to mark up your HTML.
• DOM, the Document Object Model, will be used (through JavaScript code) to work with both the structure of your HTML and (in some cases) XML returned from the server.

Let's break these down and get a better idea of what each does. I'll delve into each of these more in future articles; for now focus on becoming familiar with these components and technologies. The more familiar you are with this code, the easier it will be to move from casual knowledge about these technologies to mastering each (and really blowing the doors off of your Web application development).

The XMLHttpRequest object

The first object you want to understand is probably the one that's newest to you; it's called XMLHttpRequest. This is a JavaScript object and is created as simply as shown in Listing 1.

<script language="javascript" type="text/javascript">
var xmlHttp = new XMLHttpRequest();
</script>

I'll talk more about this object in the next article, but for now realize that this is the object that handles all your server communication. Before you go forward, stop and think about that -- it's the JavaScript technology through the XMLHttpRequest object that talks to the server. That's not the normal application flow and it's where Ajax gets much of its magic.

In a normal Web application, users fill out form fields and click a Submit button. Then, the entire form is sent to the server, the server passes on processing to a script (usually PHP or Java or maybe a CGI process or something similar), and when the script is done, it sends back a completely new page. That page might be HTML with a new form with some data filled in or it might be a confirmation or perhaps a page with certain options selected based on data entered in the original form. Of course, while the script or program on the server is processing and returning a new form, users have to wait. Their screen will go blank and then be redrawn as data comes back from the server. This is where low interactivity comes into play -- users don't get instant feedback and they certainly don't feel like they're working on a desktop application.

Ajax essentially puts JavaScript technology and the XMLHttpRequest object between your Web form and the server. When users fill out forms, that data is sent to some JavaScript code and not directly to the server. Instead, the JavaScript code grabs the form data and sends a request to the server. While this is happening, the form on the users screen doesn't flash, blink, disappear, or stall. In other words, the JavaScript code sends the request behind the scenes; the user doesn't even realize that the request is being made. Even better, the request is sent asynchronously, which means that your JavaScript code (and the user) doesn't wait around on the server to respond. So users can continue entering data, scrolling around, and using the application.

Then, the server sends data back to your JavaScript code (still standing in for the Web form) which decides what to do with that data. It can update form fields on the fly, giving that immediate feeling to your application -- users are getting new data without their form being submitted or refreshed. The JavaScript code could even get the data, perform some calculations, and send another request, all without user intervention! This is the power of XMLHttpRequest. It can talk back and forth with a server all it wants, without the user ever knowing about what's really going on. The result is a dynamic, responsive, highly-interactive experience like a desktop application, but with all the power of the Internet behind it.

Adding in some JavaScript

Once you get a handle on XMLHttpRequest, the rest of your JavaScript code turns out to be pretty mundane. In fact, you'll use JavaScript code for just a few basic tasks:

• Get form data: JavaScript code makes it simple to pull data out of your HTML form and send it to the server.
• Change values on the form: It's also simple to update a form, from setting field values to replacing images on the fly.
• Parse HTML and XML: You'll use JavaScript code to manipulate the DOM and to work with the structure of your HTML form and any XML data that the server returns.

For those first two items, you want to be very familiar with the getElementById() method as shown in Listing 2.

// Get the value of the "phone" field and stuff it in a variable called phone
var phone = document.getElementById("phone").value;

// Set some values on a form using an array called response
document.getElementById("order").value = response[0];
document.getElementById("address").value = response[1];

There's nothing particularly remarkable here and that's good! You should start to realize that there's nothing tremendously complicated about this. Once you master XMLHttpRequest, much of the rest of your Ajax application will be simple JavaScript code like that shown in Listing 2, mixed in with a bit of clever HTML. Then, every once in a while, there's a little DOM work...so let's look at that.

Finishing off with the DOM

Last but not least, there's the DOM, the Document Object Model. For some of you, hearing about the DOM is going to be a little intimidating -- it's not often used by HTML designers and is even somewhat unusual for JavaScript coders unless you're really into some high-end programming tasks. Where you will find the DOM in use a lot is in heavy-duty Java and C/C++ programs; in fact, that's probably where the DOM got a bit of its reputation for being difficult or hard to learn.

Fortunately, using the DOM in JavaScript technology is easy, and is mostly intuitive. At this point, I'd normally show you how to use the DOM or at least give you a few code examples, but even that would be misleading. You see, you can get pretty far into Ajax without having to mess with the DOM and that's the path I'm going to show you. I'll come back to the DOM in a future article, but for now, just know that it's out there. When you start to send XML back and forth between your JavaScript code and the server and really change the HTML form, you'll dig back into DOM. For now, it's easy to get some effective Ajax going without it, so put this on the back-burner for now.

Getting a Request object

With a basic overview under your belt, you're ready to look at a few specifics. Since XMLHttpRequest is central to Ajax applications -- and probably new to many of you -- I'll start there. As you saw in Listing 1, it should be pretty easy to create this object and use it, right? Wait a minute.

Remember those pesky browser wars from a few years back and how nothing worked the same across browsers? Well, believe it or not, those wars are still going on albeit on a much smaller scale. And, surprise: XMLHttpRequest is one of the victims of this war. So you'll need to do a few different things to get an XMLHttpRequest object going. I'll take your through it step by step.

Working with Microsoft browsers

Microsoft's browser, Internet Explorer, uses the MSXML parser for handling XML. So when you write Ajax applications that need to work on Internet Explorer, you need to create the object in a particular way.

However, it's not that easy. MSXML actually has two different versions floating around depending on the version of JavaScript technology installed in Internet Explorer, so you've got to write code that handles both cases. Look at Listing 3 for the code that you need to create an XMLHttpRequest on Microsoft browsers.

var xmlHttp = false;
try {
  xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
  try {
    xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
  } catch (e2) {
    xmlHttp = false;
  }
}

All of this won't make exact sense yet, but that's OK. You'll dig into JavaScript programming, error handling, conditional compilation, and more before this series is finished. For now, you want to get two core lines into your head:

xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");

and

xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");.

In a nutshell, this code tries to create the object using one version of MSXML; if that fails, it then creates the object using the other version. Nice, huh? If neither of these work, the xmlHttp variable is set to false, to tell your code know that something hasn't worked. If that's the case, you've probably got a non-Microsoft browser and need to use different code to do the job.

Dealing with Mozilla and non-Microsoft browsers

If Internet Explorer isn't your browser of choice or you write code for non-Microsoft browsers, then you need different code. In fact, this is the really simple line of code you saw back in Listing 1:

var xmlHttp = new XMLHttpRequest object;.

This much simpler line creates an XMLHttpRequest object in Mozilla, Firefox, Safari, Opera, and pretty much every other non-Microsoft browser that supports Ajax in any form or fashion.

Putting it together

The key is to support all browsers. Who wants to write an application that works just on Internet Explorer or an application that works just on non-Microsoft browsers? Worse yet, do you want to write your application twice? Of course not! So your code combines support for both Internet Explorerand non-Microsoft browsers. Listing 4 shows the code to do just that.

/* Create a new XMLHttpRequest object to talk to the Web server */
var xmlHttp = false;
/*@cc_on @*/
/*@if (@_jscript_version >= 5)
try {
  xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
  try {
    xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
  } catch (e2) {
    xmlHttp = false;
  }
}
@end @*/

if (!xmlHttp && typeof XMLHttpRequest != 'undefined') {
  xmlHttp = new XMLHttpRequest();
}

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For now, ignore the commenting and weird tags like @cc_on; those are special JavaScript compiler commands that you'll explore in depth in my next article, which will focus exclusively on XMLHttpRequest. The core of this code breaks down into three steps:

1. Create a variable, xmlHttp, to reference the XMLHttpRequest object that you will create.
2. Try and create the object in Microsoft browsers:
   • Try and create the object using the Msxml2.XMLHTTP object.
   • If that fails, try and create the object using the Microsoft.XMLHTTP object.
3. If xmlHttp still isn't set up, create the object in a non-Microsoft way.

At the end of this process, xmlHttp should reference a valid XMLHttpRequest object, no matter what browser your users run.

A word on security

What about security? Today's browsers offer users the ability to crank their security levels up, to turn off JavaScript technology, and disable any number of options in their browser. In these cases, your code probably won't work under any circumstances. For these situations, you'll have to handle problems gracefully -- that's at least one article in itself, one I will tackle later (it's going to be a long series, isn't it? Don't worry; you'll master all of this before you're through). For now, you're writing robust, but not perfect, code, which is great for getting a handle on Ajax. You'll come back to the finer details.

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