Ajax Simplified

Ajax can become complex as far as implementation, but the concept is quite simple.
Enter XML

Technically, you can create a full Ajax application without ever using XML, but you will find XML to be a virtual necessity as your Web application grows in complexity. Here is how to do the same simple Web page with XML, once again cutting every corner for the sake of simplicity.

Notice in Listing 3 that we now grab the response with the code http.responseXML and extract the value we want with the code xmlDocument.getElementsByTagName('shipping'). Note also that the XML refers to the total with the tag shipping instead of totalshipping. This difference is unnecessary, but the purpose in this tutorial is to avoid the possible implication that the XML tag name and the HTML input field id must match in order to make the application work. They do not have to match.

The only thing left is to modify our PHP code to return XML instead of plain text. See Listing 4 for the PHP code. In addition to the XML content itself, note the line of code that sends a header identifying the content as XML before returning the XML content itself. The XML places the shipping amount as a child of <order>, along with the unused data, <total>. This is simply a baby step toward representing a more realistic set of data that the page should return.

Believe it or not, that's all there is to Ajax. Just about everything else that adds complexity to Ajax application development falls into the following categories.

Validation and Error Handling

A real Ajax application would not assume that the PHP file exists. It also would check the validity of the zip code before attempting to send it as a parameter to the server in order to find the shipping cost. (You also could have the server validate the zip code or do minimal validation at the client side, such as ensuring that the user entered five full digits and then perform full validation of the zip code at the server side.)

The above example eschews all error handling in order to keep the focus on the bare bones of how Ajax works. Obviously, you need to include input validation, error detection and error handling in a real application.

Accounting for the Differences between Browsers

The above sample code works with Firefox, but there's no guarantee it will work in any other browser. If you want to write all your Ajax code from scratch, taking into account the variations between Firefox, IE and Opera, buy lots of ibuprofen—you'll need it. Fortunately, a plethora of Ajax libraries exist that manage the differences for you. One of my favorites is Dojo (see Resources).

Managing the Elements of the Document via the DOM

Ajax relies on the DOM to address the various elements within a page. As your page becomes more complex, it gets harder to keep track of all the elements, their names and ids. Firefox has a built-in DOM inspector that is enormously helpful. If that's not enough, you can install the Firebug add-on to Firefox. Firebug not only provides you with a way to examine the DOM, it also helps you debug your JavaScript code and manage your cascading stylesheets (see Resources for a link to the add-on). Figure 1 shows the XML example page as viewed through Firebug. [Reuven Lerner covers Firebug in this month's At the Forge on page 22.]

Figure 1. View elements of the DOM with Firebug.