In chapter two there's more theory on topics like JavaScript, the JavaScript DOM model, the XmlHttpRequest object and XML. These topics are thoroughly explained, with examples to demonstrate how they work. The XmlHttpRequest object is covered very in-depth, and the book takes a good look at the different "types" of the object, namely the native object in Firefox/Opera/Safari and the ActiveX object in IE. Something I didn't know is that IE actually comes with different versions of the XmlHttpRequest object, and the book shows you how to use the latest version (which supposedly comes with more features). Another good thing about chapter two is that it immediately looks at using XML in your AJAX requests ("putting the X in AJAX"), and it also shows you how to handle invalid XML in JavaScript by using various tricks. This is really useful information, because you can't catch invalid XML using standard JavaScript error handling (try {} catch).

In chapter three you are introduced to the server side part of AJAX. You will learn how to use PHP's inbuilt DOM functions to create an XML response for the AJAX request. Although this technique works fine, I've never done it like this myself, and I find it much easier to first create a standard PHP array of my data, and then to transform it into standard XML using a XML_Serializer. Each to his own of course, but the DOM method needs more code than my method. The book also shows you how to use a custom error handler (with the set_error_handler() function) to catch any PHP errors, and send it back as an XML response. This is really quite an important step, especially during development, since you won't be able to see the errors when they're printed (like a normal PHP script).

Chapter four is the first example chapter, and marks the end for most of the theory. In this chapter you take a look at form validation, and how AJAX can be helpful. Most of the chapter is spent on building a simple form validation script, and explaining the script, but this chapter also explains how to build thread-safe AJAX. Since AJAX is usually asynchronous, it can often lead to requests coming back in different order than they were sent. To learn more about this problem, have a look at "Troubles with Asynchronous Ajax Requests and PHP Sessions", but in some cases it's important that requests come back in the same order. This book will show you exactly how to do this.

In chapters 5, 6, 7, 8, 9 and 10 you are taken through more examples, like an AJAX chat, color picker, dynamic images with SVG and AJAX, an AJAX RSS reader, and more. All these chapters are pretty much about these examples and explaining them, and you won't really learn anymore new theory about AXAX itself, but these chapters do introduce a few other new topics, like using XSL to transform XML or using SVG.

Final Verdict

Although I've done countless of things with AJAX myself, and have a firm grasp on how it works, I still learnt a thing or two from this book. Most of the stuff was common knowledge for me, but there were still a few new things. If you're an absolute beginner when it comes to AJAX, and want to get started, I can definitely recommend this book. It starts off with the basics and slowly moves towards the more difficult stuff.

One thing I missed in this book was the usage of JSON. It's briefly mentioned in the beginning of the book, but an example of how it's used is never given, even though JSON is so much easier to use than XML. Another thing I was hoping to see was a bit more information on the current state of AJAX/PHP libraries, like SAJAX and My-BIC.

All in all I'd say this is an excellent book, and well worth reading and keeping as reference. It's not too long either, since the last seven chapters go really fast. Highly recommended.

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