The first letter "A" in the word AJAX stands for Asynchronous. You may wonder, however, what is asynchronous to what, and how is this possible?
When a page is loaded in the web browser, it starts its client-side life. In that period of time, many interesting things happen in modern web pages or web applications. For example, some animations could start running, some content could be updated, and so on.
True asynchronous processes in the browser do not exist, in any case, not when this book was written. This means that any of execution JavaScript code initiated, for example, by a user input event would have to be processed entirely before another event could happen. The same is true for JavaScript timeouts and intervals—they are not executed in parallel, but rather sequentially.
I/O operations can happen asynchronously though. Once initialized from synchronous JavaScript code, the request can start traveling to a remote server and back. As soon as the server replies...