All Ajax effects are interactive; the whole point of Ajax is to make a page more responsive. Some effects, though, are more interactive than others--especially those associated with providing immediate information based on some event.
If you've used either Netflix or Blockbuster Online, you've seen pop-up windows that open with information about a movie when your mouse hovers over the movie link. This is typical of an Ajax interactive effect, where the application interprets your intent. In this case, you can find out more about the movie and possibly add it to your queue.
If you've provided commentary at a site and can preview your effort as you write or before final submission, you are seeing another interactive effect. It gives you a chance to review what you've written, correct misspellings or transposed letters, and clarify sentence structure. This type of functionality isn't a requirement for the application to run; rather, it's an interpretation of what your web page readers may want--in this case, a way to preview the text.
Online applications that provide feedback when you perform an action, such as a red flashing effect when data is deleted, or a yellow flash when an update has occurred, are also interactive effects. None is essential, but they provide the page reader reassurance that an action has happened.
Each of these effects provides a signal that the application is aware of your intentions as well as your actions. It's not the same as clicking a button and having a form submitted, or dragging an item to a shopping cart, both of which are expected and essential behaviors, notable only when they don't happen.
No, the Ajax effects covered in this chapter are the web application's way of saying, "I hear you, I see what you're doing, I think I know what you want." None of the effects are essential, but in the great tool chest that is Ajax, these can be the simplest effects to implement and have the strongest positive impact for readers using your applications.
The effects are all quite different: tooltips, Just-In-Time (JIT) form help, color fades, live previews. However, they all have one thing in common--they are all responses to events. Because of this dependence on events, I'll begin the chapter with a review of event handling, particularly event handling in an Ajax environment where multiple libraries may be combined into one application.