JavaScript: The Definitive Guide, 6th Edition

Chapter 17. Handling Events

Client-side JavaScript programs use an asynchronous event-driven programming model (introduced in Event-Driven JavaScript). In this style of programming, the web browser generates an event whenever something interesting happens to the document or browser or to some element or object associated with it. For example, the web browser generates an event when it finishes loading a document, when the user moves the mouse over a hyperlink, or when the user strikes a key on the keyboard. If a JavaScript application cares about a particular type of event, it can register one or more functions to be invoked when events of that type occur. Note that this is not unique to web programming: all applications with graphical user interfaces are designed this way—they sit around waiting for something to happen (i.e., they wait for events to occur), and then they respond.

Note that the word event is not itself a technical word that requires definition. Events are simply occurrences that a web browser will notify your program about. Events are not JavaScript objects and have no manifestation in the source code of your program. There are, of course, a number of event-related objects that do appear in your source code, and these do require technical explanation. We’ll begin this chapter, therefore, with some important definitions.

The event type is a string that specifies what kind of event occurred. The type “mousemove”, for example, means that the user moved the mouse. The ...