JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

One major difference between version 4+ models and the basic model is the addition of the
Event object. This object gives event handlers a snapshot of the context in which the event
occurred. For example, it includes the screen coordinates of the event, the mouse button that
was used (if any), and any modifying keys, such as ALT or CTRL, that were depressed when it
occurred.
Another major difference is that events in newer models propagate through the document
hierarchy. In Netscape 4, events begin at the top of the hierarchy and ―trickle‖ down to the
object at which they occurred, affording enclosing objects the opportunity to modify, cancel, or
handle the event. Under Internet Explorer, events begin at the object where they occur and
―bubble‖ up the hierarchy. Under DOM2, events can trickle down and bubble up, as shown
here:
Netscape 4 Event Model
Netscape 4 implemented the first event model with advanced features not found in the basic
event model. These features permit somewhat greater flexibility with respect to how you handle
events and where you can do so in document object hierarchy.
Unfortunately, this model is ―dead‖ in the sense that it is found only in Netscape version 4
browsers. Since Mozilla-based browsers adopted the DOM2 model and Netscape 6+ are based
on Mozilla, this model is an evolutionary dead end. We present it here for those who need to
ensure backward compatibility, and to provide insight into the historical influence it had on the
DOM2 model.
Event Objects
When an event occurs in Netscape 4, the browser creates an Event object and passes it to the
handler. Some interesting properties of Event objects are listed in Table 11-6.
Table 11-6: Instance Properties of Netscape 4’s Event Object
Property Description
>data Array of strings containing the URLs of objects that were dragged and
dropped.
>modifiers Bitmask indicating which modifier keys were held down during the event.
The bitmask is a bitwise combination of the constants: ALT_MASK,
CONTROL_MASK, META_MASK, and SHIFT_MASK, which are static
(class) properties of the Event object. For example, if the ALT and CTRL
keys were depressed, modifiers will have value (Event.ALT_MASK &
Event.CONTROL_MASK).
>pageX Numeric value indicating the horizontal coordinate where the event
occurred.