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Note how the value of the attribute is used as the body of the generated function,
and note that the function is created so that the

event

parameter is available

within the generated function.

Before we move on to examining what that parameter is all about, we should

note that using the attribute mechanism of declaring

DOM

Level 0 event handlers

violates the precepts of Unobtrusive JavaScript that we explored in section 1.2.
When using jQuery in our pages, we should adhere to the principles of Unobtru-
sive JavaScript and avoid mixing behavior defined by JavaScript with display
markup. We'll see that jQuery provides a better way to declare event handlers
than either of these means before the end of this chapter.

But first, let's examine what that

event

parameter is all about.

The Event instance

When an event handler is fired, an instance of a class named

Event

is passed to

the handler as its first parameter in most browsers. Internet Explorer, always the
life of the party, does things in its own proprietary way by tacking the

Event

instance onto a

window

property named

event

.

In order to deal with this discrepancy we'll often see the following used as the

first statement in an event handler:

if (!event) event = window.event;

This levels the playing field by using object detection to check if the

event

param-

eter is undefined (or null) and assigning the value of the window's

event

property

Figure 4.1 Waving the mouse over the image and clicking it result in the event
handlers firing and emitting their messages to the console.