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Event Object

One important difference in direct assignment is that subsequent assignments to the event handler wipe

out the previous assignment:

oDiv.onclick = fnClick;

oDiv.onclick = fnDifferentClick;

In this example,

fnClick

is assigned as the

onclick

event handler first, but is then replaced by

fnDifferentClick

The Event Object

Developers for both browsers knew that it was important to pass information about an event to the

developer. The result was to create an event object that contained information specific to the event that

had just occurred such as:

The object that caused the event

Information about the mouse at the time of the event

Information about the keyboard at the time of the event

Event objects are only created when an event occurs and are made accessible to the event handlers. After

all event handlers have been executed, the event object is destroyed.

As you can probably guess, Internet Explorer and the DOM implement the event object in two

different ways.

Locating

In Internet Explorer, the

event

object is a property of the

window

object. This means that an event han-

dler must access the event object in this way:

oDiv.onclick = function () {

var oEvent = window.event;

}

Even though it is a property of the

window

object, the

event

object is only accessible when an event

occurs. After all event handlers have been executed, the

event

object is destroyed.

The DOM standard says that the

event

object must be passed in as the sole argument of the event han-

dler. So, to access the event object in a DOM-compliant browser (such as Mozilla, Safari, or Opera), you

do the following:

oDiv.onclick = function () {

var oEvent = arguments[0];

}

Of course, you can also name the argument for easier access:

oDiv.onclick = function (oEvent) {

}

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