JavaScript: The Definitive Guide, 6th Edition

Event Handler Invocation

Once you’ve registered an event handler, the web browser will invoke it automatically when an event of the specified type occurs on the specified object. This section describes event handler invocation in detail, explaining event handler arguments, the invocation context (the this value), the invocation scope, and the meaning of the return value of an event handler. Unfortunately, some of these details are different for IE8 and before than for other browsers.

In addition to describing how individual handlers are invoked, this section also explains how events propagate: how a single event can trigger the invocation of multiple handlers on the original event target and also on containing elements of the document.

Event Handler Argument

Event handlers are normally (there is one exception, described below) invoked with an event object as their single argument. The properties of the event object provide details about the event. The type property, for example, specifies the type of the event that occurred. Types of Events mentioned a number of other event object properties for various event types.

In IE8 and before, event handlers registered by setting a property are not passed an event object when they are invoked. Instead, the event object is available through the global variable window.event. For portability, you can write event handlers like this, so that they use the window.event if no argument is supplied:

function handler(event) {
    event = event || window.event ...