javascript

Chapter 12: Events
The addHandler() and removeHandler() methods don ’ t equalize all functionality across all browsers,
such as the IE scope issue, but it does allow the seamless addition and removal of event handlers.
The Event Object
When an event related to the DOM is fired, all of the relevant information is gathered and stored on an
object called event . This object contains basic information such as the element that caused the event, the
type of event that occurred, and any other data that may be relevant to the particular event. For example,
an event caused by a mouse action generates information about the mouse ’ s position, whereas an event
caused by a keyboard action generates information about the keys that were pressed. All browsers
support the event object, though not in the same way.
The DOM Event Object
In DOM - compliant browsers, the event object is passed in as the sole argument to an event handler.
Regardless of the method used to assign the event handler, DOM Level 0 or DOM Level 2, the event
object is passed in. Here is an example:
var btn = document.getElementById(“myBtn”);
btn.onclick = function(event){
alert(event.type); //”click”
};
btn.addEventListener(“click”, function(event){
alert(event.type); //”click”
}, false);
Both event handlers in this example pop up an alert indicating the type of event being fired by using
the event.type property. This property always contains the type of event that was fired, such as
“ click ” (it is the same value that you pass into addEventListener() and removeEventListener() ).
When an event handler is assigned using HTML attributes, the event object is available as a variable
called event . Here’s an example:
< input type=”button” value=”Click Me” onclick=”alert(event.type)” / >
Providing the event object in this way allows HTML attribute event handlers to perform the same as
JavaScript functions.
The event object contains properties and methods related to the specific event that caused its
creation. The available properties and methods differ based on the type of event that was fired, but all
events have the members listed in the following table.
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