JavaScript The Definitive Guide 4th Edition-@Team LiB

Event

details about an event

Availability

JavaScript 1.2; incompatible versions supported by Netscape 4 and IE 4

Synopsis

function handler(event) { ... } // Event handler argument in Netscape 4 
window.event   // Window property in IE 4

Constants

In Netscape 4, the Event object defines bitmask constants for each of the supported event types. These static properties are used to form the bitmasks that are passed to captureEvents( ) and releaseEvents( ). The available constants are:

Event.ABORT

Event.BLUR

Event.CHANGE

Event.CLICK

Event.DBLCLICK

Event.DRAGDROP

Event.ERROR

Event.FOCUS

Event.KEYDOWN

Event.KEYPRESS

Event.KEYUP

Event.LOAD

Event.MOUSEDOWN

Event.MOUSEMOVE

Event.MOUSEOUT

Event.MOUSEOVER

Event.MOUSEUP

Event.MOVE

Event.RESET

Event.RESIZE

Event.SELECT

Event.SUBMIT

Event.UNLOAD

Netscape 4 Properties

height: Set only in events of type "resize". Specifies the new height of the window or frame that was resized.
layerX, layerY: Specify the X- and Y-coordinates, relative to the enclosing layer, at which an event occurred.
modifiers: Specifies which keyboard modifier keys were held down when the event occurred. This numeric value is a bitmask consisting of any of the constants Event.ALT_MASK, Event.CONTROL_MASK, Event.META_MASK, or Event.SHIFT_MASK. Due to a bug, this property is not defined in Netscape 6 or 6.1.
pageX, pageY: Specify the X- and Y-coordinates, relative to the web browser page, at which the event occurred. Note that these coordinates are relative to the top-level page, not to any enclosing layers.
screenX, screenY: Specify the X- and Y-coordinates, relative to the screen, at which the event occurred. Note that, unlike most Event properties, these properties are supported by and have the same meaning in both Netscape 4 and Internet Explorer 4.
target: Specifies the Window, Document, Layer, or HTMLElement object on which the event occurred.
type: A string property that specifies the type of the event. Its value is the name of the event handler minus the "on" prefix. So when the onclick( ) event handler is invoked, the type property of the Event object is "click".
which: For keyboard and mouse events, which specifies which key or mouse button was pressed or released. For keyboard events, this property contains the character encoding of the key that was pressed. For mouse events, it contains 1, 2, or 3, indicating the left, middle, or right buttons.
width: Set only in events of type "resize". Specifies the new width of the window or frame that was resized.
x, y: Specify the X- and Y-coordinates at which the event occurred. In Netscape 4, these properties are synonyms for layerX and layerY and specify the position relative to the containing layer (if any). Their meaning is different in Internet Explorer (see below).

Internet Explorer 4 Properties

altKey: A boolean value that specifies whether the Alt key was held down when the event occurred.
button: For mouse events, button specifies which mouse button or buttons were pressed. This read-only integer is a bitmask: the 1 bit is set if the left button was pressed; the 2 bit is set if the right button was pressed; and the 4 bit is set if the middle button (of a three-button mouse) was pressed.
cancelBubble: If an event handler wants to stop an event from being propagated up to containing objects, it must set this property to true.
clientX, clientY: Specify the X- and Y-coordinates, relative to the web browser page, at which the event occurred.
ctrlKey: A boolean value that specifies whether the Ctrl key was held down when the event occurred.
fromElement: For mouseover and mouseout events, fromElement refers to the object from which the mouse pointer is moving.
keyCode: For keyboard events, keyCode specifies the Unicode character code generated by the key that was struck.
offsetX, offsetY: Specify the X- and Y-coordinates at which the event occurred within the coordinate system of the event's source element (see srcElement).
reason: For the datasetcomplete event, reason contains a code that specifies the status of the data transfer. A value of 0 indicates a successful transfer. A value of 1 indicates that the transfer was aborted. A value of 2 indicates that an error occurred during data transfer.
returnValue: If this property is set, its value takes precedence over the value actually returned by an event handler. Set this property to false to cancel the default action of the source element on which the event occurred.
screenX, screenY: Specify the X- and Y-coordinates, relative to the screen, at which the event occurred. Note that, unlike most Event properties, these two properties are supported by and have the same meaning in both Netscape 4 and Internet Explorer 4.
shiftKey: A boolean value that specifies whether the Shift key was held down when the event occurred.
srcElement: A reference to the Window, Document, or HTMLElement object that generated the event.
srcFilter: For filterchange events, srcFilter specifies the filter that changed.
toElement: For mouseover and mouseout events, toElement refers to the object into which the mouse pointer is moving.
type: A string property that specifies the type of the event. Its value is the name of the event handler minus the "on" prefix. So when the onclick( ) event handler is invoked, the type property of the Event object is "click".
x, y: Specify the X- and Y-coordinates at which the event occurred. In Internet Explorer 4, this property specifies the X position relative to the innermost containing element that is dynamically positioned using CSS. The interpretation of these properties is different in Netscape 4 (see the previous section).

Description

The Event object provides details about an event that has occurred. Unfortunately, these details are not standardized, and Netscape 4 and IE 4 define Event objects that are almost entirely incompatible. Besides having different properties, Netscape 4 and IE 4 provide access to Event objects in different ways. In Netscape, an Event object is passed as an argument to every event handler. For event handlers defined by HTML attributes, the name of the event argument is event. In IE, the Event object of the most recent event is instead stored in the event property of the Window object.