Detailed explanation and usage examples of javascript Event object_Basic knowledge

Event represents event status, such as the element on which the event occurred, keyboard status, mouse position, and mouse button status. Once an event occurs, an Event object will be generated. For example, when a button is clicked, the corresponding event object will be generated in the browser's memory.
The event object is only valid during the event.
Some attributes of event are only meaningful for specific events. For example, the fromElement and toElement properties are only meaningful for onmouseover and onmouseout events.

[event attribute]:
altKey, button, cancelBubble, clientX, clientY, ctrlKey, fromElement, keyCode, offsetX, offsetY, propertyName, returnValue, screenX, screenY, shiftKey, srcElement, srcFilter, toElement, type, x, y
--------------------------------------------- -------------------------------------
1.altKey
Description: Check The state of the alt key.
Syntax: event.altKey
Possible values:
The value is TRUE when the alt key is pressed, FALSE otherwise. Read only.

2.button
Description: Check the pressed mouse button.
Syntax: event.button
Possible values:
0 No button pressed
1 Press the left button
2 Press the right button
3 Press the left and right buttons
4 Press the middle button
5 Press left and middle keys
6 Press right and middle keys
7 Press all keys
This attribute is only used for onmousedown, onmouseup, and onmousemove events. For other events, 0 is returned regardless of the mouse status (such as onclick).

3.cancelBubble
Description: Detect whether to accept the control of the event of the upper element.
Syntax:
event.cancelBubble[ = cancelBubble]
Possible values:
This is a readable and writable Boolean value:
TRUE is not controlled by events on the upper element.
FALSE allows to be controlled by events of upper element. This is the default value.
Example:
The following code snippet demonstrates that when clicking on the image (onclick), if the shift key is also pressed at the same time, canceling the showSrc() event triggered by the onclick event on the upper element (body) function.

Copy code The code is as follows:

4.clientX
Description: Returns the X coordinate of the mouse in the client area of ​​the window.
Syntax: event.clientX
Comments:
This is a read-only property. This means that you can only use it to get the current position of the mouse, but you cannot use it to change the position of the mouse.

5.clientY
Description: Returns the Y coordinate of the mouse in the client area of ​​the window.
Syntax: event.clientY
Comments:
This is a read-only property. This means that you can only use it to get the current position of the mouse, but you cannot use it to change the position of the mouse.

6.ctrlKey
Description: Check the status of the ctrl key.
Syntax: event.ctrlKey
Possible values:
When the ctrl key is pressed, the value is TRUE, otherwise it is FALSE. Read only.

7.fromElement
Description: Detect the element that the mouse leaves when the onmouseover and onmouseout events occur. Reference: 18.toElement
Syntax: event.fromElement
Comments:
This is a read-only property.

8.keyCode
Description: Detect the internal code corresponding to keyboard events. This property is used for onkeydown, onkeyup, and onkeypress events.
Syntax: event.keyCode[=keyCode]
Possible values:
This is a readable and writable value, which can be any Unicode keyboard internal code. If no keyboard event was raised, the value is 0 .

9.offsetX
Description: Check the horizontal coordinate of the mouse position relative to the object that triggered the event
Syntax: event.offsetX

10.offsetY
Description: Check Relative to the object that triggered the event, the vertical coordinate of the mouse position
Syntax: event.offsetY

11.propertyName
Description: Sets or returns the name of the changed property of the element.
Syntax: event.propertyName [= sProperty]
Possible values:
sProperty is a string that specifies or returns the name of the property that changed in the event of the element that triggered the event.
This property is readable and writable. No default value.
Note:
You can get the value of propertyName by using the onpropertychange event.
Example:
The following example uses the onpropertychange event to pop up a dialog box to display the value of propertyName.

Copy code The code is as follows:

<script> <BR>function changeProp() <BR>{ <BR>btnProp.value = "This is the new VALUE"; <BR>} <BR>function changeCSSProp() <BR>{ <BR>btnStyleProp.style.backgroundColor = "aqua"; <BR>} <BR></script>

The event object property propertyName is
used here to return which property has been
altered.

VALUE="Click to change the VALUE property of this button"
onpropertychange='alert(event.propertyName " property has changed value")'>
onclick="changeCSSProp()"
VALUE="Click to change the CSS backgroundColor property of this button"
onpropertychange='alert(event.propertyName " property has changed value")'>

12.returnValue
描述：设置或检查从事件中返回的值
语法：event.returnValue[ = Boolean]
可能的值：
true 事件中的值被返回
false 源对象上事件的默认操作被取消

13.screenX
描述：检测鼠标相对于用户屏幕的水平位置
语法：event.screenX
注释：
这是个只读属性。这意味着，你只能通过它来得到鼠标的当前位置，却不能用它来更改鼠标的位置。

14.screenY
描述：检测鼠标相对于用户屏幕的垂直位置
语法：event.screenY
注释：
这是个只读属性。这意味着，你只能通过它来得到鼠标的当前位置，却不能用它来更改鼠标的位置。

15.shiftKey
描述：检查shift键的状态。
语法：event.shiftKey
可能的值：
当shift键按下时，值为 TRUE ，否则为 FALSE 。只读。

16.srcElement
描述：返回触发事件的元素。只读。例子见本文开头。
语法：event.srcElement

17.srcFilter
描述：返回触发 onfilterchange 事件的滤镜。只读。
语法：event.srcFilter

18.toElement
描述：检测 onmouseover 和 onmouseout 事件发生时，鼠标所进入的元素。
语法：event.toElement
注释：
这是个只读属性。
例子：下面的代码演示了当鼠标移到按钮上时，弹出一个对话框，显示“mouse arrived”

复制代码 代码如下:

<script> <BR>function testMouse(oObject) { <BR>if(oObject.contains(event.toElement)) <BR>{ <BR>alert("mouse arrived"); <BR>} <BR>} <BR></script>
:
Mouse Over This.

19.type
描述：返回事件名。
语法：event.type
注释：
返回没有“on”作为前缀的事件名，比如，onclick事件返回的type是click
只读。

20. x
描述：返回鼠标相对于css属性中有position属性的上级元素的x轴坐标。如果没有css属性中有position属性的上级元素，默认以BODY元素作为参考对象。
语法：event.x
注释：
如果事件触发后，鼠标移出窗口外，则返回的值为 -1
这是个只读属性。这意味着，你只能通过它来得到鼠标的当前位置，却不能用它来更改鼠标的位置。

21. y
描述：返回鼠标相对于css属性中有position属性的上级元素的y轴坐标。如果没有css属性中有position属性的上级元素，默认以BODY元素作为参考对象。
语法：event.y
注释：
如果事件触发后，鼠标移出窗口外，则返回的值为 -1
这是个只读属性。这意味着，你只能通过它来得到鼠标的当前位置，却不能用它来更改鼠标的位置。