MouseEvent.buttons read-only property indicates which buttons are pressed on the mouse when the event is triggered.
Each button that can be pressed is represented by a given number (see below). If more than one button is pressed, the value of the buttons is combined to produce a new number. For example, if the right button (
2) and the wheel button (
4) are pressed, the value is equal to
2|4, which is
Note: Do not confuse this property with the
MouseEvent.button property. The
MouseEvent.buttons property indicates the state of buttons pressed during any kind of mouse event, while the
MouseEvent.button property only guarantees the correct value for mouse events caused by pressing or releasing one or multiple buttons.
var buttonPressed = instanceOfMouseEvent.buttons
8: 第四按键 (通常是“浏览器后退”按键)
16: 第五按键 (通常是“浏览器前进”)
|Document Object Model (DOM) Level 3 Events Specification
|Feature||Edge||Firefox (Gecko)||Chrome||Internet Explorer||Opera||Safari|
|Feature||Edge||Firefox Mobile (Gecko)||Android||IE Mobile||Opera Mobile||Safari Mobile|
 Gecko supports the
buttons attribute on Windows, Linux (GTK), and Mac OS with the following restrictions:
- On Windows, if you install a mouse driver, and its utility software allows you to customize button actions (e.g., IntelliPoint and SetPoint), the middle (wheel) button, the 4th button, and the 5th button might not be set, even when they are pressed.
- On Linux (GTK), the 4th button and the 5th button are not supported. In addition, a mouseup event always has the releasing button information in this attribute value.
- On Mac OS X 10.5, the buttons attribute always returns 0 because there is no platform API for implementing this feature.
 This feature got implemented in bug 276941.