我们知道标准鼠标有左，中，右三个键。鼠标按下时如何判断按下的是哪个键呢？

W3C DOM-Level-2 定义如下 W3C DOM 写道 During mouse events caused by the depression or release of a mouse button, button is used to indicate which mouse button changed state. The values for button range from zero to indicate the left button of the mouse, on