myMouseEvent.button in DOM Level 2 Events

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 0 to indicate the left button of the mouse, 1 to indicate the middle button if present, and 2 to indicate the right button.

For mice configured for left handed use in which the button actions are reversed the values are instead read from right to left.