This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
var buttonPressed = instanceOfMouseEvent.which
A number representing a given button:
0: No button
1: Left button
2: Middle button (if present)
3: Right button
For a mouse configured for left-handed use, the button actions are reversed. In this case, the values are read from right to left.
This is not part of any specification.
BCD tables only load in the browser