I would expect event.which
to work for events involving a click such as mousedown
, mouseup
, click
, for example.
If you need this information during a mouse move, you would have to pre-record it during one of these other events and use it when you detect the move.
The fragility of this, as always, is if a click event happens outside of your target area (such as not even in a browser) as you won't find out about that.