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.

The MouseEvent.which read-only property indicates which button was pressed on the mouse to trigger the event. The standard alternatives to this property are MouseEvent.button and MouseEvent.buttons.


var buttonPressed = instanceOfMouseEvent.which

Return value

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.

Browser compatibility

Feature Edge Firefox (Gecko) Chrome Internet Explorer Opera Safari
Basic support (Yes) 1.0 (1.7 or earlier)[1] 1.0 9.0 5.0 1.0
Feature Edge Firefox Mobile (Gecko) Android IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) 1.0 (1) [1] (Yes) (Yes) (Yes) (Yes)

[1] On mousemove events, the which property is incorrectly always set to 1 bug 1048294.

See also

Document Tags and Contributors

 Contributors to this page: libbymc, Ac1521, cvrebert, Sebastianz, teoli, Marijn
 Last updated by: libbymc,