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

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support1 Yes11951
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes Yes41 Yes Yes ?

1. On mousemove events, the which property is incorrectly always set to 1.

See also

Document Tags and Contributors

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