Multiple mag callbacks issue

If you have the function handy you can do Puck.removeListener('mag', onMag) : http://www.espruino.com/Reference#l_Object_removeListener

Note that if you do:

function main() {
  function onMag(m) {
   m.x -= zero.x;
    m.y -= zero.y;
    m.z -= zero.z;
    var v=Math.sqrt(m.x*m.x+m.y*m.y+m.z*m.z).toF­ixed(0);
   console.log(v);
  }
   Puck.removeListener('mag', onMag);
   Puck.on('mag', onMag);
   Puck.magOn();
}

It won't work though, because onMag is basically a different function each time.

Or you can just do Puck.removeAllListeners('mag'): http://www.espruino.com/Reference#l_Object_removeAllListeners

But personally if you can I'd just make sure you only call Puck.on('mag', once - there's no need to call it at startup each time because Espruino will remember.