ncaught TypeError: Cannot read property 'offSetX' of undefined

function onMouseMove(e){
 if(e.offSetX || e.layerX){
  mx = e.offSetX == undefined ? e.layerX : e.offSetX;
  my = e.offSetY == undefined ? e.layerY : e.offSetY;
  console.log('mx');
 }
}