Uncaught TypeError: Cannot read property 'offsetX' of undefined一直错啊。。求求求解

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);

}

}

为何一直提示错误呢。求指导