Capturing Mouse Movement in javascript

document.onmousemove = mouseMove;

function mouseMove(ev){
 ev           = ev || window.event;
 var mousePos = mouseCoords(ev);
}

function mouseCoords(ev){
 if(ev.pageX || ev.pageY){
  return {x:ev.pageX, y:ev.pageY};
 }
 return {
  x:ev.clientX + document.body.scrollLeft - document.body.clientLeft,
  y:ev.clientY + document.body.scrollTop  - document.body.clientTop
 };
}