getMousePos.js

//let's cache the event
var priorEvt;
 
function getMousePos(evt) {
 //set and store..
  priorEvt = evt = evt || priorEvt;
  //get the bounding rectangle
  var rect = canvas.getBoundingClientRect();
  //lastly, return the x and y coordinates
  if (evt)
    return {
      x: evt.clientX - rect.left,
      y: evt.clientY - rect.top
  };
  return {
    x: 0,
    y: 0
  };
}