get element coords

getCoords.js

function getCoords(elem) {
  var box = elem.getBoundingClientRect();
    
  var body = document.body;
  var docEl = document.documentElement;
    
  var scrollTop = window.pageYOffset || docEl.scrollTop || body.scrollTop;
  var scrollLeft = window.pageXOffset || docEl.scrollLeft || body.scrollLeft;
    
  var clientTop = docEl.clientTop || body.clientTop || 0;
  var clientLeft = docEl.clientLeft || body.clientLeft || 0;
    
  var top  = box.top +  scrollTop - clientTop;
  var left = box.left + scrollLeft - clientLeft;
    
  return { top: Math.round(top), left: Math.round(left) };
}