code: function getElementRect(elem) {
    const docElem = elem.ownerDocument.documentElement;
  
    // FF 2, Safari 3 and Opera 9.5- do not support getBoundingClientRect().
    // IE9- will throw if the element is not in the document.
    if (!('getBoundingClientRect' in elem) || !containsNode(docElem, elem)) {
      return {
        left:   0,
        right:  0,
        top:    0,
        bottom: 0
      };
    }
  
    // Subtracts clientTop/Left because IE8- added a 2px border to the
    // <html> element (see http://fburl.com/1493213). IE 7 in
    // Quicksmode does not report clientLeft/clientTop so there
    // will be an unaccounted offset of 2px when in quirksmode
    const rect = elem.getBoundingClientRect();
  
    return {
      left:   Math.round(rect.left) - docElem.clientLeft,
      right:  Math.round(rect.right) - docElem.clientLeft,
      top:    Math.round(rect.top) - docElem.clientTop,
      bottom: Math.round(rect.bottom) - docElem.clientTop
    };
  }
docs: Gets an element's bounding rect in pixels relative to the viewport.
  
  @param {DOMElement} elem
  @return {object}