scroll-spy/lib/getOffsetRect.js

A light weight scroll spy library in vanilla js

module.exports = function getOffsetRect(elem) {

  // (1)
  var box = elem.getBoundingClientRect();

  var body = document.body;
  var docElem = document.documentElement;

  // (2)
  var scrollTop = window.pageYOffset || docElem.scrollTop || body.scrollTop;
  var scrollLeft = window.pageXOffset || docElem.scrollLeft || body.scrollLeft;

  // (3)
  var clientTop = docElem.clientTop || body.clientTop || 0;
  var clientLeft = docElem.clientLeft || body.clientLeft || 0;

  // (4)
  var top = box.top + scrollTop - clientTop;
  var left = box.left + scrollLeft - clientLeft;

  return { top: Math.round(top), left: Math.round(left) };
};