Get an Element's Offset relative to the document

getElementOffset.js

const getElementOffset = el => {
  const rect = el.getBoundingClientRect(),
        scrollLeft = window.scrollX || document.documentElement.scrollLeft,
        scrollTop = window.scrollY || document.documentElement.scrollTop;

  return {
    top: rect.top + scrollTop,
    left: rect.left + scrollLeft
  }
}