JavaScript: Is element in viewport?

isElementInViewport.js

/*
No jQuery necessary.
Thanks to Dan's StackOverflow answer for this:
http://stackoverflow.com/questions/123999/how-to-tell-if-a-dom-element-is-visible-in-the-current-viewport
*/

function isOneHundredPercentOfElementInViewport(el) {
  var rect = el.getBoundingClientRect();
  return (
    rect.top >= 0 &&
    rect.left >= 0 &&
    rect.bottom <= (window.innerHeight || document. documentElement.clientHeight) &&
    rect.right <= (window.innerWidth || document. documentElement.clientWidth)
  );
}

function isAnyPartOfElementInViewport(el) {

    const rect = el.getBoundingClientRect();
    // DOMRect { x: 8, y: 8, width: 100, height: 100, top: 8, right: 108, bottom: 108, left: 8 }
    const windowHeight = (window.innerHeight || document.documentElement.clientHeight);
    const windowWidth = (window.innerWidth || document.documentElement.clientWidth);

    // http://stackoverflow.com/questions/325933/determine-whether-two-date-ranges-overlap
    const vertInView = (rect.top <= windowHeight) && ((rect.top + rect.height) >= 0);
    const horInView = (rect.left <= windowWidth) && ((rect.left + rect.width) >= 0);

    return (vertInView && horInView);
}