test if dom element is in viewport either fully or fully/partially/none

isInViewport.js

    function isInViewport (elem, partially) {
      let box = elem.getBoundingClientRect(),
          top = box.top,
          bottom = box.bottom,
          right = box.right,
          left = box.left,
          h = window.innerHeight || document.documentElement.clientHeight,
          w = window.innerWidth || document.documentElement.clientWidth,
          fully = (top >= 0 && box.left >= 0 && bottom <= h && box.right <= w);

      return partially ? fully && 2 || (
            ((top >= 0 && top < h) ||
            (bottom >= 0 && bottom < h) ||
            (top < 0 && bottom > h)) &&
            ((box.left >= 0 && box.left < w) ||
            (box.right >= 0 && box.right < w) ||
            (box.left < 0  && box.right > w))
          ) && 1 : fully;
    }