要素がビジュアル領域内にあるかどうかを判断する

getBoundingClientRect()

 function isElementInViewport (el, offset = 0) {
        const box = el.getBoundingClientRect(),
              top = (box.top >= 0),
              left = (box.left >= 0),
              bottom = (box.bottom <= (window.innerHeight || document.documentElement.clientHeight) + offset),
              right = (box.right <= (window.innerWidth || document.documentElement.clientWidth) + offset);
        return (top && left && bottom && right);
    }