判断元素是否在可视区域

isElementInViewport(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)
            );
}