ele 요소가 뷰포트에 표시되면 다음 함수는 true를 반환합니다.

https://htmldom.dev/check-if-an-element-is-in-the-viewport 

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