IntersectionObserver 检测元素是不是可见

const intersectionObserver = new IntersectionObserver((entries, observer) => {
entries.forEach(item => {
const { intersectionRatio, target } = item
// intersectionRatio为目标元素的可见比例，大于0代表可见
if (intersectionRatio) {
observer.unobserve(target);
const src = target.getAttribute('lazy');
target.src = src;
}
});
});

document.querySelectorAll('img[lazy]').forEach((el) => intersectionObserver.observe(el))