IntersectionObserver实现监听dom进入浏览器视口

let img = document.getElementsByTagName("img");
 
const observer = new IntersectionObserver(entries => {
  //entries 是被观察的元素集合
  for(let i = 0, len = entries.length; i < len; i++) {
    let entries= entries[i];
    // 通过这个属性判断是否在视口中
    if(entries.isIntersecting) {
      const imgElement = entries.target;
      imgElement.src = imgElement.getAttribute("data-src");
      observer.unobserve(imgElement);
    }
  }
})
Array.from(img).forEach(item => observer.observe(item));