原生js图片懒加载

用data-src自定义属性，让浏览器暂不加载图片

html
<img data-src='./img1.png'/>
<img data-src='./img2.png'/>
<img data-src='./img3.png'/>

 js
 const images = document.querySelectAll('img');
 const callback = entries =>{
    entries.forEach( entry =>{
        if(entry.isIntersecting){
          const image = entry.target
          const data_src = image.getAttrbute('data-src')
          img.setAttribute('src',data_src);
          // 结束观察 防止触发多次
          observer.unobserve( image );
          console.log('触发')
        }
    })
}
const observe = new IntersectionObserver(callback);
images.forEach( image =>{
  //开启观察模式
  observe.observe(image)
})