知识库

html
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    html,
    body {
      height: 500%;
    }

    div {
      width: 200px;
      height: 200px;
      background-color: #ddd;
      margin: 0 auto;
      margin-top: 1000px;
    }

    div img {
      width: 100%;
      height: 100%;
      opacity: 0;
      transition: opacity .3s;
      /* 
        开始图片是隐藏的,在某些IE浏览器中,如果图片src是空的或者加载的图片是错误的,图片
        不隐藏,会显示一个 X 的效果,很难看.
        方法1: display: none => 加载真实图片后设置display: block会触发DOM的回流重绘.
        性能消耗较大
        方法2: opacity: 0 => 加载真实图片后设置opacity: 1不会引发DOM回流重绘,另一方面
        还可以结合css3动画实现渐现效果.推荐.
      */
    }
  </style>
</head>

<body>
  <div>
    <img src="" alt="" data-img="./demo.png">
  </div>

  <div>
    <img src="" alt="" data-img="./demo2.webp">
  </div>

  <div>
    <img src="" alt="" data-img="./demo3.webp">
  </div>

  <script type="module">
    import throttle from './throttle.js'

    const containers = document.querySelectorAll('div')
    const html = document.documentElement

    function lazyLoading() {
      for (let i = 0, l = containers.length; i < l; i++) {
        const img = containers[i].querySelector('img')
        const container = containers[i]
        if (img.isLoaded) {
          continue
        }

        let { bottom, top } = container.getBoundingClientRect()
        // top >= 0实现了刷新时只加载在可视区内的图片
        if (bottom <= html.clientHeight && top >= 0) { 
          img.src = img.getAttribute('data-img')
          img.onload = () => {
            // 图片加载成功
            img.style.opacity = 1
            img.isLoaded = true
            img.removeAttribute('data-img')
          }
        }
      }
    }

    window.addEventListener(
      'scroll', 
      throttle(lazyLoading, 200, { trailing: true })
    )

  </script>
</body>

</html>