console 命令行工具 X clear

console

  // 获取所有的图片标签
  const imgs = document.getElementsByTagName("img");
  // 获取可视区域的高度
  const viewHeight =
    window.innerHeight || document.documentElement.clientHeight;
  // num用于统计当前显示到了哪一张图片，避免每次都从第一张图片开始检查是否露出
  let num = 0;

  function lazyload() {
    console.log("滚动...");
    for (let i = num; i < imgs.length; i++) {
      // 用可视区域高度减去元素顶部距离可视区域顶部的高度
      let distance = viewHeight - imgs[i].getBoundingClientRect().top;
      // 如果可视区域高度大于等于元素顶部距离可视区域顶部的高度，说明元素露出
      if (distance >= 0) {
        // 给元素写入真实的src，展示图片
        imgs[i].src = imgs[i].getAttribute("data-src");
        // 前i张图片已经加载完毕，下次从第i+1张开始检查是否露出
        num = i + 1;
      }
    }
  }

  // 防抖函数
  function debounce(fn, delay = 500) {
    let timer = null;
    return function (...args) {
      if (timer) clearTimeout(timer);
      timer = setTimeout(() => {
        fn.call(this, args);
      }, delay);
    };
  }

  // 是的页面初始化是加载首屏图片
  window.onload = lazyload;
  // 监听Scroll事件，为了防止频繁调用，使用防抖函数优化一下
  window.addEventListener("scroll", debounce(lazyload, 600), false);

<head>
  <style>
    .img {
      width: 200px;
      height: 200px;
      background-color: gray;
      margin-bottom: 20px;
    }

    .pic {
      width: 100%;
      height: 100%;
    }
  </style>
</head>
<!-- 图片来自网络，侵删。 -->

<body>
  <div class="container">
    <div class="img">
      <!-- 注意我们并没有为它引入真实的src -->
      <img
        class="pic"
        alt="加载中"
        data-src="https://tse1-mm.cn.bing.net/th/id/OIP.8OrEFn_rKe82kqAWFjTuMwHaEo?pid=Api&rs=1"
      />
    </div>
    <div class="img">
      <img
        class="pic"
        alt="加载中"
        data-src="https://ssl.tzoo-img.com/images/tzoo.94911.0.910013.seoul-nami.jpg?width=1080"
      />
    </div>
    <div class="img">
      <img
        class="pic"
        alt="加载中"
        data-src="https://tse4-mm.cn.bing.net/th/id/OIP.ZitgAuABnwkrGn4lid2ZmQHaEK?pid=Api&rs=1"
      />
    </div>
    <div class="img">
      <img
        class="pic"
        alt="加载中"
        data-src="http://pic34.photophoto.cn/20150315/0034034862056002_b.jpg"
      />
    </div>
    <div class="img">
      <img
        class="pic"
        alt="加载中"
        data-src="http://img.mp.sohu.com/upload/20170724/32d4409f34194b029ed287abf1c99b70_th.png"
      />
    </div>
    <div class="img">
      <img
        class="pic"
        alt="加载中"
        data-src="https://pic6.wed114.cn/20180829/2018082910075991913520.jpg"
      />
    </div>
    <div class="img">
      <img
        class="pic"
        alt="加载中"
        data-src="https://tse4-mm.cn.bing.net/th/id/OIP.PZdPKj3sXEX2jLrepx3MUwHaEo?pid=Api&rs=1"
      />
    </div>
    <div class="img">
      <img
        class="pic"
        alt="加载中"
        data-src="https://pic6.wed114.cn/20180829/2018082910075831439349.jpg"
      />
    </div>
    <div class="img">
      <img
        class="pic"
        alt="加载中"
        data-src="https://pic6.wed114.cn/20180829/2018082910075468043336.jpg"
      />
    </div>
    <div class="img">
      <img
        class="pic"
        alt="加载中"
        data-src="https://tse2-mm.cn.bing.net/th/id/OIP.CRYz5Bv4vylsMh83G4CsLgHaFj?pid=Api&rs=1"
      />
    </div>
  </div>
</body>