JS实现图片懒加载(lazyload)过程详解

<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    img {
      display: block;
      margin-bottom: 50px;
      width: 400px;
      height: 400px;
    }
  </style>
</head>

<body>

  <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
  <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
  <img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt="">
  <img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt="">
  <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
  <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
  <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
  <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
  <img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt="">
  <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">
  <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt="">

</body>

<script>
  var num = document.getElementsByTagName('img').length;
  var img = document.getElementsByTagName("img");
  var n = 0; //存储图片加载到的位置，避免每次都从第一张图片开始遍历

  lazyload(); //页面载入完毕加载可是区域内的图片

  window.onscroll = lazyload;

  function lazyload() { //监听页面滚动事件
    var seeHeight = document.documentElement.clientHeight; //可见区域高度
    var scrollTop = document.documentElement.scrollTop || document.body.scrollTop; //滚动条距离顶部高度
    for (var i = n; i < num; i++) {
      if (img[i].offsetTop < seeHeight + scrollTop) {
        if (img[i].getAttribute("src") == "default.jpg") {
          img[i].src = img[i].getAttribute("data-src");
        }
        n = i + 1;
      }
    }
  }
</script>

<script>
  var n = 0,
    imgNum = $("img").length,
    img = $('img');

  lazyload();

  $(window).scroll(lazyload);

  function lazyload(event) {
    for (var i = n; i < imgNum; i++) {
      if (img.eq(i).offset().top < parseInt($(window).height()) + parseInt($(window).scrollTop())) {
        if (img.eq(i).attr("src") == "default.jpg") {
          var src = img.eq(i).attr("data-src");
          img.eq(i).attr("src", src);

          n = i + 1;
        }
      }
    }
  }
</script>

// 简单的节流函数
//fun 要执行的函数
//delay 延迟
//time 在time时间内必须执行一次
function throttle (fun, delay, time) {
 let timeout

 let startTime = new Date()

 return function () {
  let context = this

  let args = Array.prototype.slice.call(arguments)

  let curTime = new Date()

  clearTimeout(timeout)
  if (curTime - startTime >= time) {
   // 如果达到了规定的触发时间间隔，触发 handler
   fun.apply(context, args)
   startTime = curTime
  } else {
   // 没达到触发间隔，重新设定定时器
   timeout = setTimeout(function () {
    fun.apply(context, args)
   }, delay)
  }
 }
};
// 实际想绑定在 scroll 事件上的 handler
function lazyload(event) {}
// 采用了节流函数
window.addEventListener('scroll',throttle(lazyload,500,1000));