懒加载+函数节流

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<head>
<script src="//code.jquery.com/jquery-1.9.1.min.js"></script>
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  <title>JS Bin</title>
  <style>
    .container {
      max-width: 800px;
      margin: 0 auto;
    }
    .container:after{
      content: '';
      display: block;
      clear: both;
    }
    .container img {
      float: left;
      width: 50%;
    }
    h1{
      clear: both;
    }
  </style>
</head>
<body>
  <div class="container">
    <img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="1" data-src="http://cdn.jirengu.com/book.jirengu.com/img/1.jpg">
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    <h1 id="target">hello</h1>
    <img src="http://smashinghub.com/wp-content/uploads/2014/08/cool-loading-animated-gif-3.gif" alt="15" data-src="http://cdn.jirengu.com/book.jirengu.com/img/15.jpg">
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  </div>
</body>
</html>


start()

function isShow($node) {
    var $winHeight = $(window).height() //获取视口高度
    var $scrollTop = $(window).scrollTop() //获取视口滚动距离
    var $offset = $node.offset().top     //获取图片top位置与视口顶部距离
    var $nodeHeight = $node.outerHeight(true) //获取图片自身高度
    if ($winHeight + $scrollTop >= $offset + $nodeHeight) {
        return true
    } else {
        return false
    }
}

function start() {
    let timer = null 
    $(window).on('scroll', function (e) {
      return !function () {  //函数节流       
            clearTimeout(timer)
            console.log()
            timer = setTimeout(function () {
                $('.container img').not('[data-isloaded=1]').each(function () {
                    if (isShow($(this))) {
                        loadImg($(this))
                    }
                })
            }, 500)
        }()
    })
}

function loadImg($img) {
    $img.attr('src', $img.attr('data-src'))
    $img.attr('data-isloaded',1)
}
$(window).trigger('scroll')
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