懒加载图片示例-优快云博客

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
body{
    margin: 0;
}
ul{
    margin: 0;
    padding: 0;
    list-style: none;
}
img{
    border: none;
    vertical-align: middle;
}
.in{
    border: 1px solid black;
    margin: 10px;
    text-align: center;
    height: 400px;
    width: 400px;
    float: left;
}
.in img{
    height: 400px;
    width: 400px;
}
</style>
</head>
<body>
<ul class="list">
    <li class="in"><img src="img/loading.gif" alt="测试" data-original = "img/img1.gif"></li>
    <li class="in"><img src="img/loading.gif" alt="测试" data-original = "img/img2.gif"></li>
    <li class="in"><img src="img/loading.gif" alt="测试" data-original = "img/img3.gif"></li>
    <li class="in"><img src="img/loading.gif" alt="测试" data-original = "img/img4.gif"></li>
    <li class="in"><img src="img/loading.gif" alt="测试" data-original = "img/img1.gif"></li>
    <li class="in"><img src="img/loading.gif" alt="测试" data-original = "img/img2.gif"></li>
    <li class="in"><img src="img/loading.gif" alt="测试" data-original = "img/img3.gif"></li>
    <li class="in"><img src="img/loading.gif" alt="测试" data-original = "img/img4.gif"></li>  
</ul>
<script>
var oBtn = document.getElementsByTagName('button')[0];
var aImages = document.images;
loadImg(aImages);
window.onscroll = function(){
    loadImg(aImages);
};
function loadImg(arr){
    for( var i = 0,len = arr.length; i < len; i++){
        if(arr[i].getBoundingClientRect().top < document.documentElement.clientHeight && !arr[i].isLoad){
            arr[i].isLoad = true;
            arr[i].style.cssText = "transition: ''; opacity: 0;"
            if(arr[i].dataset){
                aftLoadImg(arr[i],arr[i].dataset.original);    
            }else{
                aftLoadImg(arr[i],arr[i].getAttribute("data-original"));
            }
            (function(i){
                setTimeout(function(){
                    arr[i].style.cssText = "transition: 1s; opacity: 1;"
                },16)
            })(i);
        }
    }
}
function aftLoadImg(obj,url){
    var oImg = new Image();
    oImg.onload = function(){
        obj.src = oImg.src;
    }
    oImg.src = url;
}
</script>    
</body>
</html>
效果图：
JS 图片懒加载
