javascript 图片 滚动加载

以下代码出自国永哥哥的博客呐，好的东西一定要拿下来收藏 的啦http://www.cnblogs.com/focuslgy/p/3194502.html

html :
<div id="box">
            <ul>
                <li>
                    <img width="199" height="150" src="p.png" alt="" lazy-src="test_img/1.jpg">
                </li>
               
            </ul>
 </div>

　　

  javascript:
  function ImgScrollLoad(option){
        this.oImg = document.getElementById(option.wrapID).getElementsByTagName('img');
        this.sHolder_src = option.holder_src;
        this.init();
    }
    ImgScrollLoad.prototype = {
        init:function(){
            this.loadImg();
            var _that = this,
                timer = null;
            // 滚动：添加定时器，防止频繁调用loadImg函数
            window.onscroll = function(){
                clearTimeout(timer);
                timer = setTimeout(function(){
                    _that.loadImg();
                },100);
            }
        },        
        loadImg:function(){
            for(var n=0;n<this.oImg.length;n++){
                //获取图片占位符图片地址
                var hSrc = this.oImg[n].getAttribute(this.sHolder_src);
                if(hSrc){
                    var scrollTop = document.body.scrollTop||document.documentElement.scrollTop,
                        windowHeight = document.documentElement.clientHeight,
                        offsetTop = this.oImg[n].getBoundingClientRect().top + scrollTop,
                        imgHeight = this.oImg[n].clientHeight,
                        justTop = offsetTop + imgHeight;

                    if(justTop>scrollTop&&offsetTop<(scrollTop+windowHeight) || offsetTop == scrollTop){
                        this.isLoad(hSrc,n);
                    }
                }

            }
        },
        isLoad:function(src,n){
            var src = src,
                o_img = new Image(),
                _that = this;
            o_img.onload = function(){
                _that.oImg[n].setAttribute('src',src);
                _that.oImg[n].removeAttribute(_that.sHolder_src);
            }
            o_img.src = src;

        }

    }
    //实例化
    new ImgScrollLoad({'wrapID':'box','holder_src':'lazy-src'});