原生javascript实现图片滚动、延时加载功能

//保存document在变量里，减少对document的查询
            var doc = document;
            for(var n=0,i = this.oImg.length;n<i;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 = doc.body.scrollTop||doc.documentElement.scrollTop,
                        windowHeight = doc.documentElement.clientHeight,
                        offsetTop = this.oImg[n].getBoundingClientRect(). + scrollTop,
                        imgHeight = this.oImg[n].clientHeight,
                        justTop = offsetTop + imgHeight;
                    // 判断图片是否在可见区域
                    if(justTop>scrollTop&&offsetTop<(scrollTop+windowHeight)){

                        this.isLoad(hSrc,n);
                    }
                }

            }

function LGY_imgScrollLoad(option){
        this.oImg = document.getElementById(option.wrapID).getElementsByTagName('img');
        this.sHolder_src = option.holder_src;
        this.int();
    }
    LGY_imgScrollLoad.prototype = {
        loadImg:function(){
            //保存document在变量里，减少对document的查询
            var doc = document;
            for(var n=0,i = this.oImg.length;n<i;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 = doc.body.scrollTop||doc.documentElement.scrollTop,
                        windowHeight = doc.documentElement.clientHeight,
                        offsetTop = this.oImg[n].getBoundingClientRect(). + scrollTop,
                        imgHeight = this.oImg[n].clientHeight,
                        justTop = offsetTop + imgHeight;
                    // 判断图片是否在可见区域
                    if(justTop>scrollTop&&offsetTop<(scrollTop+windowHeight)){
                        //alert(offsetTop);
                        this.isLoad(hSrc,n);
                    }
                }

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

        },
        int: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);
            }
        }
    }