图片过多，页面可视区域懒加载的JS

<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>