SOURCE

let a = '90988222231';
let b = '33312202873';

function sum(x, y) {
    let i = x.length - 1;
    let j = y.length - 1;
    let res = '';
    let sum = 0;
    let flag = 0;
    while(i >= 0 || j>=0 || flag) {
        let num1 = +(x[i--]) || 0
        let num2 = +(y[j--]) || 0
        sum = num1 + num2 + flag
        if(sum >= 10) {
            sum = sum - 10
            flag = 1
        } else {
            flag = 0
        }
        res = sum + res
    }
    return res
}

let re = sum(a, b)
console.log(re)


 let num = document.getElementsByTagName('img').length;
 let imgs = document.getElementsByTagName("img");
 let n = 0; //存储图片加载到的位置，避免每次都从第一张图片开始遍历

 lazyload(); //页面载入完毕加载可是区域内的图片
 window.onscroll = lazyload;

    function lazyload() { //监听页面滚动事件
        var seeHeight = document.documentElement.clientHeight; //可见区域高度
        var scrollTop = document.documentElement.scrollTop || document.body.scrollTop; //滚动条距离顶部高度
        for (var i = n; i < num; i++) {
            if (imgs[i].getBoundingClientRect().top < seeHeight) {
                if (imgs[i].getAttribute("src") == "default.jpg") {
                    imgs[i].src = imgs[i].getAttribute("data-src");
                }
                n = i + 1;
            }
        }
    }

    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt=""/>
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt=""/>
    <img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt=""/>
    <img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt=""/>
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt=""/>
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt=""/>
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt=""/>
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt=""/>
    <img src="default.jpg" data-src="http://ww1.sinaimg.cn/large/006y8mN6gw1fa7kaed2hpj30sg0l9q54.jpg" alt=""/>
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt=""/>
    <img src="default.jpg" data-src="http://ww4.sinaimg.cn/large/006y8mN6gw1fa5obmqrmvj305k05k3yh.jpg" alt=""/>