手写懒加载代码```html ``` ```js let num = document.getElementsByTag

<img src = "./imag/timg.gif" data-src = "./img/time1.jpg" alt = "">
<img src = "./imag/timg.gif" data-src = "./img/time2.jpg" alt = "">
<img src = "./imag/timg.gif" data-src = "./img/time3.jpg" alt = "">
<img src = "./imag/timg.gif" data-src = "./img/time4.jpg" alt = "">

let num = document.getElementsByTagName("img").length;
let img = document.getElementsByTagName("img");
let n = 0;
lazyload();
window.onscroll = lazyload;

function lazyload(){
    let seeHeight = document.documentElement.clientHeight;
    let scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
    
    for(let i = n; i < num; i++){
        if(img[i].offsetTop < seeHeight + scrollTop){
            if(img[i].getAttribute("src") == "./img/timg.gif"){
                img[i].src = img[i].getAttribute("data-src");
            }
            n = i + 1;
        }
    }
}