第一种

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>图片懒加载</title>
    <style>
        img {
            display: block;
            width: 100%;
            height: 300px;
            margin-bottom: 20px;
        }
    </style>
</head>
<body>
    <img data-src="./images/1.jpg" alt="">
    <img data-src="./images/2.jpg" alt="">
    <img data-src="./images/3.jpg" alt="">
    <img data-src="./images/4.jpg" alt="">
    <img data-src="./images/5.jpg" alt="">
    <img data-src="./images/6.jpg" alt="">
    <img data-src="./images/7.jpg" alt="">
    <img data-src="./images/8.jpg" alt="">
    <img data-src="./images/9.jpg" alt="">
    <img data-src="./images/10.jpg" alt="">
    <img data-src="./images/1.jpg" alt="">
    <img data-src="./images/2.jpg" alt="">
</body>
<script>
        var imgs = document.querySelectorAll('img');

        //offsetTop是元素与offsetParent的距离，循环获取直到页面顶部
        function getTop(e) {
            var T = e.offsetTop;
            while(e = e.offsetParent) {
                T += e.offsetTop;
            }
            return T;
        }

        function lazyLoad(imgs) {
            var H = document.documentElement.clientHeight;//获取可视区域高度
            var S = document.documentElement.scrollTop || document.body.scrollTop;
            for (var i = 0; i < imgs.length; i++) {
                if (H + S > getTop(imgs[i])) {
                    imgs[i].src = imgs[i].getAttribute('data-src');
                }
            }
        }

        window.onload = window.onscroll = function () { //onscroll()在滚动条滚动的时候触发
            lazyLoad(imgs);
        }
</script>
</html>

第二种

var imgs = document.querySelectorAll('img');

        //用来判断bound.top<=clientHeight的函数，返回一个bool值
        function isIn(el) {
            var bound = el.getBoundingClientRect();
            var clientHeight = window.innerHeight;
            return bound.top <= clientHeight;
        } 
        //检查图片是否在可视区内，如果不在，则加载
        function check() {
            Array.from(imgs).forEach(function(el){
                if(isIn(el)){
                    loadImg(el);
                }
            })
        }
        function loadImg(el) {
            if(!el.src){
                var source = el.dataset.src;
                el.src = source;
            }
        }
        window.onload = window.onscroll = function () { //onscroll()在滚动条滚动的时候触发
            check();
        }