懒加载是前端优化的有效方式，如果一次ajax请求了10张图片的地址，如果一次性把10张图片都加载出来，首先会加载的很慢，第二浪费流量。
懒加载实现原理：
先把img的src指向空或者一个小图片，真实的地址存储在data-src自定义的属性中，
< img src=”” data-src=”http://real.com/real.jpg” />,
等到此图片出现在视野范围内了，获取img元素，把data-src里的值赋给src。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style type="text/css">
img {
display: block;
width: 500px;
height: 400px;
}
</style>
</head>
<body>
<img src="" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158197&di=8845befd9fbda1e99e565b9c2298be50&imgtype=0&src=http%3A%2F%2Fuploads.xuexila.com%2Fallimg%2F1503%2F626-15031G42255b3.jpg">
<img src="" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158196&di=4a38d248001c145c5b4dd31474dedf41&imgtype=0&src=http%3A%2F%2Fpic29.nipic.com%2F20130512%2F11178195_152908769116_2.jpg">
<img src="" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158196&di=b39e8b14d214b7aa518d7a8328939efb&imgtype=0&src=http%3A%2F%2Fimage16-c.poco.cn%2Fmypoco%2Fmyphoto%2F20141114%2F12%2F4567377520141114124637053.jpg%3F1024x684_120">
<img src="" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158196&di=f2e7a6ffbfa3f92025c91e6180b5e317&imgtype=0&src=http%3A%2F%2Ffile31.mafengwo.net%2FM00%2F3F%2F26%2FwKgBs1gXBQuAPWGGABOK4vbGpt412.groupinfo.w600.jpeg">
<img src="" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158195&di=6a073a070b9cea1214efc0994ed6dde1&imgtype=0&src=http%3A%2F%2Fimages3.ctrip.com%2Fwri%2Fimages%2F200610%2F100602312604121954734.jpg">
<img src="" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158195&di=a9436d83f55b301d1df9cd24f6e367e7&imgtype=0&src=http%3A%2F%2Fcyjctrip.qiniudn.com%2F69015%2F1379755311203p184vt4juft5p1t8q1andgp5ijn11.jpg">
<img src="" data-src="https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1497274158195&di=2b6bc6b871d4f7f267169dfcfb727f84&imgtype=0&src=http%3A%2F%2Fcyjctrip.qiniudn.com%2F106357%2F1395933091608p18k21ek6kvj1gv2ta910kn1002m.jpg">
<script type="text/javascript">
let aImg = document.querySelectorAll('img');
let len = aImg.length;
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
let timeId;
//首屏加载
let seeHeight = document.documentElement.clientHeight;
let scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
for (let i = n; i < len; i++) {
if (aImg[i].offsetTop < seeHeight + scrollTop) {
if (aImg[i].getAttribute('src') == '') {
aImg[i].src = aImg[i].getAttribute('data-src');
}
n = i + 1;
console.log('n = ' + n);
}
else
break;
}
//使用定时器实现节流
window.onscroll = function() {
if(timeId)
clearTimeout(timeId);
timeId=setTimeout(callback,300);
};
function callback() {
let seeHeight = document.documentElement.clientHeight;
let scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
for (let i = n; i < len; i++) {
if (aImg[i].offsetTop < seeHeight + scrollTop) {
if (aImg[i].getAttribute('src') == '') {
aImg[i].src = aImg[i].getAttribute('data-src');
}
n = i + 1;
console.log('n = ' + n);
}
else
break;
}
}
</script>
</body>
</html>
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