图片预加载即提前加载图片，可保证图片快速、无缝地发布，用户需要查看时可直接从本地缓存中渲染，适用于图片占据很大比例的网站。
方法1,在CSS background中加载：会增加页面的整体加载时间
#preload-01 { background: url(-01.png) no-repeat -9999px -9999px; } #preload-02 { background: url(-02.png) no-repeat -9999px -9999px; }
方法2，JS new image对象，设置src加载：
function preloader() { if (document.images) { var img1 = new Image(); var img2 = new Image(); var img3 = new Image(); img1.src = ";; img2.src = ";; img3.src = ";; } } function addLoadEvent(func) { var oldonload = window.onload; if (typeof window.onload != 'function') { window.onload = func; } else {//onload中未挂载函数才执行该JS window.onload = function() { if (oldonload) { oldonload(); } func(); } } } addLoadEvent(preloader);
div.appendChild(img1);//插入到DOM
方法3，Ajax预加载,new Image()对象设置src
window.onload = function() { setTimeout(function() { // XHR to request a JS and a CSS var xhr = new XMLHttpRequest(); xhr.open('GET', url;); xhr.send(); xhr = new XMLHttpRequest(); xhr.open('GET', url;); xhr.send(); // preload image new Image().src = ";; }, 1000); };
懒加载：将图片src赋值为一张默认图片，当用户滚动滚动条到可视区域图片时候，再去加载真正的图片
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Lazyload 2</title>
<style>
img {
display: block;
margin-bottom: 50px;
height: 200px;
}
</style>
</head>
<body>
<img src="images/loading.gif" data-src="images/1.png">
<img src="images/loading.gif" data-src="images/2.png">
<img src="images/loading.gif" data-src="images/3.png">
<img src="images/loading.gif" data-src="images/4.png">
<img src="images/loading.gif" data-src="images/5.png">
<img src="images/loading.gif" data-src="images/6.png">
<img src="images/loading.gif" data-src="images/7.png">
<img src="images/loading.gif" data-src="images/8.png">
<img src="images/loading.gif" data-src="images/9.png">
<img src="images/loading.gif" data-src="images/10.png">
<img src="images/loading.gif" data-src="images/11.png">
<img src="images/loading.gif" data-src="images/12.png">
<script>
function throttle(fn, delay, atleast) {//函数绑定在 scroll 事件上，当页面滚动时，避免函数被高频触发，
var timeout = null,//进行去抖处理
startTime = new Date();
return function() {
var curTime = new Date();
clearTimeout(timeout);
if(curTime - startTime >= atleast) {
fn();
startTime = curTime;
}else {
timeout = setTimeout(fn, delay);
}
}
}
function lazyload() {
var images = document.getElementsByTagName('img');
var len = images.length;
var 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
return function() {
var seeHeight = document.documentElement.clientHeight;
var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
for(var i = n; i < len; i++) {
if(images[i].offsetTop < seeHeight + scrollTop) {
if(images[i].getAttribute('src') === 'images/loading.gif') {
images[i].src = images[i].getAttribute('data-src');
}
n = n + 1;
}
}
}
}
var loadImages = lazyload();
loadImages(); //初始化首页的页面图片
window.addEventListener('scroll', throttle(loadImages, 500, 1000), false);
//函数节流（throttle）与函数去抖（debounce）处理,
//500ms 的延迟，和 1000ms 的间隔，当超过 1000ms 未触发该函数，则立即执行该函数，不然则延迟 500ms 执行该函数
</script>
</body>
</html>