js——图片懒加载

<img class="js-lazy-image centered" src="./img/dog-running.svg" width="400" height="400" data-src="./img/dog-running.jpg">
<img class="js-lazy-image centered" src="./img/dog-running.svg" width="400" height="400" data-src="./img/dog-running.jpg">
<img class="js-lazy-image centered" src="./img/dog-running.svg" width="400" height="400" data-src="./img/dog-running.jpg">

    .fade-in {
      animation-name: fadeIn;
      animation-duration: 1.3s;
      animation-timing-function: cubic-bezier(0, 0, 0.4, 1);
      animation-fill-mode: forwards;
    }

    @keyframes fadeIn {
      from {
        opacity:;
      }

      to {
        opacity:;
      }
    }

.centered {
   display:block;
   margin:0 auto;
}

'use strict';
var images = document.querySelectorAll('.js-lazy-image'), config = {
    rootMargin: '50px 0px',
    threshold: 0.01
}, imageCount = images.length, observer;
if (!('IntersectionObserver'in window))
    loadImagesImmediately(images);
else {
    observer = new IntersectionObserver(onIntersection,config);
    for (var image, i = 0; i < images.length; i++)
        (image = images[i],
        !image.classList.contains('js-lazy-image--handled')) && observer.observe(image)
}
function fetchImage(a) {
    return new Promise(function(b, c) {
        var d = new Image;
        d.src = a,
        d.onload = b,
        d.onerror = c
    }
    )
}
function preloadImage(a) {
    var b = a.dataset.src;
    return b ? fetchImage(b).then(function() {
        applyImage(a, b)
    }) : void 0
}
function loadImagesImmediately(a) {
    for (var d, b = Array.from(a), c = 0; c < a.length; c++)
        d = a[c],
        preloadImage(d)
}
function disconnect() {
    observer && observer.disconnect()
}
function onIntersection(a) {
    0 === imageCount && observer.disconnect();
    for (var c, b = 0; b < a.length; b++)
        c = a[b],
        0 < c.intersectionRatio && (imageCount--,
        observer.unobserve(c.target),
        preloadImage(c.target))
}
function applyImage(a, b) {
    a.classList.add('js-lazy-image--handled'),
    a.src = b,
    a.classList.add('fade-in')
}