hsmusic-wiki/static/lazy-loading.js

// Lazy loading! Roll your own. Woot.
// This file includes a 8unch of fall8acks and stuff like that, and is written
// with fairly Olden JavaScript(TM), so as to work on pretty much any 8rowser
// with JS ena8led. (If it's disa8led, there are gener8ted <noscript> tags to
// work there.)

var observer;

function loadImage(image) {
    image.src = image.dataset.original;
}

function lazyLoad(elements) {
    for (var i = 0; i < elements.length; i++) {
        var item = elements[i];
        if (item.intersectionRatio > 0) {
            observer.unobserve(item.target);
            loadImage(item.target);
        }
    }
}

function lazyLoadMain() {
    // This is a live HTMLCollection! We can't iter8te over it normally 'cuz
    // we'd 8e mutating its value just 8y interacting with the DOM elements it
    // contains. A while loop works just fine, even though you'd think reading
    // over this code that this would 8e an infinitely hanging loop. It isn't!
    var elements = document.getElementsByClassName('js-hide');
    while (elements.length) {
        elements[0].classList.remove('js-hide');
    }

    var lazyElements = document.getElementsByClassName('lazy');
    if (window.IntersectionObserver) {
        observer = new IntersectionObserver(lazyLoad, {
            rootMargin: '200px',
            threshold: 1.0
        });
        for (var i = 0; i < lazyElements.length; i++) {
            observer.observe(lazyElements[i]);
        }
    } else {
        for (var i = 0; i < lazyElements.length; i++) {
            var element = lazyElements[i];
            var original = element.getAttribute('data-original');
            element.setAttribute('src', original);
        }
    }
}

document.addEventListener('DOMContentLoaded', lazyLoadMain);