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@crullian
Created March 11, 2019 12:12
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Vanilla JS Lazy image loading component using Intersection Observer.
const defaults = {
imageLoadedClass: 'js-lazy-image--handled',
imageSelector: '.js-lazy-image',
// If the image gets within 50px in the Y axis, start the download.
rootMargin: '50px 0px',
threshold: 0.01
};
let config,
images,
imageCount,
observer;
/**
* Fetches the image for the given URL
* @param {string} url
*/
function fetchImage(url) {
return new Promise((resolve, reject) => {
const image = new Image();
image.src = url;
image.onload = resolve;
image.onerror = reject;
});
}
/**
* Preloads the image
* @param {object} image
*/
function preloadImage(image) {
const src = image.dataset.src;
if (!src) {
return;
}
return fetchImage(src).then(() => { applyImage(image, src); });
}
/**
* Load all of the images immediately
* @param {NodeListOf<Element>} images
*/
function loadImagesImmediately(images) {
// foreach() is not supported in IE
for (let i = 0; i < images.length; i++) {
let image = images[i];
preloadImage(image);
}
}
/**
* Disconnect the observer
*/
function disconnect() {
if (!observer) {
return;
}
observer.disconnect();
}
/**
* On intersection
* @param {array} entries
*/
function onIntersection(entries) {
// Disconnect if we've already loaded all of the images
if (imageCount === 0) {
disconnect();
return;
}
// Loop through the entries
for (let i = 0; i < entries.length; i++) {
let entry = entries[i];
// Are we in viewport?
if (entry.intersectionRatio > 0) {
imageCount--;
// Stop watching and load the image
observer.unobserve(entry.target);
preloadImage(entry.target);
}
}
}
/**
* Apply the image
* @param {object} img
* @param {string} src
*/
function applyImage(img, src) {
// Prevent this from being lazy loaded a second time.
img.classList.add(config.imageLoadedClass);
img.src = src;
}
let LazyLoad = {
init: (options) => {
config = {...defaults, ...options};
images = document.querySelectorAll(config.imageSelector);
imageCount = images.length;
// If we don't have support for intersection observer, loads the images immediately
if (!('IntersectionObserver' in window)) {
loadImagesImmediately(images);
} else {
// It is supported, load the images
observer = new IntersectionObserver(onIntersection, config);
// foreach() is not supported in IE
for (let i = 0; i < images.length; i++) {
let image = images[i];
if (image.classList.contains(config.imageLoadedClass)) {
continue;
}
observer.observe(image);
}
}
}
};
export default LazyLoad;
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