lazy-load.js

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;