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Lazy load multiple images

Follow up from lazy load image example.

Automatically include all images from a folder with require.context and load them only when needed using dynamic import. A great way to optimize your site (or application) bandwidth, loading time and caching.

How to

Lazy import an image

We can import an image as module thanks to asset/resource which parse it and return its resolved path.

Create a function to dynamic import() an image, returns its parsed path and set it to <img> src attribute:

const lazyLoadImage = (imageName, img) => {
  import(
    /* webpackMode: "lazy-once" */
    `./images/${imageName}`
  )
  .then(src => img.src = src.default)
  .catch(err => console.error(err));
};

export default lazyLoadImage;

By default dynamic import create a new chunk for each imageName, instead we want to create a single chunk for all images (dist/src_images_js). To do so we use a webpack magic comment called /* webpackMode: "lazy-once" */ which generates a single lazy-loadable chunk that can satisfy all calls to import(). This is useful to prevent additional and unnecessary network requests when images are called.

Generate an image element

Create a function which generate an <img> element, append it to a dom container, and run lazyLoadImage function to set its src attribute:

import lazyLoadImage from "./lazyLoadImage";

const generateImage = (container, imageName) => {
  const img = document.createElement("img");
  container.appendChild(img);

  lazyLoadImage(imageName, img);
};

export default generateImage;

Get images names

We need to get each imageName to use them in lazyLoadImage function. Use require.contex to import all .jpg images from ./images folder and match only their name with extension:

const getImagesNames = () => {
  const r = require.context("./images", false, /\.jpg$/);

  // Return an array list of filenames (with extension)
  const importAll = (r) => r.keys().map(file => file.match(/[^\/]+$/)[0]);

  return importAll(r);
};

export default getImagesNames;

Generate all images

Create a javascript file called images.js that, when imported, will lazy-load and generate all images:

import getImagesNames from "./getImagesNames";
import generateImage from "./generateImage";

const images = document.querySelector("#images");
const imagesNames = getImagesNames();

// Generate <img> element
// Lazy load each image and set its src attribute
// Append <img> to #images container
imagesNames.forEach(name => generateImage(images, name));

Lazy load all images

You can now lazy-import images.js to generate all the images at once when needed:

if (somethingHappen) import(
  /* webpackPrefetch: true */
  "./images"
);

By using the magic comment webpackPrefetch: true, a link to the lazy-chunk will be placed in the <head> tag. A prefetched chunk is downloaded while browser is idle, and stored in the browser cache for later use:

<link rel="prefetch" as="script" href="0.js">

More information about prefetch:

Notes

Inspect devtools network tab while viewing the app. Make sure to not disable cache in devtools, otherwise the prefetched chunk will not be served from the cache.

You can add webpackMode: "eager" instead of webpackPrefetch: true if you want to include images.js inside the main bundle.

Further reading