Web Resources - Lazy Loading

A Lazy loading method will fetch only the visible resource (image, advertisements) in the web browser window

Lazy loading is technique that defers loading of non-critical resources at page load time. Instead, these non-critical resources are loaded at the moment of need. Where images are concerned, “non-critical” is often synonymous with “off-screen”.

HTML Element may have a lazy loading attribute.

Example with the loading attribute for img and iframe

<img src="image.png" srcset="image.png 1000w, image-2x.png 2000w" loading="lazy">

Solutions for detecting if an element is visible in the viewport (in order to lazy-load its content)

Solutions have often listened for scroll or resize events, then used DOM APIs like getBoundingClientRect() to calculate where elements are relative to the viewport. This works, but is not efficient. It have been error-prone, often causing the browser to become sluggish.

IntersectionObserver is a browser API that allows us to efficiently detect when an observed element enters or exits the browser's viewport.

Library for image and video by popularity

• vanilla-lazyload
• https://github.com/ApoorvSaxena/lozad.js (43)
• https://github.com/aFarkas/lazysizes (98)
• More …
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Example from Lazy Loading Images and Video after the DOMContentLoaded event.

<img class="lazy" src="placeholder-image.jpg" data-src="image-to-lazy-load-1x.jpg" data-srcset="image-to-lazy-load-2x.jpg 2x, image-to-lazy-load-1x.jpg 1x" alt="I'm an image!">

document.addEventListener("DOMContentLoaded", function() {
  var lazyImages = [].slice.call(document.querySelectorAll("img.lazy"));

  if ("IntersectionObserver" in window) {
    let lazyImageObserver = new IntersectionObserver(function(entries, observer) {
      entries.forEach(function(entry) {
        if (entry.isIntersecting) {
          let lazyImage = entry.target;
          lazyImage.src = lazyImage.dataset.src;
          lazyImage.srcset = lazyImage.dataset.srcset;
          lazyImage.classList.remove("lazy");
          lazyImageObserver.unobserve(lazyImage);
        }
      });
    });

    lazyImages.forEach(function(lazyImage) {
      lazyImageObserver.observe(lazyImage);
    });
  } else {
    // Possibly fall back to a more compatible method here
  }
});

More … https://webpack.js.org/guides/lazy-loading/

Lazy loading of ads ad

• Ad Sense
• https://github.com/osvaldasvalutis/adsenseLoader.js

• disqus

• https://www.npmjs.com/package/react-lazyload

• lazy-loading resources.
• https://developers.google.com/web/fundamentals/performance/lazy-loading-guidance/images-and-video/