<lazy-image>

📖 Docs

<lazy-image> is a custom element that contains an image that has the option to be loaded only on-demand, for performance reasons. If a <lazy-image> is inactive, then its source is not loaded (the XHR won't be made until the active attribute is set on the element). See the code on GitHub.
<lazy-image> has 3 attributes:

🎬 On-demand loading

For an image to load, it must have the active property set to true. In the example below, the images will only load when clicked (you can check the network tab in your favourite developer tools to see that there's no initial request for these files). This happens because they each start off with the active property set to false, and have a click event listener, that sets it to true:

Code

  <lazy-image src="..." alt="..." id="i"></lazy-image>

  <script>
    i.addEventListener('click', function() {
      if (!this.active)
        this.active = true;
    });
  </script>
    
Demo


If you want to have some global setting that controlls all <lazy-image> on the page (i.e. activates or deactivates all of them), you can set the window.LazyImageSiteDefaultActive global before loading the lazy-image.js script.

🎬 Intersection Observer

Intersection observers let you figure out when an element enters into view. Combined with a <lazy-image>, this lets you only load images that are scrolled into view, while leaving images at the bottom of the page that haven't been seen inactive.

Code

  // Create an observer.
  var observer = new IntersectionObserver(onChange, {
    threshold: [0.5]  // rootMargin: '50% 0%'
  });

  // That observes all the random images we've created.
  els.forEach(el => observer.observe(el));

  // Whenever we scroll...
  function onChange(changes) {
    changes.forEach(change => {
      var el = change.target;
      if (!el.active)
        el.active = true;
      observer.unobserve(el);  // Don't care anymore.
    });
  }
IntersectionObserver demo (only works in Chrome, Edge and FF nightly. 👀 caniuse for updates)

😘, monica