Astute Readers, Lazy Loaders

Astute readers of this blog may notice that I have been employing certain CSS properties on this site as I write about them. Today I added lazy loading for images using the Intersection Observer API, and I will tell you about it.

Since this site uses infinite scroll for posts, I decided to load the images as they come into view rather than all at once. Here is a great overview of the Intersection Observer API with some examples. In brief, it observes if a target element intersects an ancestor—in this case when an image enters the viewport—and performs a callback. Because the API is still experimental, not all browsers support it and you will have to write a fallback behavior.

First, in HTML images are given the class lazy-image, and instead of src attribute, they use data-src to define the sources to be loaded as they scroll into view.

<img class="lazy-image" data-src="/blog/images/lacroy.jpg" alt="enjoy la croy">

With Javascript, an event listener is set up on window load that collects all lazy-image class images and creates an intersection observer.

let images;
window.addEventListener("load", function(event) {
  images = document.querySelectorAll(".lazy-image");
}, false);

The observer is attached to each target image. The callback function handleIntersect is executed whenever a target is observed to cross a certain threshold as defined in options. Here root: null refers to the viewport and has a margin around it of 0px. A threshold of 0.05 means that the callback is run when 5% of the target is visible within the viewport. The fallback behavior is to load all images at once.

function createObserver() {
  let observer;
  const options = {
    root: null,
    rootMargin: "0px",
    threshold: 0.05
  if ('IntersectionObserver' in window) {
    observer = new IntersectionObserver(handleIntersect, options);
    images.forEach(image => {
  } else {
    images.forEach(image => {
      image.src = image.dataset.src;

The intersectionRatio is a number between 0.0 and 1.0 that indicates how much of the target is visible within the root. The callback checks that the target is visible and sets src to the data-src to load the image. It also adds fade-in class to the image for a tiny animation effect and removes the observer. Since I’m only checking whether the image is visible in the viewport and do not need to know by how much, I could instead use the Boolean property entry.isIntersecting for the if statement.

function handleIntersect(entries, observer) {
  entries.forEach(entry => {
    if (entry.intersectionRatio > 0) {
      let img =;
      img.src = img.dataset.src;

And this is the CSS animation. The image fades in from invisible and moves up slightly as if sliding into view.

.fade-in {
  animation: fade-in 1.2s ease-in-out;
@keyframes fade-in {
  0% {top: 8px; opacity: 0;}
  100% {top: 0; opacity: 1}

You can see this lazy loader in action as you scroll through this blog.

Aug 23 2017