Intersection Observer can improve your web applications performance by helping you to implement lazy loading of images.

In the last few months I worked hard to improved the page speed of my website (yeah, the one you're visiting right now : heart_eyes:). I improved all my client side code in order to be able to reach a performance score above 90 points on Lighthouse, the official Google Chrome tool to measure performance, accessibility, progressive web apps compliance and more on your web application. One of the last thing that was contained in the report was a warning about offscreen images, like the one contained in the following screenshot:

So I followed the link contained in the report that points to a page where are contained the official Google Guidelines about offscreen images loading . The main topic of the page is the Intersection Observer API and how it can help you to load specific content only when it becomes visible in the viewport. I found also another article on the official Google developer website that explains in details how to leverage the power of Intersection Observer to lazy load the images in your web applications. So as you may imagine I "accepted the challenge" (like only Barney Stinson in how I met your mother is used to do : stuck_out_tongue_winking_eye:) and I started to implement the lazy load of images for my website.

Implementation

First of all let's start by creating a function called lazyLoadImages. This function takes two parameter:

• selector, that is a string that I will use to select all the document Element objects that I wanted to observe
• loadCompleted, a function that will be executed after the image has been downloaded

This function will create a new instance of the IntersectionObserver object from the Intersection Observer API. This object constructor takes two parameter:

• a callback, that is the function called when an object become visible given the current configuration
• a configuration options, that let the developer customize how the Intersection Observer calculate the intersection with the viewport

After the creation of the IntersectionObserver object I attached it to the DOM elements I want to observe by calling its observer(element) method on the document Element objects selected using querySelectorAll method with the selector received as parameter.

const lazyLoadImages = (selector, loadCompleted) => {
  const intersectionObserver: IntersectionObserver = new IntersectionObserver(
    (entries, observer) => onIntersection(entries, observer, loadCompleted),
    { rootMargin: '50px 0px', threshold: 0.01 }
  )
  document.querySelectorAll(selector).forEach(image => intersectionObserver.observe(image))
}

As you can see in the snippet above, in the intersection callback I'm calling the onIntersection function. What does it do? This function checks the IntersectionObserverEntry received from the Intersection Observer as parameter. If a target Element is inside the viewport it would have the intersectionRatio > 0. When this happen I can remove the observer and start the load of the image with the loadImage function.

const onIntersection = (entries, observer, loadCompleted) => {
  entries.forEach(entry => {
    if (entry.intersectionRatio > 0) {
      observer.unobserve(entry.target)
      loadImage(entry.target, loadCompleted)
    }
  })
}

The loadImage function downloads the image by setting the image src field with the data contained in the data-src field. At the end of the download I remove the lazy css class, that I used to hide the image until it has been download. Then the loadCompleted function is called, where the caller can do anything it want with the image (for example I'm doing a custom animation in order to avoid a flash out effect when the image is show).

const loadImage = (image, loadCompleted) => {
  image.src = image.dataset.src
  image.onload = () => {
    removeCssClass(image, 'lazy')
    loadCompleted(image)
  }
}

This is the final script with the complete flow.

import 'intersection-observer'
import { removeCssClass } from './css-class'

const lazyLoadImages = (selector, loadCompleted) => {
  const intersectionObserver: IntersectionObserver = new IntersectionObserver(
    (entries, observer) => onIntersection(entries, observer, loadCompleted),
    { rootMargin: '50px 0px', threshold: 0.01 }
  )
  document.querySelectorAll(selector).forEach(image => intersectionObserver.observe(image))
}

const onIntersection = (entries, observer, loadCompleted) => {
  entries.forEach(entry => {
    if (entry.intersectionRatio > 0) {
      observer.unobserve(entry.target)
      loadImage(entry.target, loadCompleted)
    }
  })
}

const loadImage = (image, loadCompleted) => {
  image.src = image.dataset.src
  image.onload = () => {
    removeCssClass(image, 'lazy')
    loadCompleted(image)
  }
}

export { lazyLoadImages }

There's still one thing that I didn't discuss yet. How can we support this type of lazy loading for the browser that doesn't still have implemented the IntersectionObserver API? The answer is the Intersection Observer Polyfill. I installed it as a dependency of my project.

npm install --save intersection-observer

This polyfill exposes an ad hoc implementation in vanilla javascript of the Intersection Observer, or it exposes the current browser implementation if it is already in place for the browser that the user is using.

Conclusion

Intersection Observer is a powerful API. It lets you implement lazy loading for resources loading and reach performance and architectural application pattern that I had a chance to see only in mobile native apps. The web is filling the gap with native apps, and Intersection Observer are another demonstration that the 90% of the existing native mobile apps could become powerful web apps. As a consequence of the fact that in my daily job I'm still a native mobile app developer, I'm still following the iOS, Android and React Native scene and I'm still studying all the new tools and SDKs improvement released by Apple, Google and Facebook. But, you know, technology goes fast I have to be prepared for the future . Sooo, long live Intersection Observer!!! Web applications will be much more performant with your help .