Yes, this is when you try to improve your Page Speed Insight scrore. I use ReactJS and meet this issue. I search for solution, and yes, there is a lot of solutions.

Offscreen image? Defer?

Offscreen image is that image that not showing in current view (Eg: you have to scroll down to see it).

Defer is that load the image only when you need to see it.

Solutions I found on Google

1. react-lazy-load-image-component (not work for me)

Many answers lead to this package. Simple install, simple use, easy to understand. But not work for me. I tried setup following the guide, use @loadable/components` for dynamic import, wrap the offscreen images in it's wrapper.

1. lazysides (not compatible to my current code)

Following the post at https://web.dev/offscreen-images/, I tried this one. Watching the network tab on Chrome's developer tool, I can see it works, the image is loaded only when I scroll to it's section. But it didn't display properly, wrong css, and I have to rewrite css for each image if I apply this.

I want a solution that I don't need to modify the css, because my project is a bunch of specific-custom for every image.

Get my hand dirty

I decided to do it on my own. I found this article, and thanks god, it works for me.

For image that load by <img /> tag

1. Write a function to load the image correctly

export const loadLazyImage = function () {
   if ("IntersectionObserver" in window) {
   const observer = new IntersectionObserver((entries) => {
      entries.forEach((entry) => {
        if (entry.intersectionRatio > 0) {
          if (entry.target.hasAttribute("data-src")) {
            entry.target.setAttribute(
              "src",
              entry.target.getAttribute("data-src")
            );
            observer.unobserve(entry.target);
          }
        }
      });
    });
    document.querySelectorAll(".lazy-image").forEach((imageElement) => {
      if (imageElement.getAttribute("is-observed") != "true" && imageElement.getAttribute("data-src") != null)
      {
        imageElement.setAttribute("is-observed", "true")
        observer.observe(imageElement);  
      }
    });
  } else {
    var imgList = document.querySelectorAll(".lazy-image");
    Array.prototype.forEach.call(imgList, function (image) {
      image.setAttribute("src", image.getAttribute("data-src"));
    });
  }
};

1. In the component contain off screen images, call the loadLazyImage when it mounted, and modify the <img /> tag as comment in the code

export function OffscreenComponent() {
  useEffect(() => {
    loadLazyImage();
  }, []);

  return (
    ...
    <img className="lazy-image" data-source="your-image" src="" />
    // Put the real src in data-src field
    // You can set the src to placeholder image
    ...
  );
}

Explaination: When the web loaded, all the components were mounted, it will register an Intersection Observer to watch elements we want (in this case, all elements with class lazy-image). When the observer see that the element is visible, we set the src with data-src 's value, so now the image is loaded completely.

For image that load by background-image property

Similar to with img tag, set data-src with the correct image. Then change

image.setAttribute("src", image.getAttribute("data-src"));

to

image.setAttribute("style", `background-image:${image.getAttribute("data-src")}`);