Web Resources - Lazy Loading
Table of Contents
1 - About
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”.
2 - Articles Related
3 - Solutions
Solutions for detecting if an element is visible in the viewport (in order to lazy-load its content)
3.1 - Dom API
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.
3.2 - Web Api: IntersectionObserver
IntersectionObserver is a browser API that allows us to efficiently detect when an observed element enters or exits the browser's viewport.
4 - Example
4.1 - Image and video
Library for image and video by popularity
- More …
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
}
});
4.2 - Ads
Lazy loading of ads ad