Combining Lazy Loading with Asynchronous Delivery

1. Introduction

Lazy loading and asynchronous delivery are techniques used to optimize web performance. Lazy loading defers the loading of images and other media until they are needed, while asynchronous delivery allows resources to load without blocking the rendering of the page. This lesson explores how to effectively combine these two techniques for optimal media handling.

2. Key Concepts

2.1 Lazy Loading

Lazy loading is a design pattern that postpones loading non-essential resources at the point in time when they are needed. This can significantly improve the initial loading time of a page.

2.2 Asynchronous Delivery

Asynchronous delivery allows web pages to load resources in parallel without blocking the display of content. This enhances user experience by ensuring that critical content is rendered quickly.

3. Step-by-Step Process

3.1 Implementing Lazy Loading

To implement lazy loading, use the loading="lazy" attribute on image tags.

<img src="image.jpg" alt="Sample Image" loading="lazy">

3.2 Integrating Asynchronous Delivery

To load images asynchronously, consider using the Intersection Observer API to detect when an image enters the viewport.

const images = document.querySelectorAll('img[data-src]');
const config = { rootMargin: '0px 0px 200px 0px', threshold: 0 };

let imageObserver = new IntersectionObserver((entries, observer) => {
    entries.forEach(entry => {
        if (entry.isIntersecting) {
            const img = entry.target;
            img.src = img.dataset.src;
            img.onload = () => img.classList.add('loaded');
            observer.unobserve(img);
        }
    });
}, config);

images.forEach(image => {
    imageObserver.observe(image);
});

Note: Ensure images have a placeholder or a low-resolution version to improve user experience while they load.

4. Best Practices

Use descriptive alt attributes for accessibility.
Implement a fallback for browsers that do not support lazy loading.
Optimize images for size and format for faster loading times.
Consider using a content delivery network (CDN) for faster media delivery.

5. FAQ

What browsers support lazy loading?

Most modern browsers support lazy loading, but it’s a good idea to check compatibility and provide fallbacks for older browsers.

Does lazy loading affect SEO?

When implemented correctly, lazy loading should not negatively affect SEO, especially when using proper src attributes and alt tags.

Can I lazy load other media types?

Yes, lazy loading can be applied to videos, iframes, and other media types using similar techniques.