Science Knowings: JavaScript Course For Social Media

Lazy Loading

Lazy Loading: Introduction

Welcome to our session on Lazy Loading, a powerful technique to optimize website performance and enhance user experience. Let's dive in!

What is Lazy Loading?

Lazy Loading is a technique that defers the loading of non-critical resources (e.g., images, videos) until they are needed in the visible viewport. This approach reduces initial page load time and improves perceived performance.

Benefits of Lazy Loading

  • Reduced initial page load time
  • Enhanced user experience (faster page loads)
  • Improved page speed scores
  • Lower bandwidth usage

How Lazy Loading Works

Lazy Loading uses intersection observers to monitor when an element enters the visible viewport. When detected, the element and its resources are asynchronously loaded and displayed.

Intersection Observer API

The Intersection Observer API allows you to create callback functions that trigger when an element intersects the visible viewport. This API is crucial for implementing Lazy Loading.

Lazy Loading Images

<img src="placeholder.jpg" data-src="image.jpg" class="lazy-image">

Using the data-src attribute, you can specify the actual image source, while the placeholder image (placeholder.jpg) is initially displayed.

Lazy Loading Videos


Similarly, for videos, use the data-src attribute to provide the actual video source and a placeholder source for initial display.

Lazy Loading Other Resources

Lazy Loading can also be applied to other resource-heavy elements, such as iframes and scripts, to further optimize page load performance.

Browser Support

Lazy Loading is supported by modern browsers, including Chrome, Firefox, Safari, and Edge. For older browsers, polyfills can be used to provide support.

Best Practices

  • Use lazy loading for non-critical resources
  • Consider image and video dimensions
  • Monitor performance metrics
  • Test thoroughly across browsers
  • Combine lazy loading with other optimization techniques

Tools for Lazy Loading

  • IntersectionObserver.js
  • Lazysizes
  • vanilla-lazyload

These libraries provide pre-built solutions to easily implement Lazy Loading on your website.

Case Study

Lazy Loading was implemented on a news website, resulting in a 25% reduction in initial page load time and a 10% increase in user engagement.


Lazy Loading is a powerful technique that can significantly improve the performance and user experience of your website. By adopting best practices and using the tools available, you can optimize your website's loading process.

Next Topic: Code Splitting

In our next session, we'll discuss Code Splitting, a technique to reduce the initial bundle size of your JavaScript applications. Follow us to continue learning!