Intersection Observer

Summary

An efficient browser API for detecting when an element enters or leaves the viewport — powers lazy-loading, infinite scroll and scroll-triggered animations without expensive scroll listeners. It lets the browser efficiently determine visibility relationships without repeatedly calculating element positions inside high-frequency scroll handlers.

• 1Use it to lazy-load images only as they scroll into view. Verify the behavior with realistic data and handle the failure or empty state as well.
• 2Build a simple 'fade in on scroll' effect for page sections. Keep the first version small, then add one improvement after the core behavior works.
• 3Learn `threshold` and `rootMargin` options for fine-tuning when it triggers.