Most Developers Still Use Scroll Events Instead of Intersection Observer

It’s 2025, stop manually calculating scroll positions. The browser can handle visibility detection for you automatically.

Introduction

If you’ve ever built “lazy loading,” “infinite scrolling,” or “animate on scroll” features,
you’ve probably written something like this:

window.addEventListener("scroll", () => {
  const rect = el.getBoundingClientRect();
  if (rect.top < window.innerHeight) loadImage();
});

It works… but it’s not pretty.

Every scroll triggers dozens of function calls.
You throttle, debounce, optimize, yet it still feels clunky, especially on mobile.

The truth: most developers are still using outdated scroll logic when a better, built-in alternative has existed for years the Intersection Observer API.

In this guide, we’ll compare the old vs modern way, explain how Intersection Observer actually works, and show why switching will make your web app instantly smoother.

1️. The Old Way: Manual Scroll Tracking

Before Intersection Observer, you had to do everything yourself:

Listen for the scroll event
Manually calculate element positions
Optimize with throttling/debouncing
Handle resize and zoom events too

Example:

window.addEventListener("scroll", () => {
  const rect = image.getBoundingClientRect();
  if (rect.top < window.innerHeight) {
    loadImage();
  }
});

Even with optimizations, the browser still fires 60 scroll events per second.
Each one triggers layout calculations, not cheap operations.

It’s like checking your inbox every millisecond instead of letting the mailman notify you when something arrives.

2️. The Modern Way: Intersection Observer

The Intersection Observer API lets you watch when elements enter or leave the viewport automatically, efficiently, and asynchronously.

const observer = new IntersectionObserver((entries) => {
  entries.forEach((entry) => {
    if (entry.isIntersecting) {
      loadImage(entry.target);
      observer.unobserve(entry.target);
    }
  });
});

document.querySelectorAll(".lazy").forEach((img) => observer.observe(img));

✅ No scroll listener
✅ No manual math
✅ Automatically optimized by the browser

The browser batches intersection checks and runs them on a low-priority thread,
so your UI stays buttery-smooth.

3️. Why Scroll Events Are Problematic

Here’s why manual scroll logic hurts performance:

✅ Intersection Observer = less work for you and your browser.

4️. Lazy Loading Images (Real Comparison)

The Scroll-Based Version

window.addEventListener("scroll", () => {
  document.querySelectorAll("img[data-src]").forEach((img) => {
    const rect = img.getBoundingClientRect();
    if (rect.top < window.innerHeight) {
      img.src = img.dataset.src;
      img.removeAttribute("data-src");
    }
  });
});

Needs cleanup logic
Fires on every scroll
Slow for long pages

The Intersection Observer Version

const observer = new IntersectionObserver((entries, obs) => {
  entries.forEach((entry) => {
    if (entry.isIntersecting) {
      const img = entry.target;
      img.src = img.dataset.src;
      img.classList.add("loaded");
      obs.unobserve(img);
    }
  });
});

document.querySelectorAll("img[data-src]").forEach((img) => observer.observe(img));

✅ Loads only when visible
✅ Stops observing automatically
✅ Zero scroll handling required

This is how Medium, Unsplash, and modern frameworks handle lazy loading today.

5️. Infinite Scroll Without Scroll Tracking

You can even build infinite scrolling without touching scrollTop.

const sentinel = document.querySelector("#sentinel");

const observer = new IntersectionObserver(async (entries) => {
  if (entries[0].isIntersecting) {
    const newPosts = await fetchMorePosts();
    renderPosts(newPosts);
  }
});

observer.observe(sentinel);

✅ Trigger data fetch when the “sentinel” div becomes visible
✅ No math, no pixel counting
✅ Perfect for feeds, search results, or chat apps

6️. Triggering Animations Smoothly

Want to fade elements in as they scroll into view?
No more scroll Listeners just observe.

const boxes = document.querySelectorAll(".fade-in");

const observer = new IntersectionObserver((entries) => {
  entries.forEach((entry) => {
    if (entry.isIntersecting) {
      entry.target.classList.add("visible");
    }
  });
});

boxes.forEach((box) => observer.observe(box));

.fade-in {
  opacity: 0;
  transform: translateY(20px);
  transition: all 0.6s ease;
}
.fade-in.visible {
  opacity: 1;
  transform: translateY(0);
}

✅ Smooth animations
✅ Efficient, hardware-accelerated
✅ Zero frame drops

7️. How Intersection Observer Works Internally

The browser’s rendering engine already knows when elements are visible during paint cycles.
Intersection Observer simply exposes that visibility data to you via callbacks.

It tracks element boundaries relative to a root (usually the viewport) and fires an event when those boundaries intersect.

You can customize it with:

const options = {
  root: null, // default: viewport
  rootMargin: "50px", // preload slightly before visible
  threshold: 0.25, // trigger when 25% visible
};

✅ Gives fine-grained control over when to trigger

8️. Real-World Performance Benefits

In Chrome’s performance profiling, replacing scroll handlers with Intersection Observer:

Reduced layout reflows by up to 70%
Lowered CPU time on scroll by ~60%
Improved frame rate consistency on mobile

You’ll feel the difference instantly smoother scrolls, faster paint, happier users.

9️. Where Scroll Events Still Make Sense

Intersection Observer isn’t a total replacement.
Use scroll events when you need:

Continuous effects (like a parallax background)
Real-time scroll progress indicators
Custom gestures tied to scroll position

Otherwise, 95% of “scroll logic” should be replaced with Intersection Observer.

10. Quick Visual Recap

Conclusion

If you’re still using scroll events for lazy loading, infinite lists, or reveal animations, you’re making the browser work harder than it should.

✅ Intersection Observer is simpler
✅ It’s native and widely supported
✅ It’s performance-first by design

Stop listening to scroll events start listening to the browser.
It already knows exactly when your elements appear.

Switching takes minutes, but your app will feel like it’s running on rails.

Call to Action

Have you migrated from scroll events to Intersection Observer yet?
Share your before-and-after performance results in the comments 👇

And if your teammate’s still throttling scrollTop in 2025…
Send them this post before they throttle your frame rate

Skill Stuff