Infinite Scroll: Intersection Observer vs Events

Stop using scroll events for infinite scroll.

I see developers still listening to window scroll events to trigger data fetches. Please stop. It triggers on every pixel, forcing the browser to recalculate layout constantly (which is a performance nightmare).

The old way versus the new way

Scroll listeners require you to calculate scrollTop and innerHeight against scrollHeight. It is math-heavy and feels fragile. If you don’t wrap it in a debounce or throttle, your main thread locks up.

Then there is the Intersection Observer API. It is built exactly for this. You define a target element at the bottom of your list, and the browser tells you when it enters the viewport.

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const observer = new IntersectionObserver((entries) => {
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  if (entries[0].isIntersecting) {
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    loadMoreData();
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  }
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}, { rootMargin: '200px' });
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observer.observe(document.querySelector('#sentinel'));

Which one is better?

“But scroll events give me precise control over progress,” you might say. Sure, that is fair if you are building a complex progress bar tied to the scroll position (I’ve been there, it is frustrating). But for fetching more items, that precision is overkill.

Intersection Observer is asynchronous and offloaded to the browser’s own internals. It is vastly more efficient for infinite lists. (I should note that it might not trigger correctly if your container has no height, which is a common footgun).

My recommendation

I use Intersection Observer for 99 percent of infinite scroll tasks. It removes the need for manual calculations and keeps the UI snappy.

One caveat. If you need to support extremely old browsers without a polyfill, you might be stuck with the old way. But for any modern project, the choice is clear.

Use the Observer API. It is cleaner, faster, and keeps your code readable. Why would you want to manually manage scroll offsets in 2024?