Could the Native scrollend Event Finally Kill Your Most Fragile Debounce Hack?

We’ve all spent years convinced that a 100ms debounce on a scroll listener is the "correct" way to handle post-scroll logic. It’s a lie. It’s a fragile, guess-work-filled hack that we’ve collectively accepted because, for a long time, the browser simply refused to tell us when it was actually done moving.

If you’ve ever built a carousel, a sticky header that hides on scroll, or an infinite loader, you’ve felt this pain. You attach a listener to scroll, it fires 120 times a second, and you have to use a timer to "guess" when the user has finally stopped flicking their thumb.

The native scrollend event is finally here to kill that mess.

The Debounce Era (Or: Why we were guessing)

Before scrollend, if you wanted to trigger an action after a scroll finished—say, snapping to a specific slide—you usually reached for something like this:

let isScrolling;

window.addEventListener('scroll', () => {
  // Clear the timeout throughout the scroll
  window.clearTimeout(isScrolling);

  // Set a timeout to run after scrolling ends
  isScrolling = setTimeout(() => {
    console.log('Scrolling has (probably) stopped!');
    // Do something expensive here
  }, 100); 
});

It looks fine on the surface, but it's a performance nightmare.

First, you’re firing code on every single tick of the scroll. Second, that 100ms is a total stab in the dark. Set it too low, and the event fires while the user’s finger is still on the screen during a slow drag. Set it too high, and your UI feels laggy and unresponsive. And don’t even get me started on how this handles momentum scrolling on trackpads or mobile devices, where the "stop" is a gradual deceleration that mocks your fixed timeout.

Enter scrollend

The scrollend event is exactly what it sounds like. It’s a native browser event that fires when a scroll operation has completed. This includes:
- The user’s finger leaving the screen.
- The momentum animation finishing.
- A programmatic scroll (like scrollTo()) reaching its destination.

Here is how much simpler your life becomes:

document.addEventListener('scrollend', (event) => {
  console.log('Scroll finished. No timers, no hacks.');
  // Safe to trigger your heavy logic or UI updates now
});

The browser is essentially saying, "I’m done moving pixels, you can take over now." Because it's handled at the engine level, it's significantly more accurate than our setTimeout best-guesses.

A Practical Example: The "Active Section" Highlight

Imagine you have a side navigation that highlights the current section. Using a standard scroll listener is overkill and jittery. Using scrollend makes it smooth and efficient.

const sections = document.querySelectorAll('section');
const navLinks = document.querySelectorAll('.nav-link');

document.addEventListener('scrollend', () => {
  let currentSectionId = "";

  sections.forEach((section) => {
    const sectionTop = section.offsetTop;
    if (window.scrollY >= sectionTop - 100) {
      currentSectionId = section.getAttribute('id');
    }
  });

  navLinks.forEach((link) => {
    link.classList.toggle('active', link.href.includes(currentSectionId));
  });
});

In this case, we only run the calculation logic once—when the user has actually finished moving. No more recalculating 60 times per second just to find out the user is still in the middle of Section 2.

Why this is a performance win (The "Why")

The real magic isn't just the cleaner code; it's the main thread.

When you use a debounce hack, you’re still putting a burden on the event loop. The scroll event is still firing, your anonymous function is still being called, and clearTimeout is still being invoked repeatedly.

With scrollend, the browser's compositor thread handles the heavy lifting of tracking the scroll position. It only notifies the main thread (where your JavaScript lives) when the state actually changes to "stopped." This keeps the scroll smooth and the CPU happy.

The "Gotchas" and Browser Support

As of late 2024, support is actually pretty great. Chrome, Firefox, and Safari (as of 17.4) all support it.

However, if you're supporting older browsers, you can't just delete your debounce utility yet. You’ll want a progressive enhancement approach. I usually check for support and fall back if necessary:

if ('onscrollend' in window) {
  document.addEventListener('scrollend', handleScrollEnd);
} else {
  // Back to the 2015 life
  document.addEventListener('scroll', debounce(handleScrollEnd, 100));
}

One weird detail: scrollend does not bubble. This means if you have a scrollable div inside your page, you need to attach the listener to that specific element, or use the capture phase if you're trying to delegate it.

// This works for the whole window
window.addEventListener('scrollend', () => {});

// This works for a specific container
const myCoolList = document.querySelector('.scrollable-list');
myCoolList.addEventListener('scrollend', () => {
  console.log('List stopped moving');
});

Is the Debounce Dead?

Not entirely. You still need debouncing for input fields, window resize events, and API searches. But for scrolling? Its days are numbered.

The scrollend event represents a shift in Web APIs toward giving us "intent-based" events rather than just raw data streams. It’s more reliable, it’s easier to read, and it makes our apps feel less like a collection of timers held together by duct tape.

Stop fighting the browser and let it tell you when it's done. Your users (and your CPU) will thank you.