javascript 31 lines · 9 steps

Building a scroll-driven reading progress bar

A throttled scroll listener measures how far you've read an article and fills a progress bar to match.

Explained by highlit
Intermediate dom scroll-events throttling requestanimationframe accessibility
1const progressBar = document.getElementById('reading-progress');
2const article = document.querySelector('article');
3 
4function updateProgress() {
5 const { top, height } = article.getBoundingClientRect();
6 const viewport = window.innerHeight;
7 const scrolled = Math.min(Math.max(-top, 0), height - viewport);
8 const total = Math.max(height - viewport, 1);
9 const percent = (scrolled / total) * 100;
10 
11 progressBar.style.width = `${percent}%`;
12 progressBar.setAttribute('aria-valuenow', Math.round(percent));
13}
14 
15function throttle(fn, limit) {
16 let queued = false;
17 return function throttled() {
18 if (queued) return;
19 queued = true;
20 requestAnimationFrame(() => {
21 fn();
22 queued = false;
23 });
24 };
25}
26 
27const onScroll = throttle(updateProgress, 100);
28 
29window.addEventListener('scroll', onScroll, { passive: true });
30window.addEventListener('resize', onScroll, { passive: true });
31updateProgress();
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STEP 01

Walkthrough

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Three takeaways
  1. 1getBoundingClientRect gives live scroll offsets relative to the viewport without tracking scroll math yourself.
  2. 2Throttling with requestAnimationFrame caps expensive work to once per frame, keeping scroll handlers smooth.
  3. 3Updating aria-valuenow alongside the visual width keeps the progress bar accessible to screen readers.

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