javascript 32 lines · 7 steps

Building a scroll spy with IntersectionObserver

Highlight the nav link for whichever section is currently in view, driven by the browser's IntersectionObserver.

Explained by highlit
Intermediate intersectionobserver dom event-driven cleanup closures
1function initScrollSpy() {
2 const links = Array.from(document.querySelectorAll('.nav a[href^="#"]'));
3 const sections = links
4 .map((link) => document.querySelector(link.getAttribute('href')))
5 .filter(Boolean);
6 
7 const setActive = (id) => {
8 links.forEach((link) => {
9 link.classList.toggle('is-active', link.getAttribute('href') === `#${id}`);
10 });
11 };
12 
13 const observer = new IntersectionObserver(
14 (entries) => {
15 const visible = entries
16 .filter((entry) => entry.isIntersecting)
17 .sort((a, b) => b.intersectionRatio - a.intersectionRatio);
18 
19 if (visible.length > 0) {
20 setActive(visible[0].target.id);
21 }
22 },
23 {
24 rootMargin: '-40% 0px -55% 0px',
25 threshold: [0, 0.25, 0.5, 0.75, 1],
26 }
27 );
28 
29 sections.forEach((section) => observer.observe(section));
30 
31 return () => observer.disconnect();
32}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1IntersectionObserver lets the browser report visibility changes instead of you polling scroll position on every frame.
  2. 2Sorting entries by intersectionRatio picks the most-visible section when several qualify at once.
  3. 3Returning a disconnect function gives callers a clean way to tear down the observer.

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