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Highlighting navigation items on scroll in React with IntersectionObserver

This week I was working on my personal website (https://www.thomasledoux.be), and I needed my navigation items to be highlighted when scrolling to the linked section. I found some solutions with a scroll listener, but none using the widely supported Intersection Observer (https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API).

So I decided to write the logic myself. What has to be done first, is referencing the <section>’s using React.useRef.

import * as React from "react";
const personalRef = React.useRef(null);
const portfolioRef = React.useRef(null);
const contactRef = React.useRef(null);

Now we have the reference to the <section>’s we can fire up the IntersectionObserver. I prefer to do this using the React.useEffect hook. The ref objects are added as dependencies, so we can reference these when they are ready. I use the 0.5 threshold, this will cause the observer to be triggered when the <section> is >50% visible. The navElement gets the navigation element which has a href pointing to the <section>’s id.

React.useEffect(() => {
  let observer;
  if (personalRef.current && portfolioRef.current && contactRef.current) {
    const options = {
      threshold: 0.5,
    };
    observer = new IntersectionObserver((entries, observer) => {
      entries.forEach((entry) => {
        const navElement = document.querySelector(
          `a[href="/#${entry.target.id}"]`
        );
        if (entry.isIntersecting) {
          if (!navElement.classList.contains("active")) {
            navElement.classList.add("active");
          }
        } else if (navElement.classList.contains("active")) {
          navElement.classList.remove("active");
        }
      });
    }, options);
    observer.observe(personalRef.current);
    observer.observe(portfolioRef.current);
    observer.observe(contactRef.current);
  }
  return () => observer.disconnect();
}, [personalRef, portfolioRef, contactRef]);

And that’s it! The active class will be added to the navigation element which points to the <>. By adding the return function at the end of the useEffect hook, we make sure the IntersectionObserver is cleaned up correctly.

Full code can be found on https://github.com/thomasledoux1/website-thomas

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