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The result

First things first, here’s a recording of the end result so that you get an idea of what I’m talking about:

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The Markup

The markup is really basic:

• A main element surrounds our content div and nav.
• Each piece of content is wrapped in a section which gets an id attribute. The sidebar navigation then links to its id

<main>
	<div>
		<h1>Smooth Scrolling Sticky ScrollSpy Navigation</h1>
		<section id="introduction">
			<h2>Introduction</h2>
			<p>…</p>
		</section>
		<section id="request-response">
			<h2>Request &amp; Response</h2>
			<p>…</p>
		</section>
		<section id="authentication">
			<h2>Authentication</h2>
			<p>…</p>
		</section>
		…
		<section id="filters">
			<h2>Filters</h2>
			<p>…</p>
		</section>
	</div>
	<nav class="section-nav">
		<ol>
			<li><a href="#introduction">Introduction</a></li>
			<li><a href="#request-response">Request &amp; Response</a></li>
			<li><a href="#authentication">Authentication</a></li>
		…
			<li class=""><a href="#filters">Filters</a></li>
		</ol>
	</nav>
</main>

Sprinkle some CSS on top to lay everything out – using CSS Grid here – and you have a fully working – albeit dull – page:

See the Pen
Smooth Scrolling Sticky ScrollSpy Navigation (base layer) by Bramus (@bramus)
on CodePen.

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1. Smooth Scrolling

Enabling smooth scrolling is really easy, it you can enable it using a single line of CSS:

html {
	scroll-behavior: smooth;
}

😱 Yes, that’s it!

In the demo embedded below, click any of the links in the nav and see how smooth it scrolls:

See the Pen
Smooth Scrolling Sticky ScrollSpy Navigation (base layer) by Bramus (@bramus)
on CodePen.

For browsers that don’t support this you could add this JS fallback:

// Smooth scrolling for browsers that don't support CSS smooth scrolling
if (window.getComputedStyle(document.documentElement).scrollBehavior !== 'smooth') {
    document.querySelectorAll('a[href^="#"]').forEach(internalLink => {
        const targetElement = document.querySelector(internalLink.getAttribute('href'));
        if (targetElement) {
            internalLink.addEventListener('click', (e) => {
                targetElement.scrollIntoView({
                    behavior: 'smooth',
                });
                e.preventDefault();
            });
        }
    });
}

However, browser’s that don’t support scroll-behavior: smooth; also don’t support behavior: "smooth" for Element#scrollIntoView, so there’s not real advantage to adding this JS fallback.

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2. Sticky Navigation

To make the navigation stay in place as you scroll we can rely on position: sticky;. As with Smooth Scrolling, this is a really simple CSS addition:

main > nav {
	position: sticky;
	top: 2rem;
	align-self: start;
}

In the demo embedded below, click any of the links in the nav and see how the nav now also stays in view while the rest of the page scrolls:

See the Pen
Smooth Scrolling Sticky ScrollSpy Navigation (base layer + smooth scrolling + sticky nav) by Bramus (@bramus)
on CodePen.

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3. ScrollSpy with IntersectionObserver

Thanks to the almighty IntersectionObserver we can implement a ScrollSpy. Basically we use it to watch all section["id"] elements. If they are intersecting, we add the .active class to any link that links to it. For styling purposes we don’t add .active to the link itself, but to its surrounding li element.

In code, that becomes this little snippet:

window.addEventListener('DOMContentLoaded', () => {

	const observer = new IntersectionObserver(entries => {
		entries.forEach(entry => {
			const id = entry.target.getAttribute('id');
			if (entry.intersectionRatio > 0) {
				document.querySelector(`nav li a[href="#${id}"]`).parentElement.classList.add('active');
			} else {
				document.querySelector(`nav li a[href="#${id}"]`).parentElement.classList.remove('active');
			}
		});
	});

	// Track all sections that have an `id` applied
	document.querySelectorAll('section[id]').forEach((section) => {
		observer.observe(section);
	});
	
});

.section-nav a {
    transition: all 100ms ease-in-out;
}

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Complete Demo

Putting everything together, we end up with this:

See the Pen
Smooth Scrolling Sticky ScrollSpy Navigation by Bramus (@bramus)
on CodePen.

Delightful, no? 😊

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