Astro Table Of Contents Component + Sample Usage

[...slug].astro

---
import type { CollectionEntry } from 'astro:content';
import { getCollection, render } from 'astro:content';

// @ - alias for "src" directory
import TableOfContents from '@/components/TableOfContents.astro';

export async function getStaticPaths() {
  const posts = await getCollection('YOUR_COLLECTION_NAME_HERE');

  return posts.map((post) => ({
		params: { slug: post.id },
		props: post,
	}));
}

type Props = CollectionEntry<'YOUR_COLLECTION_NAME_HERE'>;

const post = Astro.props;
const { Content, headings } = await render(post);
---

<h1>{post.data.title}</h1>
<TableOfContents headings={headings} />
<article>
	<Content />
</article>

TableOfContents.astro

---
import type { MarkdownHeading } from 'astro';

type Props = {
  headings: MarkdownHeading[];
};

type HeadingWithSubheadings = MarkdownHeading & {
  subheadings: MarkdownHeading[];
};

const { headings } = Astro.props;

const grouppedHeadings = headings.reduce((array, heading) => {
  if (heading.depth === 2) {
    array.push({ ...heading, subheadings: [] });
  } else if (heading.depth === 3) {
    array.at(-1)?.subheadings.push(heading);
  }

  return array;
}, [] as HeadingWithSubheadings[]);
---

<nav id="table-of-contents" aria-label="Table Of Contents">
  <ol>
    {
      grouppedHeadings.map((h) => (
        <li>
          <a href={`#${h.slug}`}>{h.text}</a>
          {h.subheadings.length > 0 && (
            <ol>
              {h.subheadings.map((sub) => (
                <li>
                  <a href={`#${sub.slug}`}>{sub.text}</a>
                </li>
              ))}
            </ol>
          )}
        </li>
      ))
    }
  </ol>
</nav>

<script is:inline>
  // This script tag is useful only if you want to display the TOC alongside the blog post...
  // ... and highlight the section that the user is currently reading through.
  // Feel free to remove this tag if you don't need this type of functionality.

  const observer = new IntersectionObserver(
    (entries) => {
      for (const entry of entries) {
        const headingFragment = `#${entry.target.id}`;
        const tocItem = document.querySelector(`a[href="${headingFragment}"]`);

        if (entry.isIntersecting) {
          const previouslyActivatedItem =
            document.querySelector('.active-toc-item');

          previouslyActivatedItem?.classList.remove('active-toc-item');
          tocItem.classList.add('active-toc-item');
        } else {
          const isAnyOtherEntryIntersecting = entries.some(
            (e) => e.target.id !== entry.target.id && e.isIntersecting
          );

          if (isAnyOtherEntryIntersecting) {
            tocItem.classList.remove('active-toc-item');
          }
        }
      }
    },
    { root: null, rootMargin: '0px', threshold: [1] }
  );

  const sectionHeadings = document.querySelectorAll(
    'article > h2, article > h3'
  );

  for (const heading of sectionHeadings) {
    observer.observe(heading);
  }
</script>

<style>
.active-toc-item {
  font-weight: bold;
}
</style>

Thank you so much 😭😭😭

Thank you so much 😭😭😭

You're welcome! Glad this gist is still helpful all these years later.