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import type { MarkdownHeading } from 'astro';
import type { FunctionalComponent } from 'preact';
import { unescape } from 'html-escaper';
import { useState, useEffect, useRef } from 'preact/hooks';
type ItemOffsets = {
id: string;
topOffset: number;
};
const TableOfContents: FunctionalComponent<{ headings: MarkdownHeading[] }> = ({
headings = [],
}) => {
const toc = useRef<HTMLUListElement>();
const onThisPageID = 'on-this-page-heading';
const itemOffsets = useRef<ItemOffsets[]>([]);
const [currentID, setCurrentID] = useState('overview');
useEffect(() => {
const getItemOffsets = () => {
const titles = document.querySelectorAll('article :is(h1, h2, h3, h4)');
itemOffsets.current = Array.from(titles).map((title) => ({
id: title.id,
topOffset: title.getBoundingClientRect().top + window.scrollY,
}));
};
getItemOffsets();
window.addEventListener('resize', getItemOffsets);
return () => {
window.removeEventListener('resize', getItemOffsets);
};
}, []);
useEffect(() => {
if (!toc.current) return;
const setCurrent: IntersectionObserverCallback = (entries) => {
for (const entry of entries) {
if (entry.isIntersecting) {
const { id } = entry.target;
if (id === onThisPageID) continue;
setCurrentID(entry.target.id);
break;
}
}
};
const observerOptions: IntersectionObserverInit = {
// Negative top margin accounts for `scroll-margin`.
// Negative bottom margin means heading needs to be towards top of viewport to trigger intersection.
rootMargin: '-100px 0% -66%',
threshold: 1,
};
const headingsObserver = new IntersectionObserver(setCurrent, observerOptions);
// Observe all the headings in the main page content.
document.querySelectorAll('article :is(h1,h2,h3)').forEach((h) => headingsObserver.observe(h));
// Stop observing when the component is unmounted.
return () => headingsObserver.disconnect();
}, [toc.current]);
const onLinkClick = (e) => {
setCurrentID(e.target.getAttribute('href').replace('#', ''));
};
return (
<>
<h2 id={onThisPageID} className="heading">
On this page
</h2>
<ul ref={toc}>
{headings
.filter(({ depth }) => depth > 1 && depth < 4)
.map((heading) => (
<li
className={`header-link depth-${heading.depth} ${
currentID === heading.slug ? 'current-header-link' : ''
}`.trim()}
>
<a href={`#${heading.slug}`} onClick={onLinkClick}>
{unescape(heading.text)}
</a>
</li>
))}
</ul>
</>
);
};
export default TableOfContents;
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