docs/src/components/RightSidebar/TableOfContents.tsx


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import type { FunctionalComponent } from 'preact';
import { h, Fragment } from 'preact';
import { useState, useEffect, useRef } from 'preact/hooks';

const TableOfContents: FunctionalComponent<{ headers: any[] }> = ({
  headers = [],
}) => {
  const itemOffsets = useRef([]);
  const [activeId, setActiveId] = useState<string>(undefined);

  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);
    };
  }, []);

  return (
    <>
      <h2 class="heading">On this page</h2>
      <ul>
        <li
          class={`header-link depth-2 ${
            activeId === 'overview' ? 'active' : ''
          }`.trim()}
        >
          <a href="#overview">Overview</a>
        </li>
        {headers
          .filter(({ depth }) => depth > 1 && depth < 4)
          .map((header) => (
            <li
              class={`header-link depth-${header.depth} ${
                activeId === header.slug ? 'active' : ''
              }`.trim()}
            >
              <a href={`#${header.slug}`}>{header.text}</a>
            </li>
          ))}
      </ul>
    </>
  );
};

export default TableOfContents;