upgrading-ethereum-book/integrations/my_autolink_headings.js

import { CONTINUE, SKIP, visit } from 'unist-util-visit';
import { fromHtmlIsomorphic } from 'hast-util-from-html-isomorphic'
import { toString } from 'hast-util-to-string';

// Add IDs and SVG permalinks to headings h3 to h6
// (rehype-autolink-headings is good, but can't be configured to ignore h1 and h2)

const anchor = fromHtmlIsomorphic('<a class="anchor" ariaHidden="true"><svg aria-hidden="true" focusable="false" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>', {fragment: true}).children[0];

// The headings to process
const headings = ['h3', 'h4', 'h5', 'h6'];

// Should match the method in bin/build/checks/links.pl
function slugIt(heading) {
  return (
    toString(heading)
      .trim()
      .toLowerCase()
      .replace(/\s+/g, '-')
      .replace(/[^a-z0-9_-]/g, '')
  );
}

function autolinkHeadings() {
  return function(tree) {
    try {
      visit(tree, 'element', node => {
        if (headings.indexOf(node.tagName) === -1) {
          return CONTINUE;
        }
        const newAnchor =  structuredClone(anchor);
        if (node.properties.id) {
          newAnchor.properties = { ...newAnchor.properties, href: '#' + node.properties.id };
        } else {
          const id = slugIt(node);
          newAnchor.properties = { ...newAnchor.properties, href: '#' + id };
          node.properties.id = id;
        }
        node.children = [ newAnchor ].concat(node.children);
        return SKIP;
      })
    } catch (err) {
      console.error(err);
    }
  }
}

export default function() {
  return {
    name: 'myAutolinkHeadings',
    hooks: {
      'astro:config:setup': ({ updateConfig }) => {
        updateConfig({
          markdown: {
            rehypePlugins: [
              autolinkHeadings,
            ],
          },
        });
      },
    },
  };
}