(() => {
  const headers = document.querySelectorAll('h2');
  for (const el of headers) {
    el.id = el.textContent;
  }
  const links = Array.from(headers, el => el.textContent)
                     .map(t => `<li><a href="#${t}">${t}</a></li>`)
                     .join('');
  document.body.insertAdjacentHTML('afterbegin', `<ul>${links}</ul>`);
})();