@xiee/utils/js/heading-anchor.js

Miscellaneous tools and utilities to manipulate HTML pages

document.querySelectorAll('h1,h2,h3,h4,h5,h6').forEach(h => {
  if (h.id && !h.querySelector('.anchor'))
    h.insertAdjacentHTML('beforeend', ` <span class="anchor"><a href="#${h.id}"></a></span>`);
});