cupper-hugo-theme/layouts/partials/toc.html

{{ $headings := findRE "<h2.*?>(.|\n])+?</h2>" .Content }}
{{ if ge (len $headings) 2 }}
  <nav class="toc" aria-labelledby="toc-heading">
    <h2 id="toc-heading">Table of contents</h2>
    <ol>
      {{ range $headings }}
        <li>
          {{ $id := findRE "id=\".*\"" . }}
          {{ $id = index $id 0 }}
          {{ $id = strings.TrimPrefix "id=\"" $id }}
          {{ $id = strings.TrimSuffix "\"" $id }}
          <a href="#{{ $id }}">
            {{ . | htmlUnescape | plainify }}
          </a>
        </li>
      {{ end }}
    </ol>
  </nav>
{{ end }}