Hugo Partial for Generating the Table of Contents

single.html

...

{{- partial "toc.html" . -}}

...

toc.html

{{/* https://github.com/gohugoio/hugo/issues/1778 */}}

{{/* ignore empty links with + */}}
{{- $headers := findRE "<h[2-4].*?>(.|\n])+?</h[2-4]>" .Content -}}
{{ .Scratch.Set "last_level" 1 }}

{{/* at least one header to link to */}}
{{- $has_headers := ge (len $headers) 1 -}}
{{- if $has_headers -}}
<aside class="table-of-contents">
    <details>
    <summary>
    <b>Table of Contents</b>
    </summary>
    {{- range $headers -}}
        {{- $header := . -}}
        {{- $base := ($.Page.File.LogicalName) -}}
        {{- $anchorId := ($header | plainify | htmlUnescape | anchorize) -}}
        {{- $href := delimit (slice $base $anchorId) "#" | string -}}
        {{- range findRE "[2-4]" . 1 -}}
            {{- $next_heading := (int .) -}}
            {{- if gt $next_heading ($.Scratch.Get "last_level") -}}
                <ul class="toc-h{{ . }}">
            {{- else if lt $next_heading ($.Scratch.Get "last_level") -}}
                </ul>
            {{- end -}}
            <li><a href="{{ relref $.Page $href }}">{{- $header | plainify | htmlUnescape -}}</a></li>
            {{ $.Scratch.Set "last_level" $next_heading }}
        {{- end -}}
    {{- end -}}
    </details>
</aside>
{{- end -}}

Neither of these work correctly for me - they both leave <ul> tags unclosed. However I found one one disaev.me that works.

I've edited it so that it works with custom anchors which none of the others do, and removed all extra HTML so it just returns nested <ul>

### some heading {#custom-anchor}

{{- $headers := findRE "<h[2-4].*?>(.|\n])+?</h[2-4]>" .Content -}}
{{- $has_headers := ge (len $headers) 1 -}}
{{- if $has_headers -}}
<ul >
  {{- range $i, $header := $headers -}}
    {{- $headerLevel := index (findRE "[2-4]" . 1) 0 -}}
    {{- $headerLevel := len (seq $headerLevel) -}}

    {{/* get id="xyz" */}}
    {{ $id := index (findRE "(id=\"([^\"]*?)\")" $header 9) 0 }}

    {{/* strip id="" to leave xyz (no way to get regex capturing groups in hugo :( */}}
    {{ $cleanedID := replace (replace $id "id=\"" "") "\"" "" }}

    {{- if ne $i 0 -}}
      {{- $prevHeaderLevel := index (findRE "[2-4]" (index $headers (sub $i 1)) 1) 0 -}}
      {{- $prevHeaderLevel := len (seq $prevHeaderLevel) -}}
        {{- if gt $headerLevel $prevHeaderLevel -}}
          {{- range seq (sub $headerLevel $prevHeaderLevel) -}}
            <ul>
          {{- end}}
        {{- end}}
        {{- if lt $headerLevel $prevHeaderLevel -}}
          {{- range seq (sub $prevHeaderLevel $headerLevel) -}}
            </li></ul></li>
          {{- end}}
        {{- end}}
        {{- if eq $headerLevel $prevHeaderLevel -}}
          </li>
        {{- end}}
        <li>
          <a href="#{{- $cleanedID  -}}">{{- $header | plainify | htmlEscape | safeHTML -}}</a>
        {{- if eq $i (sub (len $headers) 1) -}}
          {{- range seq (sub $prevHeaderLevel $headerLevel) -}}
            </li></ul></li>
          {{- end}}
        {{- end}}
    {{- else}}
    <li>
      <a href="#{{- $cleanedID -}}">{{- $header | plainify | htmlEscape | safeHTML -}}</a>
    {{- end -}}
  {{- end -}}
  {{- $firstHeaderLevel := len (seq (index (findRE "[2-4]" (index $headers 0) 1) 0)) -}}
  {{- $lastHeaderLevel := len (seq (index (findRE "[2-4]" (index $headers (sub (len $headers) 1)) 1) 0)) -}}
  {{- range seq (sub $lastHeaderLevel $firstHeaderLevel) -}}
    </li></ul></li>
  {{- end -}}
</ul>
{{- end -}}

But @Iooeee 's code rendered too many end tags 😂

I have investigated into it and wrote a blog post

If the blog is too abstract, here is the code

{{- $headers := findRE "<h[1-4].*?>(.|\n])+?</h[1-4]>" .Content -}}
{{- $has_headers := ge (len $headers) 1 -}}
{{- if $has_headers -}}

{{- $largest := 6 -}}
{{- range $headers -}}
  {{- $headerLevel := index (findRE "[1-4]" . 1) 0 -}}
  {{- $headerLevel := len (seq $headerLevel) -}}
  {{- if lt $headerLevel $largest -}}
    {{- $largest = $headerLevel -}}
  {{- end -}}
{{- end -}}

{{- $firstHeaderLevel := len (seq (index (findRE "[1-4]" (index $headers 0) 1) 0)) -}}

{{- $.Scratch.Set "bareul" slice -}}
<div id="TableOfContents">
<ul>
  {{- range seq (sub $firstHeaderLevel $largest) -}}
    <ul>
    {{- $.Scratch.Add "bareul" (sub (add $largest .) 1) -}}
  {{- end -}}
  {{- range $i, $header := $headers -}}
    {{- $headerLevel := index (findRE "[1-4]" . 1) 0 -}}
    {{- $headerLevel := len (seq $headerLevel) -}}

    {{/* get id="xyz" */}}
    {{ $id := index (findRE "(id=\"(.*?)\")" $header 9) 0 }}

    {{/* strip id="" to leave xyz (no way to get regex capturing groups in hugo :( */}}
    {{ $cleanedID := replace (replace $id "id=\"" "") "\"" "" }}
    {{- $header := replaceRE "<h[1-4].*?>((.|\n])+?)</h[1-4]>" "$1" $header -}}

    {{- if ne $i 0 -}}
      {{- $prevHeaderLevel := index (findRE "[1-4]" (index $headers (sub $i 1)) 1) 0 -}}
      {{- $prevHeaderLevel := len (seq $prevHeaderLevel) -}}
        {{- if gt $headerLevel $prevHeaderLevel -}}
          {{- range seq $prevHeaderLevel (sub $headerLevel 1) -}}
            <ul>
            {{/* the first should not be recorded */}}
            {{- if ne $prevHeaderLevel . -}}
              {{- $.Scratch.Add "bareul" . -}}
            {{- end -}}
          {{- end -}}
        {{- else -}}
          </li>
          {{- if lt $headerLevel $prevHeaderLevel -}}
            {{- range seq (sub $prevHeaderLevel 1) -1 $headerLevel -}}
              {{- if in ($.Scratch.Get "bareul") . -}}
                </ul>
                {{/* manually do pop item */}}
                {{- $tmp := $.Scratch.Get "bareul" -}}
                {{- $.Scratch.Delete "bareul" -}}
                {{- $.Scratch.Set "bareul" slice}}
                {{- range seq (sub (len $tmp) 1) -}}
                  {{- $.Scratch.Add "bareul" (index $tmp (sub . 1)) -}}
                {{- end -}}
              {{- else -}}
                </ul></li>
              {{- end -}}
            {{- end -}}
          {{- end -}}
        {{- end -}}
        <li>
          <a href="#{{- $cleanedID  -}}">{{- $header | safeHTML -}}</a>
    {{- else -}}
    <li>
      <a href="#{{- $cleanedID -}}">{{- $header | safeHTML -}}</a>
    {{- end -}}
  {{- end -}}
  <!-- {{- $firstHeaderLevel := len (seq (index (findRE "[1-4]" (index $headers 0) 1) 0)) -}} -->
  {{ $firstHeaderLevel := $largest }}
  {{- $lastHeaderLevel := len (seq (index (findRE "[1-4]" (index $headers (sub (len $headers) 1)) 1) 0)) -}}
  </li>
  {{- range seq (sub $lastHeaderLevel $firstHeaderLevel) -}}
    {{- if in ($.Scratch.Get "bareul") (add . $firstHeaderLevel) -}}
      </ul>
    {{- else -}}
      </ul></li>
    {{- end -}}
  {{- end -}}
</ul>
</div>
{{- end -}}