hugo-book-theme/layouts/partials/docs/menu-filetree.html

{{ $bookSection := default "docs" .Site.Params.BookSection  }}
{{ if eq $bookSection "*" }}
  {{ $bookSection = "/" }}{{/* Backward compatibility */}}
{{ end }}

{{ with .Site.GetPage $bookSection }}
  {{ template "book-section-children" (dict "Section" . "CurrentPage" $) }}
{{ end }}

{{ define "book-section-children" }}{{/* (dict "Section" .Section "CurrentPage" .CurrentPage) */}}
  {{ $self := eq .CurrentPage .Section }}
  {{ $ancestor := strings.HasPrefix .CurrentPage.RelPermalink .Section.RelPermalink }}
  {{ $collapsed := .Section.Params.bookCollapseSection }}

  {{ if or $self $ancestor (not $collapsed) }}
  <ul>
    {{ range (where .Section.Pages "Params.bookhidden" "ne" true) }}
      {{ if .IsSection }}
        <li {{- if .Params.BookFlatSection }} class="book-section-flat" {{ end -}}>
          {{ template "book-page-link" (dict "Page" . "CurrentPage" $.CurrentPage) }}
          {{ template "book-section-children" (dict "Section" . "CurrentPage" $.CurrentPage) }}
        </li>
      {{ else if and .IsPage .Content }}
        <li>
          {{ template "book-page-link" (dict "Page" . "CurrentPage" $.CurrentPage) }}
        </li>
      {{ end }}
    {{ end }}
  </ul>
  {{ end }}
{{ end }}

{{ define "book-page-link" }}{{/* (dict "Page" .Page "CurrentPage" .CurrentPage) */}}
  {{ if .Page.Content }}
    <a href="{{ .Page.RelPermalink }}" class="
        {{- if and .Page.Params.bookCollapseSection .Page.IsSection }}collapsed {{ end }}
        {{- if eq .CurrentPage .Page }}active{{ end }}">
      {{- partial "docs/title" .Page -}}
    </a>
  {{ else }}
    <span>{{- partial "docs/title" .Page -}}</span>
  {{ end }}
{{ end }}