echo/website/layouts/partials/sidenav.html

<nav id="sidenav" class="w3-sidenav w3-collapse">
  <span class="w3-closenav w3-xxlarge w3-hide-large" onclick="closeSidenav()">
    &times;
  </span>
  <a class="support w3-btn w3-white w3-border w3-border-theme w3-round-xlarge" href="/support-echo">
    <i class="fa fa-heart" aria-hidden="true"></i> Support Echo
  </a>
	<h4>
		<select class="w3-select w3-border" onchange="version(this);">
			<option value="/v2" selected>v2</option>
			<option value="/">v3</option>
    </select>
	</h4>
  {{ $currentNode := . }}
  {{ range .Site.Menus.side }}
    <h3>{{ .Pre }} {{ .Name }}</h3>
      {{ range .Children }}
        <a{{ if $currentNode.IsMenuCurrent "side" . }} class="active"{{ end }} href="{{ .URL }}">
          {{ .Name }}
        </a>
        {{ if .HasChildren }}
          {{ range .Children }}
            <a {{ if $currentNode.IsMenuCurrent "side" . }} class="active"{{ end }} href="{{ .URL }}">
              {{ .Name }}
            </a>
          {{ end }}
        {{ end }}
      {{ end }}
  {{ end }}
</nav>