Bootstrap 4 nav collapse

bootstrap-4-nav-collapse.md

No theme

<ul id="accordionExample">
  <li data-toggle="collapse" data-target="#collapseOne">
    Lvl 1
    <ul id="collapseOne" class="collapse show" data-parent="#accordionExample">
      <li>Lvl 1.1</li>
      <li>Lvl 1.2</li>
    </ul>
  </li>
  <li data-toggle="collapse" data-target="#collapseTwo">
    Lvl 2
    <ul id="collapseTwo" class="collapse" data-parent="#accordionExample">
      <li>Lvl 2.1</li>
      <li>Lvl 2.2</li>
    </ul>
  </li>
</ul>

Dashboard theme example

<ul id="accordionExample" class="nav flex-column">
  <li class="nav-item" data-toggle="collapse" data-target="#collapseOne">
    <a class="nav-link" href="#">
      Lvl 1
    </a>

    <ul id="collapseOne" class="collapse show" data-parent="#accordionExample">
      <li>Lvl 1.1</li>
      <li>Lvl 1.2</li>
    </ul>
  </li>
  <li class="nav-item" data-toggle="collapse" data-target="#collapseTwo">
    <a class="nav-link" href="#">
      Lvl 2
    </a>

    <ul id="collapseTwo" class="collapse" data-parent="#accordionExample">
      <li>Lvl 2.1</li>
      <li>Lvl 2.2</li>
    </ul>
  </li>
</ul>