Bootstrap responsive sidebar menu to top navbar

Question

So I've been searching and searching for guidance in this with no avail.

Basically, I just want to have a sidebar, but when the screen gets small, aka smartphones screen size, it transforms into a navbar.

My HTML code for the sidebar is this:

<nav class="col-sm-3 ">
  <div class="col-sm-12" id="fixed-sidebar">
    <!-- <a href="index.html"><img id="home-logo" src="../media/logo-prueba.jpg" alt="Logo de Animanoir"></a>  -->
    <ul>
      <li class="fuente-fjalla ul-personalizada">Animación</li>
      <li class="fuente-fjalla ul-personalizada">Ilustración</li>
      <li class="fuente-fjalla ul-personalizada">Interacción</li>
      <br>
      <li class="fuente-fjalla ul-personalizada">Blog</li>
      <br>
      <li class="fuente-fjalla ul-personalizada"><a href="acerca.html">Acerca</a></li>
      <li class="fuente-fjalla ul-personalizada">Contacto</li>
    </ul>
  </div>
</nav>

My CSS:

#fixed-sidebar {
    position: fixed;
    max-width: 20%;
    color: white;
}

I have no idea how to get that into a navbar. All I know is how to make the navbar from start, but I don't want that! I don't want any of that fancy animated-overlay-multicolor-accordion-whatever things, please-

Thank you :)

Answer 1

Bootstrap 5 (update 2021)

Some of the classed have change for Bootstrap 5, but the concept is still the same. Here's an updated Bootstrap 5 sidebar to navbar example

Bootstrap 4 (original answer)

It could be done in Bootstrap 4 using the responsive grid columns. One column for the sidebar and one for the main content.

Bootstrap 4 Sidebar switch to Top Navbar on mobile

<div class="container-fluid h-100">
    <div class="row h-100">
        <aside class="col-12 col-md-2 p-0 bg-dark">
            <nav class="navbar navbar-expand navbar-dark bg-dark flex-md-column flex-row align-items-start">
                <div class="collapse navbar-collapse">
                    <ul class="flex-md-column flex-row navbar-nav w-100 justify-content-between">
                        <li class="nav-item">
                            <a class="nav-link pl-0" href="#">Link</a>
                        </li>
                        ..
                    </ul>
                </div>
            </nav>
        </aside>
        <main class="col">
            ..
        </main>
    </div>
</div>

Alternate sidebar to top
Fixed sidebar to top

For the reverse (Top Navbar that becomes a Sidebar), can be done like this example