JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="sidebar-navigation">
    <li>Overskrift
        <ul class="sidebar-inner-list">
            <li> <a href="#">Link</a>

            </li>
            <li> <a href="#">Link</a>

            </li>
        </ul>
    </li>
    <li>Overskrift 2
        <ul class="sidebar-inner-list">
            <li> <a href="#">Link 2</a>

            </li>
            <li> <a href="#">Link 2</a>

            </li>
            <li> <a href="#">Link 2</a>

            </li>
        </ul>
    </li>
</ul>

JavaScript

var allPanels = $('.sidebar-inner-list > li').hide();

$('.sidebar-navigation > li').click(function () {
    allPanels.slideUp();
    $(this).find('.sidebar-inner-list li').slideDown();
    
});