JSFiddle - React, Tailwind, and code Playground

HTML

<div class="side-nav">
    <div class="side-nav-item">
        <h3>Title 1</h3>
        <div class="side-nav-block-item">content</div>
        <div class="side-nav-block-item">content</div>
    </div>
    <div class="side-nav-item">
        <h3>Title 2</h3>
        <div class="side-nav-block-item">content</div>
    </div>
</div>

JavaScript

$('.side-nav h3').addClass('on');

$('.side-nav h3').click(function() {
    if ($(this).hasClass('on')) {
        $(this).toggleClass('on');
        $(this).siblings('.side-nav-block-item').slideUp('normal');
    } else {
        $(this).toggleClass('on');
        $(this).siblings('.side-nav-block-item').slideDown('normal');
    }
});