JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="kunder">
    <li><span class="udtalelse">Indhold 1</span></li>
    <li><span class="udtalelse">Indhold 2</span></li>
    <li><span class="udtalelse">Indhold 3</span></li>
</ul>
<span class="kunderpagination">
    <a href="#" id="prev">« Previous</a> |
    <a href="#" id="next">Next »</a>
</span>

CSS

ul{
    list-style-type: none;
}
.active{
    display: inline;
}    
.passiv{
    display: none;    
}

JavaScript

var all = $('.udtalelse').addClass("passiv");

var i = -1;

$('#prev').click(function() {
    ctrlKunder( i = !i ? all.length - 1 : --i );
});
$('#next').click(function() {
    ctrlKunder( i = ++i % all.length );
}).click();

function ctrlKunder(ele) {
    all.removeClass("active").addClass("passiv");
    all.eq(ele).removeClass("passiv").addClass("active");
}