JSFiddle - React, Tailwind, and code Playground

HTML

<div class="reservation" data-duration="10"></div>
<div class="reservation" data-duration="20"></div>
<div class="reservation" data-duration="30"></div>
<div class="reservation" data-duration="40"></div>
<div class="reservation" data-duration="50"></div>

JavaScript

$('.reservation').each(function(index,elem){
    alert($(elem).data('duration'));
});