JSFiddle - React, Tailwind, and code Playground

HTML

<select size="1" name="filter"id="priority_filter" onchange="filter_user_trainings();">
   <option value="all">All</option>
   <option value="optional">Optional</option>
   <option value="mandatory">Mandatory</option>
   <option value="essential">Essential</option>
   <option value="custom">Custom</option>
</select>

JavaScript

$(document).ready(function(){
    $("#priority_filter").change(function(){
        var val = $("#priority_filter").val();
        alert(val);
    });
});