JSFiddle - React, Tailwind, and code Playground

HTML

Username: <input type="text" id="Username" class="mandatory" />Username: <input type="text" id="Username" class="mandatory" />

JavaScript

$(document).ready(function(){
    $('.mandatory').blur(function(){
        if($(this).val() == "") {
          alert('Field: ' + $(this).attr('id') + ' is mandatory!');
        }
    });
});