JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://parsleyjs.org/dist/parsley.min.js"></script>
<form id="myform">
    <input type="text" data-parsley-myvalidator="foo" data-parsley-required />
    <input type="submit" />
</form>

JavaScript

window.ParsleyValidator.addValidator('myvalidator', 
    function (value, requirement) {
        alert('myvalidator');
        return false;
    }, 32)
    .addMessage('en', 'myvalidator', 'my validator failed');

$('#myform').parsley();