JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.20.1/moment.min.js"></script>

JavaScript

function log (msg) {
    $('body').append('<p>' + msg + '<p>');
}

log(moment(NaN).isValid());
console.log(moment("2018-01-01"));
log(moment(NaN).toString());
log(moment(NaN)._d.toString());