JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://d1l4stvdmqmdzl.cloudfront.net/4.0.0-rc2/mongoose.min.js"></script>
<div id="output">   
</div>

JavaScript

var print = function(msg) {
    document.getElementById('output').insertAdjacentHTML('beforeend', msg + '<br>');
};

var schema = new mongoose.Schema({
    name: {
        type: String,
        required: true
    }
});

var doc = new mongoose.Document({}, schema);

doc.validate(function(err) {
    print('Validation error: ' + err);
    doc.name = 'Val';
    doc.validate(function(err) {
        print('No more error: ' + err); 
    });
});