JSFiddle - React, Tailwind, and code Playground

HTML

<div id="foo"></div>

JavaScript

user = {};
div = $("#foo");
Object.observe(user, function(changes){    
    changes.forEach(function(change) {
        var fullName = (user.firstName || "") + " " + (user.lastName || "");         
        div.text(fullName);
    });
});

user.firstName = 'Bill';
user.lastName = 'Clinton';