JSFiddle - React, Tailwind, and code Playground

HTML

<div id="ok"></div>

CSS

#ok {
    font-size: 28px;
    color: red;
}

JavaScript

function Wow() {
    this.ok = document.getElementById('ok');
    this.wait();
}


Wow.prototype.wait = function() {
    var self = this;
    this.ok.innerHTML = "Waiting..."
    window.setTimeout(function() {
        self.completed();
    }, 1000);
}

var wow = new Wow();
wow.completed = function() {
    wow.ok.innerHTML = "Compelted";
};