JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>
<textarea data-bind="textInput: message"></textarea>
<p>Characters left : <span data-bind="text: charLeft"></span></p>

JavaScript

function Vm_app() {
    var self = this;
    this.message = ko.observable('');
    this.charLeft = ko.pureComputed(function(){
        return 128 - self.message().length;
    });
}

ko.applyBindings(new Vm_app());