JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://getbootstrap.com/dist/css/bootstrap.min.css">
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<textarea data-bind="value: description, valueUpdate: 'afterkeydown'" rows="6"></textarea>

<div>
  Length: <span data-bind="text: count">0</span>
</div>

JavaScript

function viewModel() {
    var self = this;
    self.description = ko.observable();

    self.count = ko.computed(function () {
        var counter self.description().length;
        return counter;
    });
}

ko.applyBindings(viewModel());