JSFiddle - React, Tailwind, and code Playground

HTML

<textarea data-bind="value: comment, valueUpdate: 'afterkyedown'"></textarea>

<br/><br/>
<span data-bind="text: getCount, valueUpdate: ['afterkeydown','propertychange','input']"></span> characters

JavaScript

var viewModel = function(){
    var self = this;
    
    self.count = ko.observable(0);
    self.comment = ko.observable("");
    self.getCount = function(){
        var countNum = 10 - self.comment().length;
        self.count(countNum);
    };

}
    
    
var viewModel12 = new viewModel();
ko.applyBindings(viewModel);