JSFiddle - React, Tailwind, and code Playground

HTML

<div>
    <input type="text" data-bind="value: message, event: { keyup: onkeyup}" />
    <label data-bind="text: status" ></label>
</div>

JavaScript

function ViewModel(model) {  
    var self = this; 

    self.message = ko.observable('');
    self.status = ko.observable('Has escrito 0 caracteres...');
  
    self.onkeyup = function(data, event) {
        self.status('Has escrito ' + event.target.value.length + ' caracteres...');           
        return true;
    }
}  
ko.applyBindings(new ViewModel());