JSFiddle - React, Tailwind, and code Playground

HTML

<p>Nombre: <input type="text" data-bind="value: firstname" /></p>
<p>Apellidos: <input type="text" data-bind="value: lastname" /></p>
<p>Nombre completo: <label data-bind="text: name"></p>

JavaScript

function ViewModel(model) {  
    var self = this;  
  
    self.firstname = ko.observable('');  
    self.lastname = ko.observable('');  
    self.name = ko.computed(function() {
        if (self.firstname() == '' || self.lastname() == '') return '';
         return self.lastname() + ', ' + self.firstname();
    });
}  

var viewModel = new ViewModel();  
ko.applyBindings(viewModel);