JSFiddle - React, Tailwind, and code Playground

HTML

<p>First name: <input data-bind="value: firstName, valueUpdate: 'afterkeydown'" /></p>
<p>Last name: <input data-bind="value: lastName, valueUpdate: 'afterkeydown'" /></p>

<p>Full name: <strong data-bind="text: fullName"></strong></p>

<button data-bind="click: capitalizeLastName">Go caps</button>

JavaScript

function AppViewModel() {
this.firstName = ko.observable("Bert");
this.lastName = ko.observable("Bertington");

this.fullName = ko.computed(function() {
    return this.firstName() + " " + this.lastName();    
}, this);

this.capitalizeLastName = function() {
    var currentVal = this.lastName();        // Read the current value
    this.lastName(currentVal.toUpperCase()); // Write back a modified value
};    
}

ko.applyBindings(new AppViewModel());