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() {
    var self = this;
    this.firstName = ko.observable("Bert");
    this.lastName = ko.observable("Bertington");

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

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

ko.applyBindings(new AppViewModel());