About data binding by knockout js and value change

There are two textboxes and a span bound by culling. The example I got is easy to understand. Here is the code.

<p>First name: <input data-bind="value: firstName" /></p>
<p>Last name: <input data-bind="value: lastName" /></p>
<h2>Hello, <span data-bind="text: fullName"> </span>!</h2>

var ViewModel = function(first, last) {
    this.firstName = ko.observable(first);
    this.lastName = ko.observable(last);

    this.fullName = ko.computed(function() {
        // Knockout tracks dependencies automatically. It knows that fullName depends on firstName and lastName, because these get called when evaluating fullName.
        return this.firstName() + " " + this.lastName();
    }, this);
};

ko.applyBindings(new ViewModel("Planet", "Earth")); 

When the code runs whatever value is passed, it is reflected, but when I change a textbox value, the changed data is not reflected immediately as it keyuphappens , but when the focus changes, the changed data is reflected in the span. So please guide me if i want to change the data on keyup then i need to change the code. thanks