KnockoutJS Computed Observables

<!DOCTYPE html>
<head>
   <title>KnockoutJS Example</title>
   <script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.3.0/knockout-min.js" type="text/javascript"></script>
</head>
<body>
<div>

   FirstName <input data-bind='value: firstname' /><br>
   LastName <input data-bind='value: lastname' /><br>
   Hello <span data-bind="text:fullname">.
    
</div>

<script>
// data model content goes here 
var ViewModel = function() {
    this.firstname = ko.observable('Dave'); 
    this.lastname = ko.observable('Smith');
    this.fullname = ko.computed(function() {
        return this.firstname() + " " + this.lastname();
    }, this);
};
// Activate Knockout.JS
ko.applyBindings(new ViewModel());
</script>
</body>
</html>