Knockout - Hello World

HTML

<script src="https://github.com/downloads/SteveSanderson/knockout/knockout-2.1.0.js"></script>
<input data-bind="value: firstName" />
<br/>
<span data-bind="text: firstName"></span>

JavaScript

var myViewModel = {
    firstName: ko.observable("Hello World")
};

ko.applyBindings(myViewModel);