Knockout checkedValue on checkboxes

HTML

<script src="http://bestware.us/knockout/875-add-checkedValue/knockout-latest.js"></script>
<input type="button" data-bind="click: addItem" value="Add Item"/>
 <div data-bind="foreach: items">
 <li>
    <label><input type="checkbox" data-bind="checkedValue: $data, checked: $root.itemsChecked" />
<strong data-bind="text: $data"></strong></li>
</div>
<div>Selected Value: <span data-bind="text: itemsChecked"></span></div>
<div>View Model: <span data-bind="text: ko.toJSON($data)"></span></div>

TypeScript

var viewModel = {
    itemsChecked: ko.observableArray<sting>(),
    items: ko.observableArray<string>([
        'One',
        'Two',
        'Three'
    ]),
    addItem: function() { this.items.push({itemValue: ko.observable('Add')}); }
};
ko.applyBindings(viewModel);