How can I use knockout's $parent/$root pseudovariables from inside a .computed() observable?

http://stackoverflow.com/questions/8640748/how-can-i-use-knockouts-parent-root-pseudovariables-from-inside-a-computed

by johnwun

HTML

<script src="https://github.com/downloads/SteveSanderson/knockout/knockout-2.0.0.debug.js"></script>
<ul data-bind="foreach: items">
    <li data-bind="css: { selected: isSelected }">
        <a href="#" data-bind="text: name, click: $root.selectedItem"></a>
    </li>
</ul>

<hr/>

<div data-bind="text: ko.toJSON($root)"></div>

CSS

.selected { background-color: #f88; }

JavaScript

var Item = function(name, parent) {
   this.name = ko.observable(name);  
   this.isSelected = ko.computed(function() {
       return this === parent.selectedItem();        
   }, this);
};

var ViewModel = function() {
    // here clicked object ref is passed implicitly to variable
   this.selectedItem = ko.observable();
   this.items = ko.observableArray([
       new Item("one", this),
       new Item("two", this),
       new Item("three", this)
       ]);
};

    
ko.applyBindings(new ViewModel());