Planning Poker Using Knockout
You are using function as definition for model and inside that function you are defined an interface to have access to those properties/methods. In complicated cases you would have inflexible usage between definition and public interface. Better solution would be to create definition for the class like this

var Model = function () {
   var self = this;
   self.selectedItems = ko.computed(function () {
        return _.filter(items(), function (item) {
            return item.active();
        });
    })
}


and then create an instance of it

var model = new Model();


because a new operator you will create new JS object and this object will be send as scope for current this content. This is commonly used patterns in order to follow OOJ see link here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Introduction_to_Object-Oriented_JavaScript