Test of valueAllowUnset

HTML

<p>
    :
    <select data-bind="options: countries,
                       optionsCaption: 'select',
                       value: selectedCountry,
                       valueAllowUnset: allowUnset
                       "></select>
</p>

<p>: <span data-bind="text: selectedCountry"></span> </p>

<label>
    <input type="checkbox" data-bind="checked: allowUnset"/> valueAllowUnset
</label>
<br/>

<button data-bind="click: changeCountry"></button>

JavaScript

function ViewModel() {
    var self = this;
    self.countries = ['kl','s','sdf'];
    self.selectedCountry = ko.observable('sdf');
    self.allowUnset = ko.observable(false);
    self.changeCountry = function() {
        self.selectedCountry('d');
    };
}

ko.applyBindings(new ViewModel());