knockoutjs valueAllowUnset 属性

<p>    Select a country:    <select data-bind="options: countries,    optionsCaption: 'Choose one...',    value: selectedCountry,    valueAllowUnset: true"></select></p> <script type="text/javascript">    var viewModel = {        countries: ['Japan', 'Bolivia', 'New Zealand'],        selectedCountry: ko.observable('Latvia')    };</script>

在这段代码中，我们将valueAllowUnset设置为true，此时我们将value设置为“Lativia”时，列表中不存在此项，那么将会把selectedCountry设置为undefined，界面显示“Choose one…”项。