var AppViewModel = {
        shouldShowMessage: ko.observable(true)  ///初始化的時候div是visible的    
    };

    AppViewModel.shouldShowMessage = ko.observable(false);   ///現在hidden勒
    ko.applyBindings( AppViewModel);

<div data-bind="visible:shouldShowMessage">
    You will see this message only when "shouldShowMessage" holds a true value.
</div>

<script type="text/javascript">
    var AppViewModel = {
        shouldShowMessage: ko.observable(true),   ///初始化的時候div是visible的
        myValues: ko.observableArray([])        
    };

    AppViewModel.shouldShowMessage = ko.observable(false);   ///現在hidden勒
    AppViewModel.myValues.push("some value");                   ///向myValues數組中 添加一個項
    ko.applyBindings( AppViewModel);
</script>

<div data-bind="visible: myValues().length > 0">   
　　You will see this message only when 'myValues' has at least one member.
</div>