JSFiddle - React, Tailwind, and code Playground

HTML

<select data-bind="value: test, visible: !test().length">
    <option></option>
    <option>Option 1</option>
    <option>Option 2</option>
</select>
<button data-bind="text: test, visible: test().length"></button>

JavaScript

var ViewModel = {
    test: ko.observable('')
};
ko.applyBindings(ViewModel);