gistfile1.html

<form data-bind="submit:addItem">
    Add item: <input type="text" data-bind='value:itemToAdd, valueUpdate: "afterkeydown"' />
    <button type="submit" data-bind="enable: itemToAdd.length > 0">Add</button>
</form>
 
<p>Your values:</p>
<select multiple="multiple" height="5" data-bind="options:allItems, selectedOptions:selectedItems"> </select>
 
<div>
    <button data-bind="click: removeSelected, enable: selectedItems.length > 0">Remove</button>
    <button data-bind="click: sort, enable: allItems.length > 1">Sort</button>
</div>