chosen-change-add-empty-option

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/chosen/1.1.0/chosen.jquery.min.js"></script>
<link rel="stylesheet" href="http://cdnjs.cloudflare.com/ajax/libs/chosen/1.1.0/chosen.min.css">
    <select multiple=" " style="width:150px">
      <option value="a">a</option>
      <option value="b">b</option>
      <option value="c">c</option>
      <option value="aa">aa</option>
      <option value="bb">bb</option>
      <option value="cc">cc</option>
    </select>

JavaScript

$( "select" ).addClass( "chosen-select" )
              .chosen( { disable_search: true, disable_search_threshold: 50 } );