check if at least one option is selected in a multiple select element

checkselect.html

<select id="mySelect" multiple="multiple">
  <option value="1">First</option>
  <option value="2">Second</option>
  <option value="3">Third</option>
  <option value="4">Fourth</option>
</select>

<script type="text/javascript">	
$(document).ready(function() {
  if (!$("#mySelect option:selected").length)
    // none is selected
});
</script>