call javascript function onchange event of dropdown list

Your code is working just fine, you have to declare javscript method before DOM ready.

using jQuery

 $("#ddl").change(function () {
                alert($(this).val());
            });

jsFiddle

I don't know why do you need this onmousedown event here, but what you have to do is put your function above actual usage. Look at the snipplet below:

<script type="text/javascript">
function jsFunction(value)
{
    alert(value);
}
</script>

<select id ="ddl" name="ddl" onmousedown="this.value='';" onchange="jsFunction(this.value);">
  <option value='1'>One</option>
  <option value='2'>Two</option>
  <option value='3'>Three</option>
</select>