how to get perticular listbox selected value using javascript

I have html listbox as, which has same name but different id

<select name="dept"  id="vrow" >
                            <option selected="selected" > - Select Department- </option>
                            <option value="GEN">computer </option>
                            <option value="SC">mac </option>
                            <option value="ST">civ </option>
                            <option value="OBC">ele </option>
                          </select>

i had atteched it to each row fetched from database,
how can i get ID if i change perticular listbox value.