怎样用JS获取弹出层中dropdownlist的selectedvalue值

 <div class=" Dialog" id="AddOrEditRoleDialog" style="top: 80px; width: 480px; display: none;
        text-align: center; margin: aotu;">
        <div class=" DialogHeader" onmousedown="javascript:Drag(this,$get('AddOrEditRoleDialog'),event)">
            <div class="DialogHeaderLeft">
                <div class="DialogExpandImage">
                    <img src="../Resourse/Theme/images/add.gif" /></div>
            </div>
            <div id="PerAddMenu" class="DialogHeaderCenter">
                添加产品分类</div>
            <div id="PerEditRole" class="DialogHeaderCenter" style="display: none;">
                编辑产品名称</div>
        </div>
        <div class=" DialogContent" style="padding-top: 15px; padding-bottom: 15px;">
            <asp:UpdatePanel ID="UpdatePanelNewPerAssetReg" runat="server" UpdateMode="Conditional"
                ChildrenAsTriggers="false">
                <ContentTemplate>
                    <table style="width: 90%;" class="styleTable">
                          <tr>
                            <td class="FormItemText">
                                 品牌名称：
                            </td>
                            <td class="FromItemLeft">
                          <asp:DropDownList ID="DDL_CompanyName" runat="server" Width="155px">
                                </asp:DropDownList>
                            </td>
                           <td class="FormItemText">
                               
                           </td>
                        </tr>
                         <tr>
                            <td class="FormItemText">
                           分类名称：
                            </td>
                            <td class="FromItemLeft">
                               <asp:TextBox ID="TB_ProType" runat="server"></asp:TextBox>
                            </td>
                           <td class="FormItemText">
                               
                           </td>
                        </tr>
                         
                        <tr>
                            <td colspan="2" align="center">
                                <asp:Button ID="SavePerRoletRegButton" AccessKey="F" onmouseover="javascript:this.className='ButtonOver'"
                                    onmouseout="javascript:this.className='Button'" runat="server" CssClass="Button"
                                    Text=" 添加(F) " OnClientClick="javascript:return checkSubmit();" OnClick="SavePerRoletRegButton_Click"
                                    UseSubmitBehavior="True" />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
                                <input class=" Button" accesskey="C" onmouseover="javascript:this.className='ButtonOver'"
                                    onmouseout="javascript:this.className='Button'" type="button" value=" 取消(C) "
                                    onclick="javascript:cancelSave();" />
                            </td>
                        </tr>
                    </table>
                </ContentTemplate>
            </asp:UpdatePanel>
        </div>
        <div class=" DialogFooter">
        </div>
    </div>
JS
 function editRole() {          
             if ($get("<%=TB_rolId.ClientID%>").value.trim() == "") {
                 alert("你没有选择要编辑的选项，请选择！");
                 return false;
             }
             
             $get('AddOrEditRoleDialog').style.display = 'block';
             $get('PerEditRole').style.display = 'block';
             $get('DialogMask').style.display = 'block';    
             var table = $get("<%=GV_RoleList.ClientID %>");
             if (table && table.rows.length > 0) {
                 var row = table.rows[table.rowIndex];
                 $get("<%=TB_ProType.ClientID %>").value = row.getAttribute("FTypeName");
                 
             }
             $get("<%=TB_addOreditRole.ClientID %>").value = "edit";
             $get("<%=SavePerRoletRegButton.ClientID %>").value = " 编辑(F) ";
             }