How to get the value of selected text in select combobox by javascript in asp.net c#

How to get the value from selected text by select combobox?
Here is the aspx.cs code for bind combobox

service.BindHTMLDropdownList(cmb_labelno, "tbl_StockRunning", "StockTagNo", "StockId", "where IsActive = 1 and StockTypePrefix = 'T' and " +
    "BranchId = " + Session["BranchId"] + "");

Combobox Name : cmb_labelno
Table Name : tbl_StockRunning
Text : StockTagNo
Value : StockId

Here is the aspx code

<select id="cmb_labelno" runat="server"></select>
<input type="button" id="btnGetStock" value="VIEW"/>


$("#btnGetStock").click(function (e) {               
    alert(document.getElementById('cmb_labelno').value);
});

Now, the place of alert(document.getElementById(‘cmb_labelno’).value I used
alert(document.getElementById(‘cmb_labelno’).val also still value is not showing.

While the same thing if I am doing in Jquery still showing the selectedtext only not value
Here is my Jquery code,

document.getElementById('<%= cmb_labelno.ClientID %>').value

Is there any problem in code…???