I noticed that you are actually accessing the .value
field of the element that the <asp:Label />
control generates, which is a <span></span>
. This type of element won't return anything for the .value
attribute. If you are actually trying to access its text then use:
alert(document.getElementById('<%=((Label)Master.FindControl("lbl")).ClientID %>').innerText);
or
alert(document.getElementById('<%=((Label)Master.FindControl("lbl")).ClientID %>').innerHTML);