javascript not setting the hidden field value in ASP.NET

Today when i tried to change the value of hidden filed in java script, its not setting the value. I am using master page for an ASPX page.

<asp:HiddenField ID="hdnTest" runat="server" Value="Before JS Call" />

In java script method, when I try to assign the value it’s not getting replaced with new value.

document.getElementById('<%=hdnTest.ClientID%>').value = 'After JS Call';

then i replaced with below code

document.getElementById('ContentPlaceHolder1_hdnTest').value = 'After JS Call';

and it worked out.

Not sure with the reason but its resolved!