Javascript - Getting Null Value Of The Hidden Field At Server Side

I am calling a Java-script function, in that i am passing the value of hidden field, that hidden field i want to use at server-side, but the value of hidden field is null.

Client Side function

function getDetails()
{
document.forms[0].HdnNode.value=tree_selected_id; //HTML Hidden Field.
str="Cmp_12";
str_array=str.split("_");
var str_array1=str_array[0];
var str_array2=str_array[1];
document.getElementById("<%=HiddenNodeId.ClientId %>").value=str_array1;
document.getElementById("<%=HiddenTreeId.ClientId %>").value=str_array2;
}

Server Side Function

Public Sub InsertNodes(ByVal NodeId As String)
Dim objErrorObj As New ErrorObj
Dim ParentID As String
ParentID = HiddenNodeParent.Value
NodeId = HiddenNodeId.Value
Dim NodeIDTree As String
NodeIDTree = HiddenTreeId.Value
End Sub