Doubt in Javascript in ASP.NET

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ASP.NET4

Dear Friends:
In ASP.NET i have created javascript function see the below coding

<script language="javascript" type="text/javascript">
        function fnassignvalues() {
            <%Session["year"] = null; %>            
            window.location = "../Default.aspx";
        }
    </script>

and i called this function in the below coding
<a id="A2" onmousemove="" önclick="javascript: fnassignvalues()" font-bold="true" runat="server" forecolor="White"
style="color: #FFFFFF; font-weight: bold; cursor:pointer;">Sign Out

but the function raised before the button clicked, how could restrict the function should raised after button click event ?