how to pass the strMessage (from codebehind to the script) to get the alert message

i.e if my strmessage from code behind is hi,

then i need

You Have Used already the message : hi

My code...

<script type="text/javascript">
     var strFileName;

    function alertShowMessage() {
    alert("You Have Used already the message :"+ <%=strFileName %>+ ");
    }
</script>


datatable dtChkFile =new datatable();
dtChkFile = objutility.GetData("ChkFileName_SMSSent", new object[] {"rahul"}).Tables[0];
         if (dtChkFile.Rows.Count > 0)
         {
             for (int i = 0; i < dtChkFile.Rows.Count; i++)
             {
                  strMessage= dtChkFile.Rows[i]["Message"].To);

             }
         }