C# - How To Pass The StrMessage (from Codebehind To The Script) To Get The Alert Message

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);
}
}