Question : Syntax Error

What is wrong with the syntax of this:

function ReturnToLoginPage()
{
var location_string = <%=session("sub_directory")%> & "login.asp";
alert(location_string);
location.href = 'location_string';
}

Answer : Syntax Error

We did not get to implement any of the Comments, as the client picked the computer up for the diagnostic charge d/t thier time constraints.