You can generate your JavaScript from ASP.NET.
Then simply write the settings at the server-side to your var defaults
like this:
var defaults = {
inactivity: <%=ConfigurationManager.AppSettings["Inactivity"] %>
}
EDIT:
If you want to keep your JavaScript in static js files, you can still initialize your var defaults
from a small <script>
rendered by your ASP.NET application. Your settings would be global, just like the AppSettings
in web.config
.