EmberJS change environment variables (e.g. rootURL) at runtime

I need to change the environment variable rootURL at loading time, depending on some global js variable (_ENV), which denotes Symfony's current environment.

So, if accessing the app via http://ift.tt/2k6kIzU it should be rootURL = '/app/'. If accessing via http://somedomain/app_dev.php/app/ it should be rootURL = '/app_dev.php/app/'.

Many thanks!