@rongcuid I'm back!
I've updated the default systemd config in our docs. In your specific case, all you'll have to add is the --no-silent flag to ExecStart and stdout from the forked process will automatically be piped to loader.js's stdout, which will be caught by systemd and logged to the journal.
Alternatively you can maybe change Type to simple and add the --no-daemon flag to ExecStart, but I have not tried this and do not know whether it will work with a multi-port setup (update: I tried it, it works fine). That is how the old Upstart config worked, though, since Upstart had no support for forking processes.