`app.listen` uses the same signature as Node's `server.listen`. You can specify the `host` [0] or else it defaults to `0.0.0.0`. This means that when you run this service, it will be accessible to anyone that can reach the server.

Typically, services allow you set the host address by configuration. This way you can set it to `127.0.0.1` so that it is only available locally.

[0]: https://nodejs.org/api/net.html#serverlistenport-host-backlo...