One can use the room returned from calling Video.connect to subscribe to a room's participantConnected event, so that all kinds of different logic can be executed when a new participant joins a room. I can also query rooms remotely without Video.connect to see how many participants are currently connected. However, I would like be able to use the participantConnected event to display in real time if someone is in a room, without actually joining the room.
Is this possible and how would I do that without becoming a participant via Video.connect and without polling twilio's api?
Preferably, I'd like to do this on the javascript side, but I could use twilio's api wrapper library on the server-side if I had to. I just want the client to get notified somehow when someone enters or leaves a room so I can display whether the room is empty in real time.
In order to receive those events with the client SDK, you would need to join the room as you would need a connection to it in order to receive events from it.
You can, however, subscribe to status callback webhooks for rooms. There are lots of events that a room emits that can be received as webhooks, such as room-created, room-ended, and, as you've asked for, participant-connected.
You can set the statusCallback when you create a room with the REST API and you can see all the parameters that are sent as part of the webhook request here.