One-line check if socket is in given room?

You can simply check like this
io.sockets.adapter.rooms['roomId']

This returns you a object with sId e.g.
{"1sEAEIZeuMgdhF35AAAA":true}

---

Updates specific to versions: 3.0+:

io.sockets.adapter.get('roomId')

1.4:

io.sockets.adapter.rooms['roomId']

1.3.x:

io.sockets.adapter.rooms['roomId']; 

1.0.x to 1.2.x:

io.adapter.rooms['roomId'];

---

**Update:**
However one can check socket Id is in a given room or not with one-line as mentioned above only if server architecture has a single node server/single node process.

If you are using multi-node server, i.e. separate node process with load balanced.

Point to note here is, that the sockets are only registered on the process that they first connected to. So, you need to use socket.io-redis to connect all your nodes together to sync events, and what you can do to maintain list of socket Ids across multi-node is broadcast an event each time a client connects/disconnects, so that each node updates & maintains the real-time list of all the clients/socket Ids.

Background/Details:
The redis adapter extends the base adapter, but it only overrides/adds the following properties:
clients broadcast add del delAll

With the following code:

io.sockets.adapter.rooms["roomId"]; //Any of the above examples specific to versions mentioned above

you are querying the rooms property on socket.io-redis adapter. This wasn't overridden by the redis adapter, so you're actually querying the base adapter, which only knows about rooms/clients in the current process.

Why didn't the redis adapter override the rooms property? Might be because it would have to query the redis database instance to construct an object containing all rooms and connections on every access of this property. Not a good idea?

So as of this writing answer, you'll have to add that functionality to the adapter itself with a method like this:

/**
   * One-Line code/property to check if the socket id is in given room.
   *
   * @param {String} room id
   * @param {Function} callback (optional)
   * @api public
   */

  Redis.prototype.isSidExistsInRoom = function(room, fn){ ... }

where you will hit the redis database instance.

This should be part of the base adapter interface for all other adapters to implement. It's a common problem everyone will face one day, when they scale their servers ;)

P.S. Just a hint on another approach is to use the customRequest/customHook methods in socket.io-redis 3.1.0.

---

**Update with ver 5.2.0: (relevant multi node servers)**
Now redis adapter gives you rooms across processes/nodes as of 5.2.0
Source: [RedisAdapter#clients(rooms:Array, fn:Function)][5] > Returns the list of client IDs connected to rooms across all nodes. See [Namespace#clients(fn:Function)][6]

io.of('/').adapter.clients((err, clients) => {
  console.log(clients); // an array containing all connected socket ids
});

io.of('/').adapter.clients(['room1', 'room2'], (err, clients) => {
  console.log(clients); // an array containing socket ids in 'room1' and/or 'room2'
});

// you can also use

io.in('room3').clients((err, clients) => {
  console.log(clients); // an array containing socket ids in 'room3'
});

Happy Coding!

For the documentation, socket.io doesn't seem to have any simple way to do that. You really need to check if the client is in the room array, or the opposite: if the room is in the client array.

This can be done with an oneliner using indexOf:

if(socket.rooms.indexOf(room) >= 0)

Or the opposite search:

if(io.sockets.manager.rooms['/' + room].indexOf(socket.id) >= 0)