Chat-room application case study

export const expectedObservation = {
  joins: [
    {
      participantId: 'alice',
      channel: 'general',
      acknowledgement: { accepted: true, channel: 'general' },
    },
    {
      participantId: 'alice',
      channel: 'support',
      acknowledgement: { accepted: true, channel: 'support' },
    },
    {
      participantId: 'bob',
      channel: 'general',
      acknowledgement: { accepted: true, channel: 'general' },
    },
    {
      participantId: 'carol',
      channel: 'support',
      acknowledgement: { accepted: true, channel: 'support' },
    },
  ],

    socket.on('join-channel', async (channel, acknowledge) => {
      if (!channels.has(channel)) {
        acknowledge({ accepted: false, reason: 'unknown-channel' });
        return;
      }

      await socket.join(channel);
      io.to(socket.id).emit('welcome', {
        channel,
        text: `Welcome to #${channel}.`,
      });
      acknowledge({ accepted: true, channel });
    });

class BroadcastOperator {
  constructor(server, rooms, excludedSocketIds = new Set()) {
    this.server = server;
    this.rooms = new Set(rooms);
    this.excludedSocketIds = excludedSocketIds;
  }

  emit(event, ...args) {
    const recipients = this.rooms.size === 0 ? new Set(this.server.sockets.keys()) : new Set();

    for (const room of this.rooms) {
      for (const socketId of this.server.rooms.get(room) ?? []) {
        recipients.add(socketId);
      }
    }

    for (const socketId of recipients) {
      if (this.excludedSocketIds.has(socketId)) continue;
      this.server.sockets.get(socketId)?.deliver(event, cloneArguments(args));
    }

    return true;
  }
}

class ServerSocket extends Emitter {
  constructor(id, server, client, auth) {
    super();
    this.id = id;
    this.server = server;
    this.client = client;
    this.handshake = { auth: auth ?? {} };
    this.rooms = new Set();
    this.connected = true;
  }

  join(roomOrRooms) {
    if (!this.connected) return;

    for (const room of asArray(roomOrRooms)) {
      this.rooms.add(room);
      const members = this.server.rooms.get(room) ?? new Set();
      members.add(this.id);
      this.server.rooms.set(room, members);
    }
  }

  to(roomOrRooms) {
    return new BroadcastOperator(this.server, asArray(roomOrRooms), new Set([this.id]));
  }