room.js

import { reset } from 'redux-form';
import { Presence } from 'phoenix'; // new line

// new function
const syncPresentUsers = (dispatch, presences) => {
  const presentUsers = [];
  Presence.list(presences, (id, { metas: [first] }) => first.user)
          .map(user => presentUsers.push(user));
  dispatch({ type: 'ROOM_PRESENCE_UPDATE', presentUsers });
};

export function connectToChannel(socket, roomId) {
  return (dispatch) => {
    if (!socket) { return false; }
    const channel = socket.channel(`rooms:${roomId}`);
    let presences = {}; // new line

    // new function
    channel.on('presence_state', (state) => {
      presences = Presence.syncState(presences, state);
      syncPresentUsers(dispatch, presences);
    });

    // new function
    channel.on('presence_diff', (diff) => {
      presences = Presence.syncDiff(presences, diff);
      syncPresentUsers(dispatch, presences);
    });

    channel.on('message_created', (message) => {
      dispatch({ type: 'MESSAGE_CREATED', message });
    });

    channel.join().receive('ok', (response) => {
      dispatch({ type: 'ROOM_CONNECTED_TO_CHANNEL', response, channel });
    });

    return false;
  };
}

// ...rest of file