session.js

const initialState = {
  willAuthenticate: true,
  isAuthenticated: false,
  currentUser: {},
  socket: null, // new line
};

// ...

    case 'LOGOUT':
      return {
        ...state,
        willAuthenticate: false,
        isAuthenticated: false,
        currentUser: {},
        socket: null, // new line
      };
    case 'SOCKET_CONNECTED': // new case
      return {
        ...state,
        socket: action.socket,
      };
    default:
      return state;
  }
}