send(data, index = null) {
    const sockets = this.getSockets(index);
    this.checkRequired(sockets);
    if (this.isCaching() && (data !== this.cache[`${index}`])) {
      this.cache[`${index}`] = data;
    }
    sockets.forEach((socket) => {
      if (!socket) { return; }
      socket.send(data);
    });
  }
@param {any} data
@param {number|null} [index]