pong(data, mask, cb) {
    if (this.readyState === WebSocket.CONNECTING) {
      throw new Error('WebSocket is not open: readyState 0 (CONNECTING)');
    }

    if (typeof data === 'function') {
      cb = data;
      data = mask = undefined;
    } else if (typeof mask === 'function') {
      cb = mask;
      mask = undefined;
    }

    if (typeof data === 'number') data = data.toString();

    if (this.readyState !== WebSocket.OPEN) {
      sendAfterClose(this, data, cb);
      return;
    }

    if (mask === undefined) mask = !this._isServer;
    this._sender.pong(data || EMPTY_BUFFER, mask, cb);
  }
Send a pong.

@param {*} [data] The data to send
@param {Boolean} [mask] Indicates whether or not to mask `data`
@param {Function} [cb] Callback which is executed when the pong is sent
@public