dataMessage(cb) {
    if (!this._fin) {
      this._state = GET_INFO;
      return;
    }

    const messageLength = this._messageLength;
    const fragments = this._fragments;

    this._totalPayloadLength = 0;
    this._messageLength = 0;
    this._fragmented = 0;
    this._fragments = [];

    if (this._opcode === 2) {
      let data;

      if (this._binaryType === 'nodebuffer') {
        data = concat(fragments, messageLength);
      } else if (this._binaryType === 'arraybuffer') {
        data = toArrayBuffer(concat(fragments, messageLength));
      } else if (this._binaryType === 'blob') {
        data = new Blob(fragments);
      } else {
        data = fragments;
      }

      if (this._allowSynchronousEvents) {
        this.emit('message', data, true);
        this._state = GET_INFO;
      } else {
        this._state = DEFER_EVENT;
        setImmediate(() => {
          this.emit('message', data, true);
          this._state = GET_INFO;
          this.startLoop(cb);
        });
      }
    } else {
      const buf = concat(fragments, messageLength);

      if (!this._skipUTF8Validation && !isValidUTF8(buf)) {
        const error = this.createError(
          Error,
          'invalid UTF-8 sequence',
          true,
          1007,
          'WS_ERR_INVALID_UTF8'
        );

        cb(error);
        return;
      }

      if (this._state === INFLATING || this._allowSynchronousEvents) {
        this.emit('message', buf, false);
        this._state = GET_INFO;
      } else {
        this._state = DEFER_EVENT;
        setImmediate(() => {
          this.emit('message', buf, false);
          this._state = GET_INFO;
          this.startLoop(cb);
        });
      }
    }
  }
Handles a data message.

@param {Function} cb Callback
@private