_compress(data, fin, callback) {
    const endpoint = this._isServer ? 'server' : 'client';

    if (!this._deflate) {
      const key = `${endpoint}_max_window_bits`;
      const windowBits =
        typeof this.params[key] !== 'number'
          ? zlib.Z_DEFAULT_WINDOWBITS
          : this.params[key];

      this._deflate = zlib.createDeflateRaw({
        ...this._options.zlibDeflateOptions,
        windowBits
      });

      this._deflate[kTotalLength] = 0;
      this._deflate[kBuffers] = [];

      this._deflate.on('data', deflateOnData);
    }

    this._deflate[kCallback] = callback;

    this._deflate.write(data);
    this._deflate.flush(zlib.Z_SYNC_FLUSH, () => {
      if (!this._deflate) {
        //
        // The deflate stream was closed while data was being processed.
        //
        return;
      }

      let data = bufferUtil.concat(
        this._deflate[kBuffers],
        this._deflate[kTotalLength]
      );

      if (fin) {
        data = new FastBuffer(data.buffer, data.byteOffset, data.length - 4);
      }

      //
      // Ensure that the callback will not be called again in
      // `PerMessageDeflate#cleanup()`.
      //
      this._deflate[kCallback] = null;

      this._deflate[kTotalLength] = 0;
      this._deflate[kBuffers] = [];

      if (fin && this.params[`${endpoint}_no_context_takeover`]) {
        this._deflate.reset();
      }

      callback(null, data);
    });
  }
Compress data.

@param {(Buffer|String)} data Data to compress
@param {Boolean} fin Specifies whether or not this is the last fragment
@param {Function} callback Callback
@private