function getGzipSize(filePath) {
  let bytesWritten = 0;
  const byteCounter = new Writable({
    write(chunk, encoding, callback) {
      // we control the input stream, will always be a Buffer
      bytesWritten += chunk.length;
      callback();
    },
  });
  return new Promise((res, rej) => {
    fs.createReadStream(filePath)
      .pipe(zlib.createGzip({ level: 9 }))
      // writable streams do not fire the 'close' event anymore
      // https://github.com/nodejs/node/issues/21122#issuecomment-414622251
      .on('close', () => res(bytesWritten))
      .pipe(byteCounter)
      .on('error', rej);
  });
}