How to handle concurrent sftp connections in Node js. getting error No sftp connection available

for sftp we are using latest version of ssh2-sftp-client module(5.1.3) service file(sftpConnectionService) with sftp connection {Error: get->get: No SFTP connection available}

class SftpConnectionService {
  constructor() {
    this.sftp = null;
  }
  async getSftpConnection(randomDigit) {
    try {
      this.sftp = new Client(`connection-${randomDigit}`);
      await this.sftp.connect({
        host: sftpconfig.host,
        port: sftpconfig.port,
        username: sftpconfig.username,
        password: sftpconfig.password,
      });
      return this.sftp;
    } catch (exception) {
        throw exception;
    }
  }
}

js file getting the connection and download the file

   downloadFile(){   
     let sftp;
      const randomDigit = generateRandomCodeSrvc.generateCode(4);
      sftp = await sftpConnectionService.getSftpConnection(randomDigit);
      const remoteDir = `month/statement/filename.pdf`
     let bufferStream = await sftp.get(remoteDir);
     if (sftp) await sftp.end();
   }