code: function onError(error) {
      if (error.syscall !== 'listen') throw error;
  
      const bind = (typeof port === 'string')
                  ? 'Pipe ' + port
                  : 'Port ' + port;
  
      // handle specific listen errors with friendly messages
      switch (error.code) {
        case 'EACCES':
          console.error(bind + ' requires elevated privileges');
          process.exit(1);
          break;
  
        case 'EADDRINUSE':
          console.error(bind + ' is already in use');
          process.exit(1);
          break;
  
        default:
          throw error;
      }
    }
docs: Event listener for HTTP server "error" event.