NodeJS ExpressJS - how to log errors for the production server?

Question

How can I log any errors from ExpressJS apps into a file?

I know I can log a Slim framework easily with monolog:

$app->get('/tickets', function (Request $request, Response $response) {
    $this->logger->addInfo("Something interesting happened");

    $mapper = new Simon\TicketMapper($this->db);
    $tickets = $mapper->getTickets();

    $response->getBody()->write(var_export($tickets, true));
    return $response;
});

In Express, I usually log the error to console for development:

Model.findOne(options, function(err, doc) {

    var output = {};
    if (err) {
        console.log("Error retrieving doc: " + err);
    }

But in my production server, I do need a helpful log file when the app goes wrong.

Any ideas?

In Express's app.js, it has this:

// development error handler
// will print stacktrace
if (app.get('env') === 'development') {

  app.use(function(err, req, res, next) {
    res.status(err.status || 500);
    res.render('error', {
        message: err.message,
        error: err
    });
  });

}

// production error handler
// no stacktraces leaked to user
app.use(function(err, req, res, next) {
    res.status(err.status || 500);
    res.render('error', {
        message: err.message,
        error: {}
    });
});

But how do I turn the production's one on? Is it the same as monolog that I need?

Answer 1

winston is a popular library for logging.

You can transport all logs to one file or keep different files for error, debug and info logs.

Example:

  var winston = require('winston');

  var logger = new winston.Logger({
    level: 'error',
    transports: [
      new (winston.transports.File)({ filename: 'error.log' })
    ]
  });

In the code:

logger.log('error', 'test error message %s', 'my string');

You can also use winston daily rotate file with winston to rotate your log files based on size or date.