Logging with Winston

Why Logging?

• Debug production issues
• Track errors
• Monitor performance
• Audit user actions

Setup

npm install winston

Basic Logger

const winston = require('winston');

const logger = winston.createLogger({
  level: 'info',
  format: winston.format.combine(
    winston.format.timestamp(),
    winston.format.json()
  ),
  transports: [
    new winston.transports.File({ filename: 'error.log', level: 'error' }),
    new winston.transports.File({ filename: 'combined.log' })
  ]
});

if (process.env.NODE_ENV !== 'production') {
  logger.add(new winston.transports.Console({
    format: winston.format.simple()
  }));
}

logger.info('User logged in', { userId: 123 });
logger.error('Payment failed', { error: err.message });

Log Levels

logger.error('Critical error');
logger.warn('Warning message');
logger.info('Info message');
logger.http('HTTP request');
logger.debug('Debug info');

Request Logging

const morgan = require('morgan');

app.use(morgan('combined', {
  stream: {
    write: (message) => logger.http(message.trim())
  }
}));

Error Logging

process.on('uncaughtException', (error) => {
  logger.error('Uncaught Exception', { error: error.stack });
  process.exit(1);
});

app.use((err, req, res, next) => {
  logger.error('Express error', {
    error: err.message,
    stack: err.stack,
    url: req.url
  });
  
  res.status(500).json({ error: 'Internal server error' });
});

Key Takeaway

Use Winston for structured logging. Log to files in production. Include context in logs. Monitor error logs. Never log sensitive data.