Winston structured logging: JSON format with custom transports

Winston is configured with the default format which produces human-readable strings. In production, log aggregators cannot parse these strings as structured data, making field-based filtering and querying impossible.

Configure Winston with JSON format and combine format for consistent structured output.

const winston = require('winston');

const logger = winston.createLogger({
  level: process.env.LOG_LEVEL || 'info',
  format: winston.format.combine(
    winston.format.timestamp(),
    winston.format.errors({ stack: true }),  // serialize Error objects with stack traces
    winston.format.json()
  ),
  defaultMeta: {
    service: 'order-service',
    environment: process.env.NODE_ENV,
    version: process.env.APP_VERSION,
  },
  transports: [
    new winston.transports.Console(),
    // In production, add a transport for your log aggregator
    // new WinstonLoki({ host: 'http://loki:3100', labels: { service: 'order-service' } }),
  ],
});

// Logging with metadata
logger.error('Payment failed', {
  orderId: '123',
  error: err,  // errors({ stack: true }) handles this
  userId: req.user.id,
});

Note: pino is 5-10x faster than winston in high-throughput scenarios — consider migrating if performance is a concern.

winston.format.errors({ stack: true }) is non-obvious but essential — without it, Error objects logged as metadata are serialized as empty objects {} because JSON.stringify ignores non-enumerable properties like message and stack.

Setting up production-grade structured logging with Winston in a Node.js application