Winston Logger - Quick Reference
When to Use This Skill
- Logging with multiple transports (file, console, HTTP)
- Structured logging with custom levels
- Automatic log file rotation
Deep Knowledge: Use mcp__documentation__fetch_docs with technology: winston for comprehensive documentation.
Basic Setup
npm install winston
Essential Patterns
Logger Base
import winston from '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' }),
],
});
// Add console in development
if (process.env.NODE_ENV !== 'production') {
logger.add(new winston.transports.Console({
format: winston.format.simple(),
}));
}
Custom Format
const customFormat = winston.format.printf(({ level, message, timestamp, ...meta }) => {
return `${timestamp} [${level.toUpperCase()}]: ${message} ${Object.keys(meta).length ? JSON.stringify(meta) : ''}`;
});
const logger = winston.createLogger({
format: winston.format.combine(
winston.format.timestamp(),
customFormat
),
transports: [new winston.transports.Console()],
});
Child Logger
const childLogger = logger.child({ requestId: req.id, module: 'auth' });
childLogger.info('User authenticated', { userId: user.id });
Daily Rotate File
import DailyRotateFile from 'winston-daily-rotate-file';
const transport = new DailyRotateFile({
filename: 'logs/app-%DATE%.log',
datePattern: 'YYYY-MM-DD',
maxSize: '20m',
maxFiles: '14d',
});
logger.add(transport);
When NOT to Use This Skill
- High-performance APIs: Use Pino instead - 10x faster than Winston
- JSON-only logging: Pino is more optimized for structured JSON output
- Serverless/Lambda: Prefer simpler console logging or Pino for lower overhead
- Python/Java projects: Use language-specific logging frameworks
- Simple scripts: Built-in console is sufficient for basic debugging
Anti-Patterns
| Anti-Pattern | Why It's Bad | Solution |
|---|
| Synchronous file logging | Blocks event loop, degrades performance | Use async transports with {stream:...} |
| Logging objects without serialization | Can log [Object] instead of data | Use JSON.stringify() or Winston JSON format |
| No log rotation | Disk fills up in production | Use winston-daily-rotate-file |
| Logging in tight loops | Overwhelms I/O, fills disks | Add conditional logic or sample logs |
| String concatenation | Always evaluated, even when disabled | Use format strings: logger.info('User %s', userId) |
| Missing error stack traces | Loses debugging context | Use {error: err} or Winston error format |
Quick Troubleshooting
| Issue | Cause | Solution |
|---|
| Logs not appearing | Wrong log level configured | Check logger.level and transport levels |
| Performance degradation | Synchronous file writes | Use async transports or reduce log verbosity |
| Disk full | No log rotation | Configure winston-daily-rotate-file with maxFiles |
[Object] in logs | Improper object formatting | Use winston.format.json() or winston.format.prettyPrint() |
| Duplicate logs | Multiple transports to same destination | Review transport configuration |
| Colors not showing | Console transport missing colorize | Add winston.format.colorize() to console transport |
