Winston Logger

Common errors

• TypeError: winston.Logger is not a constructor

  cause Attempting to instantiate `winston.Logger` directly, which was common in Winston v2 but is no longer the correct pattern in v3.
  fix Use `winston.createLogger()` to create a logger instance instead of `new winston.Logger()`.

• TS2345: Argument of type 'LogCallback' is not assignable to parameter of type '...' (e.g., 'LogEntry')

  cause Your TypeScript code is attempting to use the `LogCallback` type or pass a `LogCallback` where it's no longer expected after v3.15.0.
  fix Remove `LogCallback` from your type annotations or function signatures; the type was removed as the underlying functionality was unsupported.

• Error: EACCES: permission denied, open 'error.log'

  cause The Node.js process does not have write permissions to the directory where Winston is attempting to create or write log files.
  fix Ensure the user running the Node.js process has appropriate write permissions for the log file directory, or configure Winston to write logs to a different, accessible location.

Warnings

• breaking Winston v3 introduces significant breaking changes compared to v2, including a new API for logger instantiation and transport configuration.
  Fix: Refer to the `UPGRADE-3.0.md` guide in the official documentation for detailed migration instructions.
• gotcha Using the default logger (`require('winston')` or `import winston from 'winston'`) without explicitly adding any transports can lead to high memory usage as logs are buffered indefinitely.
  Fix: Always create your own logger instance using `winston.createLogger()` and configure transports, or explicitly add transports to the default logger before use.
• breaking The `LogCallback` type was removed from Winston's TypeScript definitions in v3.15.0 because the underlying library did not actually support this functionality.
  Fix: Remove all references to `LogCallback` from your TypeScript code; the associated callback functionality was never executed even prior to its type definition removal.

Install

• npm install winston npm
• yarn add winston yarn
• pnpm add winston pnpm

Imports

• winston

  import winston from 'winston'

  Winston supports both CommonJS (`const winston = require('winston')`) and ES Modules, as it ships TypeScript types and targets modern Node.js environments.

Quickstart

Initializes a Winston logger with file transports for error and combined logs, adding a console transport for non-production environments, and demonstrates various log levels.

import winston from 'winston';

const logger = winston.createLogger({
  level: 'info',
  format: winston.format.json(),
  defaultMeta: { service: 'user-service' },
  transports: [
    // Log all errors to error.log
    new winston.transports.File({ filename: 'error.log', level: 'error' }),
    // Log all info and higher to combined.log
    new winston.transports.File({ filename: 'combined.log' })
  ]
});

// Add a console transport if not in production
if (process.env.NODE_ENV !== 'production') {
  logger.add(new winston.transports.Console({
    format: winston.format.simple()
  }));
}

// Example logging
logger.info('User logged in', { userId: 123 });
logger.warn('Deprecated feature used');
logger.error('Failed to connect to database', { error: 'Connection refused' });