winston

A logger for just about everything.

winston@3

See the Upgrade Guide for more information. Bug reports and PRs welcome!

Looking for winston@2.x documentation?

Please note that the documentation below is for winston@3. Read the winston@2.x documentation.

Motivation

winston is designed to be a simple and universal logging library with support for multiple transports. A transport is essentially a storage device for your logs. Each winston logger can have multiple transports (see: Transports) configured at different levels (see: Logging levels). For example, one may want error logs to be stored in a persistent remote location (like a database), but all logs output to the console or a local file.

winston aims to decouple parts of the logging process to make it more flexible and extensible. Attention is given to supporting flexibility in log formatting (see: Formats) & levels (see: Using custom logging levels), and ensuring those APIs decoupled from the implementation of transport logging (i.e. how the logs are stored / indexed, see: Adding Custom Transports) to the API that they exposed to the programmer.

Quick Start

TL;DR? Check out the quick start example in ./examples/. There are a number of other examples in ./examples/*.js. Don't see an example you think should be there? Submit a pull request to add it!

Usage

The recommended way to use winston is to create your own logger. The simplest way to do this is using winston.createLogger:

const winston = require('winston');
 
const logger = winston.createLogger({
  level: 'info',
  format: winston.format.json(),
  defaultMeta: { service: 'user-service' },
  transports: [
    //
    // - Write all logs with level `error` and below to `error.log`
    // - Write all logs with level `info` and below to `combined.log`
    //
    new winston.transports.File({ filename: 'error.log', level: 'error' }),
    new winston.transports.File({ filename: 'combined.log' }),
  ],
});
 
//
// If we're not in production then log to the `console` with the format:
// `${info.level}: ${info.message} JSON.stringify({ ...rest }) `
//
if (process.env.NODE_ENV !== 'production') {
  logger.add(new winston.transports.Console({
    format: winston.format.simple(),
  }));
}

You may also log directly via the default logger exposed by require('winston'), but this merely intended to be a convenient shared logger to use throughout your application if you so choose.

Table of contents

• Motivation
• Quick Start
• Usage
• Table of Contents
• Logging
  • Creating your logger
  • Streams, objectMode, and info objects
• Formats
  • Combining formats
  • String interpolation
  • Filtering info Objects
  • Creating custom formats
• Logging levels
  • Using logging levels
  • Using custom logging levels
• Transports
  • Multiple transports of the same type
  • Adding Custom Transports
  • Common Transport options
• Exceptions
  • Handling Uncaught Exceptions with winston
  • To Exit or Not to Exit
• Rejections
  • Handling Uncaught Promise Rejections with winston
• Profiling
• Streaming Logs
• Querying Logs
• Further Reading
  • Using the default logger
  • Awaiting logs to be written in winston
  • Working with multiple Loggers in winston
• Installation
• Run Tests

Logging

Logging levels in winston conform to the severity ordering specified by

from most important to least important._

const levels = { 
  error: 0,
  warn: 1,
  info: 2,
  http: 3,
  verbose: 4,
  debug: 5,
  silly: 6
};

Creating your own Logger

You get started by creating a logger using winston.createLogger:

const logger = winston.createLogger({
  transports: [
    new winston.transports.Console(),
    new winston.transports.File({ filename: 'combined.log' })
  ]
});

A logger accepts the following parameters:

The levels provided to createLogger will be defined as convenience methods on the logger returned.

//
// Logging
//
logger.log({
  level: 'info',
  message: 'Hello distributed log files!'
});
 
logger.info('Hello again distributed logs');

You can add or remove transports from the logger once it has been provided to you from winston.createLogger:

const files = new winston.transports.File({ filename: 'combined.log' });
const console = new winston.transports.Console();
 
logger
  .clear()          // Remove all transports
  .add(console)     // Add console transport
  .add(files)       // Add file transport
  .remove(console); // Remove console transport

You can also wholesale reconfigure a winston.Logger instance using the configure method:

const logger = winston.createLogger({
  level: 'info',
  transports: [
    new winston.transports.Console(),
    new winston.transports.File({ filename: 'combined.log' })
  ]
});
 
//
// Replaces the previous transports with those in the
// new configuration wholesale.
//
const DailyRotateFile = require('winston-daily-rotate-file');
logger.configure({
  level: 'verbose',
  transports: [
    new DailyRotateFile(opts)
  ]
});

Creating child loggers

You can create child loggers from existing loggers to pass metadata overrides:

const logger = winston.createLogger({
  transports: [
    new winston.transports.Console(),
  ]
});
 
const childLogger = logger.child({ requestId: '451' });

Streams, objectMode, and info objects

In winston, both Logger and Transport instances are treated as objectMode streams that accept an info object.

The info parameter provided to a given format represents a single log message. The object itself is mutable. Every info must have at least the level and message properties:

const info = {
  level: 'info',                 // Level of the logging message  
  message: 'Hey! Log something?' // Descriptive message being logged.
};

Properties besides level and message are considered as "meta". i.e.:

const { level, message, ...meta } = info;

Several of the formats in logform itself add additional properties:

As a consumer you may add whatever properties you wish – internal state is maintained by Symbol properties:

• Symbol.for('level') (READ-ONLY): equal to level property. Is treated as immutable by all code.
• Symbol.for('message'): complete string message set by "finalizing formats":
  • json
  • logstash
  • printf
  • prettyPrint
  • simple
• Symbol.for('splat'): additional string interpolation arguments. Used exclusively by splat() format.

These Symbols are stored in another package: triple-beam so that all consumers of logform can have the same Symbol reference. i.e.:

const { LEVEL, MESSAGE, SPLAT } = require('triple-beam');
 
console.log(LEVEL === Symbol.for('level'));
// true
 
console.log(MESSAGE === Symbol.for('message'));
// true
 
console.log(SPLAT === Symbol.for('splat'));
// true

NOTE: any { message } property in a meta object provided will automatically be concatenated to any msg already provided: For example the below will concatenate 'world' onto 'hello':

logger.log('error', 'hello', { message: 'world' });
logger.info('hello', { message: 'world' });

Formats

Formats in winston can be accessed from winston.format. They are implemented in logform, a separate module from winston. This allows flexibility when writing your own transports in case you wish to include a default format with your transport.

In modern versions of node template strings are very performant and are the recommended way for doing most end-user formatting. If you want to bespoke format your logs, winston.format.printf is for you:

const { createLogger, format, transports } = require('winston');
const { combine, timestamp, label, printf } = format;
 
const myFormat = printf(({ level, message, label, timestamp }) => {
  return `${timestamp} [${label}] ${level}: ${message}`;
});
 
const logger = createLogger({
  format: combine(
    label({ label: 'right meow!' }),
    timestamp(),
    myFormat
  ),
  transports: [new transports.Console()]
});

To see what built-in formats are available and learn more about creating your own custom logging formats, see logform.

Combining formats

Any number of formats may be combined into a single format using format.combine. Since format.combine takes no opts, as a convenience it returns pre-created instance of the combined format.

const { createLogger, format, transports } = require('winston');
const { combine, timestamp, label, prettyPrint } = format;
 
const logger = createLogger({
  format: combine(
    label({ label: 'right meow!' }),
    timestamp(),
    prettyPrint()
  ),
  transports: [new transports.Console()]
})
 
logger.log({
  level: 'info',
  message: 'What time is the testing at?'
});
// Outputs:
// { level: 'info',
//   message: 'What time is the testing at?',
//   label: 'right meow!',
//   timestamp: '2017-09-30T03:57:26.875Z' }

String interpolation

The log method provides the string interpolation using util.format. It must be enabled using format.splat().

Below is an example that defines a format with string interpolation of messages using format.splat and then serializes the entire info message using format.simple.

const { createLogger, format, transports } = require('winston');
const logger = createLogger({
  format: format.combine(
    format.splat(),
    format.simple()
  ),
  transports: [new transports.Console()]
});
 
// info: test message my string {}
logger.log('info', 'test message %s', 'my string');
 
// info: test message 123 {}
logger.log('info', 'test message %d', 123);
 
// info: test message first second {number: 123}
logger.log('info', 'test message %s, %s', 'first', 'second', { number: 123 });

Filtering info Objects

If you wish to filter out a given info Object completely when logging then simply return a falsey value.

const { createLogger, format, transports } = require('winston');
 
// Ignore log messages if they have { private: true }
const ignorePrivate = format((info, opts) => {
  if (info.private) { return false; }
  return info;
});
 
const logger = createLogger({
  format: format.combine(
    ignorePrivate(),
    format.json()
  ),
  transports: [new transports.Console()]
});
 
// Outputs: {"level":"error","message":"Public error to share"}
logger.log({
  level: 'error',
  message: 'Public error to share'
});
 
// Messages with { private: true } will not be written when logged.
logger.log({
  private: true,
  level: 'error',
  message: 'This is super secret - hide it.'
});

Use of format.combine will respect any falsey values return and stop evaluation of later formats in the series. For example:

const { format } = require('winston');
const { combine, timestamp, label } = format;
 
const willNeverThrow = format.combine(
  format(info => { return false })(), // Ignores everything
  format(info => { throw new Error('Never reached') })()
);

Creating custom formats

Formats are prototypal objects (i.e. class instances) that define a single method: transform(info, opts) and return the mutated info:

• info: an object representing the log message.
• opts: setting specific to the current instance of the format.

They are expected to return one of two things:

• An info Object representing the modified info argument. Object references need not be preserved if immutability is preferred. All current built-in formats consider info mutable, but [immutablejs] is being considered for future releases.
• A falsey value indicating that the info argument should be ignored by the caller. (See: Filtering info Objects) below.

winston.format is designed to be as simple as possible. To define a new format simple pass it a transform(info, opts) function to get a new Format.

The named Format returned can be used to create as many copies of the given Format as desired:

const { format } = require('winston');
 
const volume = format((info, opts) => {
  if (opts.yell) {
    info.message = info.message.toUpperCase();
  } else if (opts.whisper) {
    info.message = info.message.toLowerCase();
  }
 
  return info;
});
 
// `volume` is now a function that returns instances of the format.
const scream = volume({ yell: true });
console.dir(scream.transform({
  level: 'info',
  message: `sorry for making you YELL in your head!`
}, scream.options));
// {
//   level: 'info'
//   message: 'SORRY FOR MAKING YOU YELL IN YOUR HEAD!'
// }
 
// `volume` can be used multiple times to create different formats.
const whisper = volume({ whisper: true });
console.dir(whisper.transform({
  level: 'info',
  message: `WHY ARE THEY MAKING US YELL SO MUCH!`
}, whisper.options));
// {
//   level: 'info'
//   message: 'why are they making us yell so much!'
// }

Logging Levels

Logging levels in winston conform to the severity ordering specified by

from most important to least important._

Each level is given a specific integer priority. The higher the priority the more important the message is considered to be, and the lower the corresponding integer priority. For example, as specified exactly in RFC5424 the syslog levels are prioritized from 0 to 7 (highest to lowest).

{ 
  emerg: 0, 
  alert: 1, 
  crit: 2, 
  error: 3, 
  warning: 4, 
  notice: 5, 
  info: 6, 
  debug: 7
}

Similarly, npm logging levels are prioritized from 0 to 6 (highest to lowest):

{ 
  error: 0, 
  warn: 1, 
  info: 2, 
  http: 3,
  verbose: 4, 
  debug: 5, 
  silly: 6 
}

If you do not explicitly define the levels that winston should use, the npm levels above will be used.

Using Logging Levels

Setting the level for your logging message can be accomplished in one of two ways. You can pass a string representing the logging level to the log() method or use the level specified methods defined on every winston Logger.

//
// Any logger instance
//
logger.log('silly', "127.0.0.1 - there's no place like home");
logger.log('debug', "127.0.0.1 - there's no place like home");
logger.log('verbose', "127.0.0.1 - there's no place like home");
logger.log('info', "127.0.0.1 - there's no place like home");
logger.log('warn', "127.0.0.1 - there's no place like home");
logger.log('error', "127.0.0.1 - there's no place like home");
logger.info("127.0.0.1 - there's no place like home");
logger.warn("127.0.0.1 - there's no place like home");
logger.error("127.0.0.1 - there's no place like home");
 
//
// Default logger
//
winston.log('info', "127.0.0.1 - there's no place like home");
winston.info("127.0.0.1 - there's no place like home");

winston allows you to define a level property on each transport which specifies the maximum level of messages that a transport should log. For example, using the syslog levels you could log only error messages to the console and everything info and below to a file (which includes error messages):

const logger = winston.createLogger({
  levels: winston.config.syslog.levels,
  transports: [
    new winston.transports.Console({ level: 'error' }),
    new winston.transports.File({
      filename: 'combined.log',
      level: 'info'
    })
  ]
});

You may also dynamically change the log level of a transport:

const transports = {
  console: new winston.transports.Console({ level: 'warn' }),
  file: new winston.transports.File({ filename: 'combined.log', level: 'error' })
};
 
const logger = winston.createLogger({
  transports: [
    transports.console,
    transports.file
  ]
});
 
logger.info('Will not be logged in either transport!');
transports.console.level = 'info';
transports.file.level = 'info';
logger.info('Will be logged in both transports!');

winston supports customizable logging levels, defaulting to npm style logging levels. Levels must be specified at the time of creating your logger.

Using Custom Logging Levels

In addition to the predefined npm, syslog, and cli levels available in winston, you can also choose to define your own:

const myCustomLevels = {
  levels: {
    foo: 0,
    bar: 1,
    baz: 2,
    foobar: 3
  },
  colors: {
    foo: 'blue',
    bar: 'green',
    baz: 'yellow',
    foobar: 'red'
  }
};
 
const customLevelLogger = winston.createLogger({
  levels: myCustomLevels.levels
});
 
customLevelLogger.foobar('some foobar level-ed message');

Although there is slight repetition in this data structure, it enables simple encapsulation if you do not want to have colors. If you do wish to have colors, in addition to passing the levels to the Logger itself, you must make winston aware of them:

winston.addColors(myCustomLevels.colors);

This enables loggers using the colorize formatter to appropriately color and style the output of custom levels.

Additionally, you can also change background color and font style. For example,

baz: 'italic yellow',
foobar: 'bold red cyanBG'

Possible options are below.

• Font styles: bold, dim, italic, underline, inverse, hidden, strikethrough.

• Font foreground colors: black, red, green, yellow, blue, magenta, cyan, white, gray, grey.

• Background colors: blackBG, redBG, greenBG, yellowBG, blueBG magentaBG, cyanBG, whiteBG

Colorizing Standard logging levels

To colorize the standard logging level add

winston.format.combine(
  winston.format.colorize(),
  winston.format.json()
);

where winston.format.json() is whatever other formatter you want to use. The colorize formatter must come before any formatters adding text you wish to color.

Transports

There are several core transports included in winston, which leverage the built-in networking and file I/O offered by Node.js core. In addition, there are additional transports written by members of the community.

Multiple transports of the same type

It is possible to use multiple transports of the same type e.g. winston.transports.File when you construct the transport.

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

If you later want to remove one of these transports you can do so by using the transport itself. e.g.:

const combinedLogs = logger.transports.find(transport => {
  return transport.filename === 'combined.log'
});
 
logger.remove(combinedLogs);

Adding Custom Transports

Adding a custom transport is easy. All you need to do is accept any options you need, implement a log() method, and consume it with winston.

const Transport = require('winston-transport');
const util = require('util');
 
//
// Inherit from `winston-transport` so you can take advantage
// of the base functionality and `.exceptions.handle()`.
//
module.exports = class YourCustomTransport extends Transport {
  constructor(opts) {
    super(opts);
    //
    // Consume any custom options here. e.g.:
    // - Connection information for databases
    // - Authentication information for APIs (e.g. loggly, papertrail, 
    //   logentries, etc.).
    //
  }
 
  log(info, callback) {
    setImmediate(() => {
      this.emit('logged', info);
    });
 
    // Perform the writing to the remote service
    callback();
  }
};

Common Transport options

As every transport inherits from winston-transport, it's possible to set a custom format and a custom log level on each transport separately:

const logger = winston.createLogger({
  transports: [
    new winston.transports.File({
      filename: 'error.log',
      level: 'error',
      format: winston.format.json()
    }),
    new transports.Http({
      level: 'warn',
      format: winston.format.json()
    }),
    new transports.Console({
      level: 'info',
      format: winston.format.combine(
        winston.format.colorize(),
        winston.format.simple()
      )
    })
  ]
});

Exceptions

Handling Uncaught Exceptions with winston

With winston, it is possible to catch and log uncaughtException events from your process. With your own logger instance you can enable this behavior when it's created or later on in your applications lifecycle:

const { createLogger, transports } = require('winston');
 
// Enable exception handling when you create your logger.
const logger = createLogger({
  transports: [
    new transports.File({ filename: 'combined.log' }) 
  ],
  exceptionHandlers: [
    new transports.File({ filename: 'exceptions.log' })
  ]
});
 
// Or enable it later on by adding a transport or using `.exceptions.handle`
const logger = createLogger({
  transports: [
    new transports.File({ filename: 'combined.log' }) 
  ]
});
 
// Call exceptions.handle with a transport to handle exceptions
logger.exceptions.handle(
  new transports.File({ filename: 'exceptions.log' })
);

If you want to use this feature with the default logger, simply call .exceptions.handle() with a transport instance.

//
// You can add a separate exception logger by passing it to `.exceptions.handle`
//
winston.exceptions.handle(
  new winston.transports.File({ filename: 'path/to/exceptions.log' })
);
 
//
// Alternatively you can set `handleExceptions` to true when adding transports
// to winston.
//
winston.add(new winston.transports.File({
  filename: 'path/to/combined.log',
  handleExceptions: true
}));

To Exit or Not to Exit

By default, winston will exit after logging an uncaughtException. If this is not the behavior you want, set exitOnError = false

const logger = winston.createLogger({ exitOnError: false });
 
//
// or, like this:
//
logger.exitOnError = false;

When working with custom logger instances, you can pass in separate transports to the exceptionHandlers property or set handleExceptions on any transport.

Example 1

const logger = winston.createLogger({
  transports: [
    new winston.transports.File({ filename: 'path/to/combined.log' })
  ],
  exceptionHandlers: [
    new winston.transports.File({ filename: 'path/to/exceptions.log' })
  ]
});

Example 2

const logger = winston.createLogger({
  transports: [
    new winston.transports.Console({
      handleExceptions: true
    })
  ],
  exitOnError: false
});

The exitOnError option can also be a function to prevent exit on only certain types of errors:

function ignoreEpipe(err) {
  return err.code !== 'EPIPE';
}
 
const logger = winston.createLogger({ exitOnError: ignoreEpipe });
 
//
// or, like this:
//
logger.exitOnError = ignoreEpipe;

Rejections

Handling Uncaught Promise Rejections with winston

With winston, it is possible to catch and log uncaughtRejection events from your process. With your own logger instance you can enable this behavior when it's created or later on in your applications lifecycle:

const { createLogger, transports } = require('winston');
 
// Enable rejection handling when you create your logger.
const logger = createLogger({
  transports: [
    new transports.File({ filename: 'combined.log' }) 
  ],
  rejectionHandlers: [
    new transports.File({ filename: 'rejections.log' })
  ]
});
 
// Or enable it later on by adding a transport or using `.rejections.handle`
const logger = createLogger({
  transports: [
    new transports.File({ filename: 'combined.log' }) 
  ]
});
 
// Call rejections.handle with a transport to handle rejections
logger.rejections.handle(
  new transports.File({ filename: 'rejections.log' })
);

If you want to use this feature with the default logger, simply call .rejections.handle() with a transport instance.

//
// You can add a separate rejection logger by passing it to `.rejections.handle`
//
winston.rejections.handle(
  new winston.transports.File({ filename: 'path/to/rejections.log' })
);
 
//
// Alternatively you can set `handleRejections` to true when adding transports
// to winston.
//
winston.add(new winston.transports.File({
  filename: 'path/to/combined.log',
  handleRejections: true
}));

Profiling

In addition to logging messages and metadata, winston also has a simple profiling mechanism implemented for any logger:

//
// Start profile of 'test'
//
logger.profile('test');
 
setTimeout(function () {
  //
  // Stop profile of 'test'. Logging will now take place:
  //   '17 Jan 21:00:00 - info: test duration=1000ms'
  //
  logger.profile('test');
}, 1000);