Category: 08. Winston

If you’re logging to a database or an external service, querying logs can be useful. For instance, with MongoDB and winston-mongodb, you can query logs like this:

const winston = require('winston');
require('winston-mongodb');

const logger = winston.createLogger({
  level: 'info',
  format: winston.format.combine(
winston.format.timestamp(),
winston.format.json()
  ),
  transports: [
new winston.transports.MongoDB({
  db: 'mongodb://localhost/logs',
  collection: 'log',
  level: 'info'
})
  ]
});

// Querying logs from MongoDB
const MongoClient = require('mongodb').MongoClient;

MongoClient.connect('mongodb://localhost/logs', function (err, client) {
  if (err) throw err;

  const db = client.db('logs');
  const collection = db.collection('log');

  collection.find({ level: 'info' }).toArray(function (err, docs) {
if (err) throw err;
console.log(docs);
client.close();
  });
});

For managing log file sizes and rotation, you can use the winston-daily-rotate-file transport. Install it first:

npm install winston-daily-rotate-file

Then configure it like so:

const winston = require('winston');
require('winston-daily-rotate-file');

const transport = new winston.transports.DailyRotateFile({
  filename: 'application-%DATE%.log',
  datePattern: 'YYYY-MM-DD',
  maxSize: '20m',
  maxFiles: '14d',
  level: 'info',
  format: winston.format.combine(
winston.format.timestamp(),
winston.format.json()
  )
});

const logger = winston.createLogger({
  level: 'info',
  transports: [transport]
});

logger.info('This message will be logged to a daily rotated file');

Winston supports creating custom transports to log messages in unique ways. Here’s a basic example of a custom transport:

const { TransportStreamOptions } = require('winston-transport');

class CustomTransport extends TransportStreamOptions {
  constructor(options) {
super(options);
this.level = options.level || 'info';
  }

  log(info, callback) {
setImmediate(() => this.emit('logged', info));
console.log(${info.level}: ${info.message});
if (callback) callback();
  }
}

const logger = winston.createLogger({
  level: 'info',
  format: winston.format.simple(),
  transports: [
new CustomTransport({ level: 'info' })
  ]
});

logger.info('This is a custom transport log message');

Winston can handle uncaught exceptions and unhandled rejections.

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

process.on('unhandledRejection', (ex) => {
  throw ex;
});

You can send logs to multiple destinations. For example, you might want to log errors to a file and info messages to the console.

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

Winston provides several formats to structure your logs. You can also create custom formats.

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

This example combines a timestamp and JSON format.

Winston uses different log levels. By default, they are:

• error
• warn
• info
• http
• verbose
• debug
• silly

You can configure the level of logging you want. For example, if you only want to log info and above, you set the level to info.

const logger = winston.createLogger({
  level: 'info',
  format: winston.format.json(),
  transports: [
new winston.transports.Console()
  ]
});

Here’s a simple example to get you started:

const winston = require('winston');

const logger = winston.createLogger({
  level: 'info',
  format: winston.format.simple(),
  transports: [
new winston.transports.Console()
  ]
});

logger.info('This is an info message');
logger.error('This is an error message');

Here’s a simple example to get you started:

const winston = require('winston');

const logger = winston.createLogger({
  level: 'info',
  format: winston.format.simple(),
  transports: [
new winston.transports.Console()
  ]
});

logger.info('This is an info message');
logger.error('This is an error message');

First, you need to install Winston via npm. Open your terminal and run:

npm install winston