Log4js - Appenders

Appenders serialise log events to some form of output. They can write to files, send emails, send data over the network. All appenders have a type which determines which appender gets used. For example:

const log4js = require('log4js');
log4js.configure({
  appenders: {
    out: { type: 'stdout' },
    app: { type: 'file', filename: 'application.log' }
  },
  categories: {
    default: { appenders: [ 'out', 'app' ], level: 'debug' }
  }
});

This defines two appenders named ‘out’ and ‘app’. ‘out’ uses the stdout appender which writes to standard out. ‘app’ uses the file appender, configured to write to ‘application.log’.

Core Appenders

The following appenders are included with log4js. Some require extra dependencies that are not included as part of log4js (the smtp appender needs nodemailer for example), and these will be noted in the docs for that appender. If you don’t use those appenders, then you don’t need the extra dependencies.

• categoryFilter
• console
• dateFile
• file
• fileSync
• gelf
• hipchat
• logFaces-HTTP
• logFaces-UDP
• loggly
• logLevelFilter
• logstashUDP
• mailgun
• multiFile
• multiprocess
• recording
• redis
• slack
• smtp
• stderr
• stdout

Other Appenders

Log4js can load appenders from outside the core appenders. The type config value is used as a require path if no matching appender can be found. For example, the following configuration will attempt to load an appender from the module ‘cheese/appender’, passing the rest of the config for the appender to that module:

log4js.configure({
  appenders: { gouda: { type: 'cheese/appender', flavour: 'tasty' } },
  categories: { default: { appenders: ['gouda'], level: 'debug' }}
});

Log4js checks the following places (in this order) for appenders based on the type value:

1. The core appenders: require('./appenders/' + type)
2. node_modules: require(type)
3. relative to the main file of your application: require(path.dirname(require.main.filename) + '/' + type)
4. relative to the process’ current working directory: require(process.cwd() + '/' + type)

If you want to write your own appender, read the documentation first.