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
• logLevelFilter
• multiFile
• multiprocess
• noLogFilter
• recording
• stderr
• stdout
• tcp
• tcp-server

Optional Appenders

The following appenders are supported by log4js, but are no longer distributed with log4js core from version 3 onwards.

• gelf
• hipchat
• logFaces-HTTP
• logFaces-UDP
• loggly
• logstashHTTP
• logstashUDP
• mailgun
• rabbitmq
• redis
• slack
• smtp

For example, if you were previously using the gelf appender (type: 'gelf') then you should add @log4js-node/gelf to your dependencies and change the type to type: '@log4js-node/gelf'.

Other Appenders

These appenders are maintained by its own authors and may be useful for you:

• udp
• cloudwatch
• sentry

Custom 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.

Advanced configuration

If you’ve got a custom appender of your own, or are using webpack (or some other bundler), you may find it easier to pass in the appender module in the config instead of loading from the node.js require path. Here’s an example:

const myAppenderModule = {
  configure: (config, layouts, findAppender, levels) => {
    /* ...your appender config... */
  },
};
log4js.configure({
  appenders: { custom: { type: myAppenderModule } },
  categories: { default: { appenders: ["custom"], level: "debug" } },
});