log4js

What is log4js?

The log4js npm package is a logging library for Node.js, inspired by the Java-based Log4j. It provides a flexible logging system that can be configured to log to the console, to files, or to external logging services. It supports multiple log levels, categories, and appenders, allowing for fine-grained control over logging output.

What are log4js's main functionalities?

Basic Logging

This code sets up log4js to log messages to the standard output (console). It configures an appender named 'out' that writes to stdout and sets the default logging level to 'info'.

const log4js = require('log4js');
log4js.configure({
  appenders: { 'out': { type: 'stdout' } },
  categories: { default: { appenders: ['out'], level: 'info' } }
});
const logger = log4js.getLogger();
logger.info('Informational message');

File Appender

This code configures log4js to write log messages to a file named 'app.log' in the 'logs' directory. It creates a file appender and sets the logging level to 'warn'.

const log4js = require('log4js');
log4js.configure({
  appenders: { 'file': { type: 'file', filename: 'logs/app.log' } },
  categories: { default: { appenders: ['file'], level: 'info' } }
});
const logger = log4js.getLogger();
logger.warn('Warning message');

Log Levels

This code demonstrates the use of different log levels available in log4js. Each level represents a different severity of logging, with 'trace' being the least severe and 'fatal' being the most severe.

const log4js = require('log4js');
const logger = log4js.getLogger('myCategory');
logger.trace('Trace message');
logger.debug('Debug message');
logger.info('Info message');
logger.warn('Warn message');
logger.error('Error message');
logger.fatal('Fatal message');

Multiple Appenders and Categories

This code shows how to configure multiple appenders and categories. The 'default' category logs to both the console and a file, while the 'special' category logs only to a file and only logs messages at the 'error' level or higher.

const log4js = require('log4js');
log4js.configure({
  appenders: {
    'out': { type: 'stdout' },
    'file': { type: 'file', filename: 'logs/app.log' }
  },
  categories: {
    default: { appenders: ['out', 'file'], level: 'info' },
    special: { appenders: ['file'], level: 'error' }
  }
});
const defaultLogger = log4js.getLogger();
defaultLogger.info('Default category, logged to console and file');
const specialLogger = log4js.getLogger('special');
specialLogger.error('Special category, logged to file only');

Other packages similar to log4js

winston

Winston is a multi-transport async logging library for Node.js. Like log4js, it supports multiple transports (e.g., file, console, HTTP) and custom log levels. It is highly configurable and is considered one of the most popular logging solutions in the Node.js ecosystem, often compared to log4js for its rich features and flexibility.

bunyan

Bunyan is a simple and fast JSON logging library for Node.js services. It includes a CLI tool for pretty-printing log files. Bunyan's focus on JSON logging format makes it particularly suitable for use in large-scale distributed systems where log aggregation and analysis are important. It is less configurable than log4js and winston but is valued for its simplicity and streaming capabilities.

pino

Pino is a very low-overhead Node.js logger that outputs logs in JSON format. It is designed for performance and can be significantly faster than other logging libraries like log4js, especially in high-throughput scenarios. Pino's API is relatively minimal compared to log4js, focusing on delivering the best performance possible.

README

log4js-node

This is a conversion of the log4js framework to work with node. I've mainly stripped out the browser-specific code and tidied up some of the javascript.

Out of the box it supports the following features:

• coloured console logging
• replacement of node's console.log functions (optional)
• file appender, with log rolling based on file size
• SMTP appender
• GELF appender
• hook.io appender
• multiprocess appender (useful when you've got worker processes)
• a logger for connect/express servers
• configurable log message layout/patterns
• different log levels for different log categories (make some parts of your app log as DEBUG, others only ERRORS, etc.)

NOTE: from log4js 0.5 onwards you'll need to explicitly enable replacement of node's console.log functions. Do this either by calling log4js.replaceConsole() or configuring with an object or json file like this:

{
    appenders: [
        { type: "console" }
    ],
    replaceConsole: true
}

installation

npm install log4js

usage

Minimalist version:

       var log4js = require('log4js');
       var logger = log4js.getLogger();
       logger.debug("Some debug messages");

By default, log4js outputs to stdout with the coloured layout (thanks to masylum), so for the above you would see:

[2010-01-17 11:43:37.987] [DEBUG] [default] - Some debug messages

See example.js for a full example, but here's a snippet (also in fromreadme.js):

var log4js = require('log4js'); 
//console log is loaded by default, so you won't normally need to do this
//log4js.loadAppender('console');
log4js.loadAppender('file');
//log4js.addAppender(log4js.appenders.console());
log4js.addAppender(log4js.appenders.file('logs/cheese.log'), 'cheese');

var logger = log4js.getLogger('cheese');
logger.setLevel('ERROR');

logger.trace('Entering cheese testing');
logger.debug('Got cheese.');
logger.info('Cheese is Gouda.');
logger.warn('Cheese is quite smelly.');
logger.error('Cheese is too ripe!');
logger.fatal('Cheese was breeding ground for listeria.');

Output:

[2010-01-17 11:43:37.987] [ERROR] cheese - Cheese is too ripe!
[2010-01-17 11:43:37.990] [FATAL] cheese - Cheese was breeding ground for listeria.

The first 5 lines of the code above could also be written as:

var log4js = require('log4js');
log4js.configure({
        appenders: [
                { type: 'console' },
                { type: 'file', filename: 'logs/cheese.log', category: 'cheese' }
        ]
});

configuration

You can configure the appenders and log levels manually (as above), or provide a configuration file (log4js.configure('path/to/file.json')), or a configuration object. The configuration file location may also be specified via the environment variable LOG4JS_CONFIG (export LOG4JS_CONFIG=path/to/file.json). An example file can be found in test/log4js.json. An example config file with log rolling is in test/with-log-rolling.json. By default, the configuration file is checked for changes every 60 seconds, and if changed, reloaded. This allows changes to logging levels to occur without restarting the application.

To turn off configuration file change checking, configure with:

var log4js = require('log4js');
log4js.configure('my_log4js_configuration.json', {});

To specify a different period:

log4js.configure('file.json', { reloadSecs: 300 });

For FileAppender you can also pass the path to the log directory as an option where all your log files would be stored.

log4js.configure('my_log4js_configuration.json', { cwd: '/absolute/path/to/log/dir' });

If you have already defined an absolute path for one of the FileAppenders in the configuration file, you could add a "absolute": true to the particular FileAppender to override the cwd option passed. Here is an example configuration file:

#### my_log4js_configuration.json ####
{
  "appenders": [
    {
      "type": "file",
      "filename": "relative/path/to/log_file.log",
      "maxLogSize": 20480,
      "backups": 3,
      "category": "relative-logger"
    },
    {
      "type": "file",
      "absolute": true,
      "filename": "/absolute/path/to/log_file.log",
      "maxLogSize": 20480,
      "backups": 10,
      "category": "absolute-logger"          
    }
  ]
}

Documentation for most of the core appenders can be found on the wiki, otherwise take a look at the tests and the examples.

Documentation

See the wiki. Improve the wiki, please.

Contributing

Contributions welcome, but take a look at the rules first.

License

The original log4js was distributed under the Apache 2.0 License, and so is this. I've tried to keep the original copyright and author credits in place, except in sections that I have rewritten extensively.