Introduction to Log4js

Log4js is a versatile logging library for Node.js applications, providing a variety of powerful features and configuration options to handle logging effectively. This library supports multiple appenders, log levels, and even custom layouts, making it a go-to solution for developers aiming to implement robust logging strategies in their applications.

Basic Setup

To get started with Log4js, you need to install it via npm:

  npm install log4js

Then, include Log4js in your Node.js application:

  const log4js = require('log4js');

Creating a Simple Logger

Here’s how you can create a simple logger:

  log4js.configure({
    appenders: { out: { type: 'stdout' } },
    categories: { default: { appenders: ['out'], level: 'info' } }
  });

  const logger = log4js.getLogger();
  logger.info('This is an informational message');

Using Different Log Levels

Log4js supports different log levels. You can log messages as debug, info, warn, error, or fatal:

  logger.debug('This is a debug message');
  logger.info('This is an info message');
  logger.warn('This is a warning message');
  logger.error('This is an error message');
  logger.fatal('This is a fatal message');

File Logging

To log messages to a file, configure the file appender:

  log4js.configure({
    appenders: { file: { type: 'file', filename: 'app.log' } },
    categories: { default: { appenders: ['file'], level: 'info' } }
  });

  const fileLogger = log4js.getLogger('file');
  fileLogger.info('Logging to a file');

Using Multiple Appenders

You can use multiple appenders to log messages to different destinations:

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

  const multiLogger = log4js.getLogger();
  multiLogger.info('Logging to stdout and file');

Custom Log Layouts

Log4js allows you to define custom layouts for your log messages:

  log4js.configure({
    appenders: {
      custom: {
        type: 'file',
        filename: 'custom.log',
        layout: {
          type: 'pattern',
          pattern: '%d %p %c %m%n'
        }
      }
    },
    categories: { default: { appenders: ['custom'], level: 'info' } }
  });

  const customLogger = log4js.getLogger('custom');
  customLogger.info('This is an info message with a custom layout');

Creating an Application with Log4js

Let’s put it all together in a sample application:

  const express = require('express');
  const log4js = require('log4js');

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

  const app = express();
  const logger = log4js.getLogger();

  app.use((req, res, next) => {
    logger.debug(`Received request for ${req.url}`);
    next();
  });

  app.get('/', (req, res) => {
    logger.info('Root endpoint hit');
    res.send('Hello World');
  });

  app.use((err, req, res, next) => {
    logger.error('An error occurred', err);
    res.status(500).send('Internal Server Error');
  });

  app.listen(3000, () => {
    logger.info('Application started on port 3000');
  });

With these configurations and examples, you can efficiently incorporate Log4js into your Node.js applications to handle complex logging requirements with ease.

Hash: 648960f09f0520c432b1a11cd7b1ec6cb50b6bd3eebb5c8244827c5cb7250a38