log4js singleton pattern

SharedLog.js

/**
 * Creates and returns a reference to a shared instance of log4js. Only a single instance is created
 * regardless of how many references are made.
 *
 * To set your desired log level, change the following line to suit your needs.
 * - `const config = { logLevel: 'debug' };`
 *
 * In another module or file that you wish to log, do the following:
 * - `const SharedLog = require('./path/to/SharedLog.js');`
 * - `const logger = SharedLog.getInstance().logger;`
 * - `logger.debug('whatever you want to log')`
 *   - of course, you can use:
 *     - logger.debug
 *     - logger.error
 *     - logger.XXXX - where XXXX is any of the log levels supported!
 *   - to use the custom log 'dberror':
 *     - `logger.log('dberror', 'whatever you want to log');`
 *
 * Note that the logger (regardless of which one you use) takes variadic arguments. You have to pass at least
 * one argument, or as many as you wish, including passing objects.
 *
 * The logger below also demonstrates the use of a file appender. Any logs sent to 'dberror' will be
 * written to a file called 'run.log'. Ser the log4js documentation for more details on file appenders,
 * including log file rotation and more.
 *
 * More about the Log4js framework here: https://stritti.github.io/log4js/docu/users-guide.html
 * And about the specific NodeJS implementation here: https://log4js-node.github.io/log4js-node/index.html
 *
 * This work is licensed under the MIT License as follows:
 *
 * Copyright 2018 E.A.Soto <[email protected]>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated
 * documentation files (the "Software"), to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and
 * to permit persons to whom the Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all copies or substantial portions
 * of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
 * TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF
 * CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 *
 */

// In your package.json, you need to have a dependency to 'log4js'
const log4js = require('log4js');
// Set logLevel to any of: off, fatal, error, warn, info, debug, trace, all
// In our case, we can also use dberror which will log to a db errors file!
const config = { logLevel: 'debug' };

let SharedLog = (function() {
  // Instance stores a reference to the Singleton
  let _sharedLog;
  let _logger;

  /**
   * Class instance initializer. This is not something you call externally, but is instead
   * used internally by the getInstance() method.
   *
   * @return {{logger: Logger | *}}
   */
  function init() {
    // We don't have a shared log instance yet, so let's create it and store it
    _sharedLog = this;

    // Configure the logger
    log4js.configure({
      levels: {
        dberror: { value: log4js.levels.ERROR.level + 5, colour: 'magenta' },
      },
      appenders: {
        console: {
          type: 'stdout',
          layout: { type: 'colored' },
          level: 'trace',
        },
        // File appenders support log rotation and more. See the log4js documentation for that.
        file: {
          type: 'file',
          // rename your log file here to whatever you want
          filename: 'run.log',
          compress: false,
        },
        dberrorstofile: {
          type: 'logLevelFilter',
          appender: 'file',
          level: 'dberror',
        },
      },
      categories: {
        default: { appenders: ['dberrorstofile', 'console'], level: 'off' },
      },
    });
    // Create an instance as configured
    _logger = log4js.getLogger();
    // Configure the Log Level
    _logger.level = config.logLevel;

    return {
      // Public methods and variables
      // public method: function (someParam) {
      //   console.log(someParam);
      // },
      logger: _logger,
    };
  }

  return {
    /**
     * This is the method used externally which returns an instance of SharedLog.
     * Inside SharedLog, we can then access '.logger' which is our log4js instance.
     * See the comments in the header for this class on how to reference this.
     * @return {*}
     */
    getInstance: function() {
      // Get the Singleton instance if one exists
      // or create one if it doesn't
      if (!_sharedLog) {
        _sharedLog = init();
      }

      return _sharedLog;
    },
  };
})();

module.exports = SharedLog;