Using an existing winston logger

Question

I'm new to Node.js and Javascript, so I'm still learning the ins-and-outs. I come from a python background.

I have an importable function that creates a logger, and pases the logging object back to the main script.

I'm importing other modules into the main script, and I want them to use the same logging object/settings.

In Python, the logger object is a singleton that - when instantiated again from a subscript or imported method - will pick up all the existing transports.

Can this be done as well in JS, without necessarily just passing the logger object in as a parameter of the function?

logger.js

const { createLogger, transports } = require("winston");
let   { format }                   = require("winston");
let logger;
let log_level;
let logfile; 
let screendump;

function formatParams(info) { (...snip...) }

//Set format default here
format = format.combine( ...snip ...);

module.exports = {
    set: function (argv) { // defaults
        /* Set logging. Can be set by command line or config file */
        var log_level  = ( process.env.log_level  != null ? process.env.log_level  : argv["log_level"] )  // Can end as undefined
        var logfile    = ( process.env.logfile    != null ? process.env.logfile    : argv["logfile"]   )  // Can end as undefined
        var screendump = ( process.env.screendump != null ? process.env.screendump : argv["screendump"])  // Can end as undefined

        // Put a check in here to ensure "formatting" is a format object
        if ( typeof formatting !== 'undefined') { format = formatting; }

        logger = createLogger({
                    level : log_level,
                    format: format,
                    transports: [
                    // Start with everything going to dev null...just to satisfy
                    // Winston's requirement for at least one transport.
                    // Add more later 
                    new transports.File({ filename: '/dev/null', level: 'error' }),
                  ]
                });

        // put a file check in here
        if ( typeof logfile !== 'undefined' ) { 
            logger.add(new transports.File({ filename: logfile, format: format }) )
        }

        if ( typeof screendump !== 'undefined' || screendump != false) { 
            logger.add(new transports.Console({ format: format }) );
        }
        return logger;
    },

};

main.js

var   log        = require('./logger');
require('./mySubFunction')();

logger = log.set(argv);
logger.log('info',"Logger successfully started!");

mySubFunction(param = 10);

mySubFunction.js

logger = <import existing logger object created by main?>

module.exports = function() { 
    this.mySubFunction = function (param) { // defaults
        var i;
        for (i = 0; i < param; i++) { 
            logger.log('info', "This is iteration" + String(i));
        }
        return;
    }
}

Answer 1

You can create a logger instance in one file and export it, creating a singleton.

logger.js

var createLogger = require('winston');
module.exports = createLogger();

somethingElse.js

var logger = require('./logger');

process.argv is global so you don't neccessarily have to pass that in from main.