Unable to log errors using winston in Node.js

Question

On running the following code, the console says logger.log is not a function even though I am exporting the function from index.js.

index.js

const winston = require('winston');
const error = require('./middleware/error');

const logger=winston.createLogger({transports:[new winston.transports.Console(),
                                               new winston.transports.File({filename:'logfile.log'})
                                              ]});

app.use(error);
module.exports = logger;

error.js

const winston = require('winston');
const logger = require('../index');

module.exports = function(err,req,res,next){
    //Log the exception 
    logger.log('error',err.message);

    res.status(500).send('Something failed.');
}

This is what the terminal says:

TypeError: logger.log is not a function
    at module.exports (C:\Users\hp\Desktop\YouStart\nodejs-mosh\vidly\5.13- Project- Restructure the App\vidly\middleware\error.js:8:12)
    at C:\Users\hp\Desktop\YouStart\nodejs-mosh\vidly\5.13- Project- Restructure the App\vidly\node_modules\express-async-errors\index.js:5:20
    at Layer.handle_error (C:\Users\hp\Desktop\YouStart\nodejs-mosh\vidly\5.13- Project- Restructure the App\vidly\node_modules\express\lib\router\layer.js:71

Answer 1

Circular dependencies are never a good pattern. You're forcing error.js to resolve its exports before index.js can resolve its exports - this is probably leading to your issue.

You should avoid circular dependencies basically in every situation. It's easy: You never need to have error.js require index.js, because you can instead just export a function that lets you pass whatever is needed from index.js as a parameter. It would look like this:

index.js

const winston = require('winston');

const logger = winston.createLogger({transports:[new winston.transports.Console(),
                                                 new winston.transports.File({filename:'logfile.log'})
                                              ]});

const error = require('./middleware/error')(logger); //pass logger to error middleware  
app.use(error);

error.js

module.exports = function(logger) {
    //return function configured with logger to use as middleware
    return function(err,req,res,next) {
        //Log the exception 
        logger.log('error',err.message);

        res.status(500).send('Something failed.');
    }
};