Outputting Logs inside JSON Object with Winston in Node.js

Question

So I am using winston.js to log to a file. I have set up the logger to output JSON with the Winston default JSON format.

This is my Logger configuration.

logger.util.js:

'use strict';

const winston = require('winston');
const moment = require('moment');

const logger = winston.createLogger({
    format: winston.format.json(),
    transports: [
        new winston.transports.File({
  filename: './logs/' + moment(moment.now()).format('DD-MM-YYYY') + '-API.json',
    handleExceptions: true
  })
 ],
});

module.exports = logger;

When logging an event like so:

logger.log({
  'level': 'info',
  'timestamp': moment.now(),
  'account_id': res.account_id || null,
  'action': JSON.stringify(payload),
  'request_id':  res.request_id,
  'status': success.status.code,
  'route' : res.method +  ' ' + res.route,
  'bytes': res.socket.bytesRead || null,
  'elapsed': res.elapsed || null,
});

I get the following output into my log file

Output:

{
   "level": "error",
   "timestamp": 1544708669700,
   "account_id": 7,
   "action": "SequelizeDatabaseError: column \"alert\" does not exist",
   "request_id": "27cc338b-3980-4818-a9e7-83380b1b2c3a",
   "status": 500,
   "route": "POST /post/new",
   "bytes": 714,
   "elapsed": 35
}
{
   "level": "info",
   "timestamp": 1544709322038,
   "action": "{\"device\":{\"id\":57},\"removed\":55}",
   "status": 200,
   "route": "undefined undefined",
   "bytes": 517,
   "elapsed": null
}

I want the log to out to my file like so. So the data can be searched at a later date.

Desired Output:

[
  {
     "level": "error",
     "timestamp": 1544708669700,
     "account_id": 7,
     "action": "SequelizeDatabaseError: column \"alert\" does not exist",
     "request_id": "27cc338b-3980-4818-a9e7-83380b1b2c3a",
     "status": 500,
     "route": "POST /post/new",
     "bytes": 714,
     "elapsed": 35
  },
  {
     "level": "info",
     "timestamp": 1544709322038,
     "action": "{\"device\":{\"id\":57},\"removed\":55}",
     "status": 200,
     "route": "undefined undefined",
     "bytes": 517,
     "elapsed": null
  }
]

I understand I may need to create a custom format with Winston and wondered if someone could give me an example? After my research, I haven't been able to find something which is similar enough to work off and the docs don't give a similar example either.

Answer 1

I hope this is what you were looking for.

Create a custom transport in the following way:

const Transport = require('winston-transport');
const util = require('util');
const fs = require('fs');

module.exports = class CustomTransport extends Transport {
    constructor(opts) {
        super(opts);
        this.filename = opts.filename;
        this.setup();
    }

    initialize() {
        try {
            fs.writeFileSync(this.filename, [], 'utf8');
        } catch (error) {
            console.log(error);
        }
    }

    setup() {
        // This checks if the file exists
        if (fs.existsSync(this.filename)) {
            // The content of the file is checked to know if it is necessary to adapt the array
            try {
                const data = fs.readFileSync(this.filename, 'utf8');
                // If the content of the file is not an array, it is set
                const content = JSON.parse(data);
                if (!Array.isArray(content)) {
                    this.initialize();
                }
            } catch (error) {
                this.initialize();
                console.log(error);
            }
        }
        // Otherwise create the file with the desired format
        else {
            this.initialize();
        }
    }

    readLog() {
        let data = null;
        try {
            data = fs.readFileSync(this.filename, 'utf8');
        } catch (error) {

            console.log(error);
        }
        return data;
    }

    writeLog(info) {
        const data = this.readLog();
        let arr = [];
        if (data) {
            arr = JSON.parse(data);
        }
        //add data
        arr.push(info);
        //convert it back to json
        const json = JSON.stringify(arr);
        try {
            // Writing the array again
            fs.writeFileSync(this.filename, json, 'utf8');
        } catch (error) {
            console.log(error)
        }
    }

    log(info, callback) {
        setImmediate(() => {
            this.emit('logged', info);
        });
        // Perform the writing
        this.writeLog(info);

        callback();
    }
};

and then following the code that you placed this would be the adaptation with the custom transporter:

'use strict';

const winston = require('winston');
const CustomTransport = require('./CustomTransport');
const moment = require('moment');

const logger = winston.createLogger({
    format: winston.format.json(),
    transports: [
        new CustomTransport({
            filename: moment(moment.now()).format('DD-MM-YYYY') + '-API.json',
            handleExceptions: true
        })
    ],
});

logger.log({
    'level': 'info',
    'timestamp': moment.now(),
    'account_id': res.account_id || null,
    'action': JSON.stringify(payload),
    'request_id': res.request_id,
    'status': success.status.code,
    'route': res.method + ' ' + res.route,
    'bytes': res.socket.bytesRead || null,
    'elapsed': res.elapsed || null,
});

I tried the service with dummy data and this was the result:

[
    {
        "level": "info",
        "timestamp": 1544723041103,
        "account_id": 1,
        "action": 6,
        "request_id": 2,
        "status": 3,
        "route": 4,
        "bytes": 5,
        "elapsed": 5
    },
    {
        "level": "info",
        "timestamp": 1544724862768,
        "account_id": 1,
        "action": 6,
        "request_id": 2,
        "status": 3,
        "route": 4,
        "bytes": 5,
        "elapsed": 5
    }
]