How to add prefix to socket.io logs redirected to winston?

Question

In my node.js application I successfully redirect log messages produced by a socket.io library to a winston library:

var express = require('express')
  , winston = require('winston')
  , http = require('http');
var logger = new (winston.Logger)({
  transports: [
    // ... configuring transports ...
  ]
});
var app = express();
var server = http.createServer(app);
var io = require('socket.io').listen(server, {'logger': logger});

Now I would want to add a prefix (something like "socket.io: ") to all these redirected messages for distinguishing them from log messages produced by other parts of the application. Is there a way how to achieve this?

Answer 1

Add label in logger transports.

var logger = new (winston.Logger)({
  transports: [
    new (winston.transports.Console)({
      json : false,
      timestamp : true,
      label: "socket.io:"
    })
  ]
});

Log messages will look like this -

2013-08-30T08:26:52.703Z - info: [socket.io:] socket.io started
2013-08-30T08:26:52.705Z - info: [socket.io:] hello

Check here more logging options with winston - https://github.com/flatiron/winston

Answer 2

This isn't Winston specific but a generalized JS solution to prefixing and middleware...

var log = console.log;
function x(){ // optionally pass in post/pre fix here
  // caution do not put (var log = console.log()) here, the arguments will build up [ arg1 ] -> [ [arg1] , arg2 ] -> [ [ [ arg1 ] , arg2] , arg3 ] 
  console.log = function () {
        var args = Array.from(arguments);
        args.push('post fix');
        log.apply(console, args);
    }
}

new x()
console.log(1)
new x()
console.log(2)

OUTPUTS: 
1 post fix
2 post fix

OR better yet...

const log = console.log;
export default function middleWare(event) { // optionally pass in post/pre fix here
    console.log = (...args) => {
        log(...args, event.id);
    }
}

Answer 3

I solved it by adding a following function

var winston = require('winston');

// Add prefix function
winston.prefix = function (filename)
{
    var label = '[' + path.parse(filename).name + ']';
    var override = function (lvl) {
        return function () {
            var args = [].slice.call(arguments);
            if (args[0]) {
                if (typeof(args[0]) === 'string')
                    args[0] = label + ' ' + args[0];
                else
                    args.unshift(label);
            }
            winston[lvl].apply(null, args);
        };
    };
    var log = { };
    for (var lvl in winston.levels) {
        log[lvl] = override(lvl);
    }
    return log;
}

And then in each module

var log = require('winston').prefix(__filename);
log.debug('hello');

or

var log = require('winston').prefix('socket_io');
log.debug('hello');