stream and save stdout on log files. This snippet also manage automatic deletion for files older than X days.

log-rotation.js

#!/usr/bin/env node

/*
Don't forget to:
$ chmod 755 log-rotation.js
$ mkdir logs
Then you can save any generic stdout on logs piping it with log-roation.js:
$ node whatever.js | ./log-rotation.js
*/

var glob = require('glob'),
    moment = require('moment'),
    fs = require('fs'),
    path = require('path'),
    debug = require('debug')('log-rotation'),
    stream = require('logrotate-stream');

var toLogFile = stream({ file: __dirname+'/logs/'+getDate()+'.log', size: '20m', keep: 10 });

debug('start log saving...');
removeOldLogs(2); // older than 2 days
process.stdin.resume();
process.stdin.pipe(toLogFile);


/* FUNCTIONS */

// return Date
function getDate(){
    var data = moment();
    return data.format('YYYY-MM-DD');
}

// return Data range not to delete
function getDateRange(days){
    var n = [];
    var curr = getDate();
    n.push(curr+'.log*|');
    for(var i = 1; i <= days; i++)
      n.push(moment(curr).subtract(i, 'days').format('YYYY-MM-DD')+'.log*|');
    return n.toString().replace(/,/g, '').slice(0, -1);
}

// remove old logs file
function removeOldLogs(d){
  var pattern = getDateRange(d);
  debug('avoid this deletion:', pattern);
  glob("!("+pattern+")", {cwd: __dirname+'/logs'}, function (err, files) {
    if (err) throw err;
    files.forEach(function(file){
      fs.unlinkSync(path.join(__dirname, '/logs/', file));
      debug(file, 'deleted');
    });
  });
}