gistfile1.js

var format      = require('util').format
  , MongoClient = require('mongodb').MongoClient
  , mongo_uri   = process.env.MONGOHQ_URL
  , villians    = process.argv.slice(2)

// TODO customize these for your needs
var mongo_options = {
  w: 1, // write concern
  slaveOk: true // send reads to secondaries
}

MongoClient.connect(mongo_uri, mongo_options, function(err,db) {
  // TODO do proper error handling
  if (err) { return console.error(err) }

  // ensure that superman is in collection
  // give superman a fan every time we update
  // add villians provided on command-line
  var query   = {name: 'superman'}
    , update  = {$inc: {fan_count: 1}, $addToSet: {villians: {$each: villians}}}
    , options = {upsert: true}

  db.collection('superheros').update(query, update, options, function(err, result) {
    if (err) { return console.error(err) }

    // how many fans does superman have?
    db.collection('superheros').findOne(query, function(err, doc) {
      if (err) { return console.error(err) }

      console.log(format('superman has %d fans', doc.fan_count))
      if(doc.villians.length) {
        console.log(format('superman is fighting villians: %s', doc.villians.join(', ')))
      }
      // allow program to exit by clearing up db resources
      db.close()
    })
  })
})