Node.js reuse MongoDB reference

mydb.js:

var mongodb= require('mongodb'),
  server = new mongodb.Server('staff.mongohq.com', 10030, {
    auto_reconnect: true
  }),
  db1 = new mongodb.Db('mydb', server);


// callback: (err, db)
function openDatabase(callback) {
  db1.open(function(err, db) {
    if (err)
      return callback(err);

    console.log('Database connected');

    return callback(null, db);
  });
}

// callback: (err, collection)
function authenticate(db, username, password, callback) {
  db.authenticate(username, password, function(err, result) {
    if (err) {
      return callback (err);
    }
    if (result) {
      var collection = new mongodb.Collection(db, 'test');

      // always, ALWAYS return the error object as the first argument of a callback
      return callback(null, collection);
    } else {
      return callback (new Error('authentication failed'));
    }
  });
}

exports.openDatabase = openDatabase;
exports.authenticate = authenticate;

use.js:

var mydb = require('./mydb');
// open the database once
mydb.openDatabase(function(err, db) {
  if (err) {
    console.log('ERROR CONNECTING TO DATABASE');
    console.log(err);
    process.exit(1);
  }

  // authenticate once after you opened the database. What's the point of 
  // authenticating on-demand (for each query)?
  mydb.authenticate(db, 'usernsame', 'password', function(err, collection) {
    if (err) {
      console.log('ERROR AUTHENTICATING');
      console.log(err);
      process.exit(1);
    }

    // use the returned collection as many times as you like INSIDE THE CALLBACK
    collection.find({}, {limit: 10})
    .toArray(function(err, docs) {
      console.log('\n------ 1 ------');
      console.log(docs);
    });

    collection.find({}, {limit: 10})
    .toArray(function(err, docs) {
      console.log('\n------ 2 ------');
      console.log(docs);
    });
  });
});

Result:

on success:

 Database connected
 Database user authenticated

------ 1 ------
[ { _id: 4f86889079a120bf04e48550, asd: 'asd' } ]

------ 2 ------
[ { _id: 4f86889079a120bf04e48550, asd: 'asd' } ]

on failure:

Database connected
{ [MongoError: auth fails] name: 'MongoError', errmsg: 'auth fails', ok: 0 }

---

[Original Answer]:

You're opening the db multiple times (once in each query). You should open the database just once, and use the db object in the callback for later use.

You're using the same variable name multiple times, and that might've caused some confusion.

var mongodb = require('mongodb'),
    server = new mongodb.Server('staff.mongohq.com', 10030, {
        auto_reconnect: true
    }),
    db1 = new mongodb.Db('mydb', server);

function authenticateAndGo(db, handle) {
    db.authenticate('username', 'password', function(err) {
        if (err) {
            console.log(err);
            return;
        }
        console.log('Database user authenticated');

        var collection = new mongodb.Collection(db, 'test');

        handle(collection);
    });
}

function query(handle) {
    db1.open(function(err, db2) {
        if( err ) {
            console.log(err);
            return;
        }
        console.log('Database connected');

        authenticateAndGo(db2, handle);
    });
};
exports.query = query;

I've changed the above code a little (db1 for the original db, db2 for the opened db). As you can see, you're opening db1 multiple times, which is not good. extract the code for opening into another method and use it ONCE and use the db2 instance for all your queries/updates/removes/...

You can only call "open" once. When the open callback fires, you can then do your queries on the DB object it returns. So one way to handle this is to queue up the requests until the open completes. e.g MyMongo.js

var mongodb = require('mongodb');

function MyMongo(host, port, dbname) {
    this.host = host;
    this.port = port;
    this.dbname = dbname;

    this.server = new mongodb.Server(
                              'localhost', 
                              9000, 
                              {auto_reconnect: true});
    this.db_connector = new mongodb.Db(this.dbname, this.server);

    var self = this;

    this.db = undefined;
    this.queue = [];

    this.db_connector.open(function(err, db) {
            if( err ) {
                console.log(err);
                return;
        }
        self.db = db;
        for (var i = 0; i < self.queue.length; i++) {
            var collection = new mongodb.Collection(
                                 self.db, self.queue[i].cn);
            self.queue[i].cb(collection);
        }
        self.queue = [];

    });
}
exports.MyMongo = MyMongo;

MyMongo.prototype.query = function(collectionName, callback) {
    if (this.db != undefined) {
        var collection = new mongodb.Collection(this.db, collectionName);
        callback(collection);
        return;
    }
    this.queue.push({ "cn" : collectionName, "cb" : callback});
}

and then a sample use:

var MyMongo = require('./MyMongo.js').MyMongo;

var db = new MyMongo('localhost', 9000, 'db1');
var COL = 'col';

db.query(COL, function(collection) {
    collection.find({}, {
        limit: 10
    }).toArray(function(err, docs) {
        console.log("First:\n", docs);
    });
});


db.query(COL, function(collection) {
    collection.find({}, {
        limit: 10
    }).toArray(function(err, docs) {
        console.log("\nSecond:\n", docs);
    });
});

I simply call the open function once directly after the db init:

var mongodb = require('mongodb');
var server = new mongodb.Server('foo', 3000, {auto_reconnect: true});
var db = new mongodb.Db('mydb', server);   
db.open(function(){});

After that I do not have to care about that anymore because of auto_reconnect is true.

db.collection('bar', function(err, collection) { [...] };