如何创建与node.js和MongoDB数据库的连接？

Question

var Db = require('mongodb').Db,
MongoClient = require('mongodb').MongoClient,
Server = require('mongodb').Server,
ReplSetServers = require('mongodb').ReplSetServers,
ObjectID = require('mongodb').ObjectID,
Binary = require('mongodb').Binary,
GridStore = require('mongodb').GridStore,
Grid = require('mongodb').Grid,
Code = require('mongodb').Code,
BSON = require('mongodb').pure().BSON,
assert = require('assert');

// Set up the connection to the local db
var mongoclient = new MongoClient(new Server("localhost", 27017), {native_parser: true});

// Open the connection to the server
mongoclient.open(function(err, mongoclient) {

// Get the first db and do an update document on it
var db = mongoclient.db("braj");
db.collection('logintbl').update({a:1}, {b:1}, {upsert:true}, function(err, result) {
  assert.equal(null, err);
  assert.equal(1, result);

// Get another db and do an update document on it
  var db2 = mongoclient.db("braj");
  db2.collection('logintbl').update({a:1}, {b:1}, {upsert:true}, function(err, result) {
   assert.equal(null, err);
   assert.equal(1, result);

// Close the connection
  mongoclient.close();

    });
  });
});