How to close all connections to MongoDB server

dbconn.js

var mongodb  = require('mongodb')
  , server   = new mongodb.Server('localhost', 27017, {}) 
  , db       = new mongodb.Db('test', server, {}) 
  ;

console.log('[1]', db.serverConfig._serverState);
db.open(function (err, db) {
  console.log('[2]', err, db.serverConfig._serverState);
  db.close();
  console.log('[3]', err, db.serverConfig._serverState);
});

console.log('[4]', db.serverConfig._serverState);
db.open(function (err, db) {
  console.log('[5]', err, db.serverConfig._serverState);
  db.close();
  console.log('[6]', err, db.serverConfig._serverState);
});

// Output:
// [1] disconnected
// [4] connecting
// [5] null connected
// [6] null disconnected
// [2] null disconnected
// [3] null disconnected