var settings = require('../settings'),    Db = require('mongodb').Db,    Connection = require('mongodb').Connection,    Server = require('mongodb').Server;module.exports = new Db(settings.db, new Server(settings.host, settings.port), {safe: true});我在這段代碼中有幾個不懂點。。Db = require('mongodb').Db和Connection = require('mongodb').Connection和Server = require('mongodb').Server和new Db(settings.db, new Server(settings.host, settings.port), {safe: true})它們的作用是干什么的？？？求助。麻煩知道的給解答下。謝謝！