mongodb的一个问题

Question

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});我在这段代码中有几个不懂点。。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})它们的作用是干什么的？？？求助。麻烦知道的给解答下。谢谢！