TypeError: Cannot read property 'db' of undefined

i am following a project by udemy and i am having a TypeError: Cannot read property 'db' of undefined

var express = require('express');
var router = express.Router();  
var mongo = require('mongodb');  
var monk = require('monk');  
var db = monk('localhost:27017/nodeblog');

/* GET home page. */  
router.get('/', function(req, res, next) {  
var db = req.locals.db;  
var posts = db.get('posts');  
posts.find({},{},function(err, posts){    
 res.render('index',{  
    "posts": posts  
});  
});  
});  

module.exports = router;

via Cobat