Node.js - reference db is not defined

var express = require('express');

var bodyParser = require('body-parser');

var mongoose = require('mongoose');

//var methodOverride = require("method-override");

var app = express();

// Connection to DB

mongoose

.connect (db, { useNewUrlParser: true })

.then(

    () => {console.log('Database is connected') },

    err => { console.log('Can not connect to the database' +err)

});

// Middlewares

app.use(bodyParser.urlencoded({ extended: false }));

app.use(bodyParser.json());

//app.use(methodOverride());

// Import Models and Controllers

var models = require('./models/client')(app, mongoose);

var ClientCtrl = require('./controllers/clients');

var router = express.Router();

// Index - Route

router.get('/', function(req, res) {

 res.send("Hola Mundo - www.programacion.com.py");

});

app.use(router);

// API routes

var api = express.Router();

api.route('/clients')

 .get(ClientCtrl.findAll)

 .post(ClientCtrl.add);

api.route('/clients/:id')

 .get(ClientCtrl.findById)

 .put(ClientCtrl.update)

 .delete(ClientCtrl.delete);

app.use('/api', api);

// Start server

app.listen(3000, function() {

 console.log("Node server running on http://localhost:3000");

});