unable to fix some error with mongodb - node.js

Question

Here is the error message which i got while trying to run the project using gulp. I guess the problem is caused because of mongodb.
Gulp is running my app on PORT: 8000
events.js:160
throw er; // Unhandled 'error' event
^
MongoError: failed to connect to server [localhost:27017] on first connect
[MongoError: connect ECONNREFUSED 127.0.0.1:27017]
at Pool.<anonymous> (C:\Users\H SHASHANK
KUMAR\Desktop\code\node_modules\mongodb-core\lib\topologies\server.js:336:35)
at emitOne (events.js:96:13)
at Pool.emit (events.js:188:7)
at Connection.<anonymous> (C:\Users\H SHASHANK KUMAR\Desktop\code\node_modules\mongodb-core\lib\connection\pool.js:280:12)
at Connection.g (events.js:292:16)
at emitTwo (events.js:106:13)
at Connection.emit (events.js:191:7)
at Socket.<anonymous> (C:\Users\H SHASHANK
KUMAR\Desktop\code\node_modules\mongodb-
core\lib\connection\connection.js:187:49)
at Socket.g (events.js:292:16)
at emitOne (events.js:96:13)
at Socket.emit (events.js:188:7)
at emitErrorNT (net.js:1277:8)
at _combinedTickCallback (internal/process/next_tick.js:80:11)
at process._tickCallback (internal/process/next_tick.js:104:9)
Below is the code which may contain the error.
var express = require('express')
mongoose = require('mongoose'); // creating reference for express
var db = mongoose.connect('mongodb://localhost/bookAPI'); //using mongoose
to connect to mongo db.
var Book = require('./models/bookModel'); //mongo db uees models to retrive
data.
var app = express(); // creating an instance of express to run app.
var port = process.env.PORT || 3000;
var bookRouter = express.Router();
bookRouter.route('/Books')
.get(function(req, res){
Book.find(function(err, books){
if(err)
console.log(err);
else
res.json(books);
});
});
app.use('/api', bookRouter);
app.get('/', function(req, res){
res.send('welcome to my API');
});
app.listen(port, function(){
console.log('Gulp is running my app on PORT: '+port);
});
I have tried searching for the error in other possible post on stack overflow but i didn't find any fix for my problem.