MongoError: failed to connect to server [localhost:27017] on first connect?

I am new in UI field. I have an issue connecting the database locally. Is there any error in my code? I went through many threads but not resolved yet. Please help! This is server.js :

var express = require('express');     
var app = express();

var mongodb = require('mongodb');
var MongoClient = mongodb.MongoClient
var dbOperation = null;

MongoClient.connect('mongodb://localhost:27017/training', function(err, db) {
  if (err) throw err
  dbOperation = db;                           
})

app.use('/mahadev', express.static('src'));

app.get('/todo', function(req, res) {
  dbOperation.collection('todo').find().toArray(function (err, result){
    if (err) throw err
    res.send(result);
    console.log(result)
})
});
app.listen(3000, function() {
  console.log('Example app listening on port 3000!')
})

command prompt says:

C:\Users\dhava>node server.js
Example app listening on port 3000!

C:\Users\dhava\node_modules\mongodb\lib\mongo_client.js:336
          throw err
          ^
MongoError: failed to connect to server [localhost:27017] on first connect
    at Pool.<anonymous> (C:\Users\dhava\node_modules\mongodb-core\lib\topologies\server.js:326:35)
    at emitOne (events.js:96:13)
    at Pool.emit (events.js:188:7)
    at Connection.<anonymous> (C:\Users\dhava\node_modules\mongodb-core\lib\connection\pool.js:270:12)
    at Connection.g (events.js:291:16)
    at emitTwo (events.js:106:13)
    at Connection.emit (events.js:191:7)
    at Socket.<anonymous> (C:\Users\dhava\node_modules\mongodb-core\lib\connection\connection.js:175:49)
    at Socket.g (events.js:291:16)
    at emitOne (events.js:96:13)

Browser says:

This site can’t be reached

localhost refused to connect.
Search Google for localhost 3000 todo
ERR_CONNECTION_REFUSED

Thank you in advanced :)

via Dhaval J. Patel