MongoNetworkError: failed to connect to server [localhost:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017]

 I'm new in nodeJS, started learning by following a trailer on youtube, everything goes well until I added the connect function if mongodb,

mongo.connect("mongodb://localhost:27017/mydb")

when I run my code on cmd (node start-app), get the following error,

MongoNetworkError: failed to connect to server [localhost:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017]
Could someone explain to me which step I missed? my code :
var express = require("express");
var MongoClient = require('mongodb');
var url = "mongodb://localhost:27017/mydb";
var webService = require("./webService");
var server = express();
MongoClient.connect(url, function (err, db) {
    if (err) throw err;
    console.log("Database created!");
    db.close();
});
server.use(express.urlencoded({ extended: true }));
server.set('views', __dirname);
server.get('/', function (request, response) {
    response.sendFile(__dirname + '/MainPage.html');
});
server.get('/Sign', function (request, response) {
    response.render(__dirname + '/Sign.ejs');
});
server.post("/signUp", webService.signUp);
server.post("/createUser", webService.createUser);
server.listen(5500);