Rest api with node.js

Hi fellows! Getting this nasty error.
node:159192) UnhandledPromiseRejectionWarning: TypeError: callback is not a function
at C:\Users\ddobrikov\Desktop\cashierles\node_modules\mongoose\lib\connection.js:785:13
at processTicksAndRejections (internal/process/task_queues.js:93:5)
(node:159192) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:159192) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

my code

const express = require('express'); 
const app = express(); 
const mongoose = require('mongoose'); 
const bodyParser = require('body-parser');
require('dotenv/config');


//import routes
const postsRoute = require('./routes/posts');  

app.use(bodyParser);

app.use('/posts', postsRoute); 

//routes 

app.get('/',(req, res) => { 
   res.send('We are on home');
}) 


// connect to a database
mongoose.connect(process.env.DB_CONNECTION,{ useUnifiedTopology: true }, {useNewUrlParser: true}, () => console.log('Connect to Db'));

//server 
app.listen(3000); 
const express = require('express'); 
const router  = express.Router();  
const Post = require('../models/Post');

router.get('/',(req, res) => { 
    res.send('We are on posts');
})   

router.get('/specific',(req, res) => { 
    res.send('We are on admin');
})  


router.post('/',(req, res) => { 
    console.log(req.body);
}) 
module.exports= router;
`type or paste code here`

Hello!

That’s because the mongoose.connect accepts only three parameters.

Even if delete the function and leave three parameters still getting the error. Have you got another idea why getting it?