This is a mongodb localhost:27017 connection code in node js express. We can connect simply mongodb(PORT: localhost:27017 ) to our node js project as this code...

express-app.js

db.connect((err)=>{
  if (err) {
    console.log("Database connection Error :"+err);
  }else{
    console.log("Database connected to port 27017");
  }
})

in using files include this.js

var db = require('./config/connection')

mongodb-connection.js

const mongoClient = require('mongodb').MongoClient;
const state={
    db:null
}

module.exports.connect=(done)=>{
    const url='mongodb://localhost:27017';
    const dbname='DB_NAME';

    mongoClient.connect(url,{ useNewUrlParser: true, useUnifiedTopology: true },(err,data)=>{
        if (err) {
            return done(err)
        }else{
            state.db=data.db(dbname)
            done()
        }
    })
}
module.exports.get=()=>{
    return state.db
}

mongoose-connection.js

const mongoose = require("mongoose");

const server = "mongodb://localhost:27017";
const dbName = "DB_NAME_HERE";
const dbURI = server + "/" + dbName;

function connect() {
  mongoose
    .connect(dbURI, {
      useNewUrlParser: true,
      useUnifiedTopology: true,
      useFindAndModify: false,
      useCreateIndex: true,
    })
    .then(() => {
      console.log("Database connection successful.");
    })
    .catch((err) => {
      console.log("Database connection error : " + err);
    });
}

module.exports = { connect };