UnhandledPromiseRejectionWarning: MongooseServerSelectionError

Try this It seems like you are using MongoDB atlas

mongoose
     .connect( uri, { useNewUrlParser: true, useCreateIndex: true, useUnifiedTopology: true })
     .then(() => console.log( 'Database Connected' ))
     .catch(err => console.log( err ));

and go to your MongoDB Atlas -> NetworkAccess-> Edit-> and add Current IP address it works!!