Working fine for me on Mongodb 4.2 and Mongoose 5.7.13
Node.js
const Connect = async () => {
    let url = "mongodb://localhost:27017/test_db";
    try {
        let client = await Mongoose.connect( url, {
            poolSize: 10,
            authSource: "admin",
            user: "root",
            pass: "root123", 
            useCreateIndex: true,
            useNewUrlParser: true,
            useUnifiedTopology: true
        } );
        log( "Database is connected!" );
    } catch ( error ) {
        log( error.stack );
        process.exit( 1 );
    }
}
Connect();
/etc/mongod.conf
systemLog:
  destination: file
  logAppend: true
  path: /var/log/mongodb/mongod.log
storage:
  dbPath: /var/lib/mongo
  journal:
    enabled: true
processManagement:
  fork: true  # fork and run in background
  pidFilePath: /var/run/mongodb/mongod.pid  # location of pidfile
  timeZoneInfo: /usr/share/zoneinfo
net:
  port: 27017
  bindIp: 0.0.0.0 
setParameter:
   enableLocalhostAuthBypass: false
security:
  authorization: enabled
Database User
use admin;
db.createUser(
  {
    user: "root",
    pwd: "root123",
    roles: [ { role: "userAdminAnyDatabase", db: "admin" }, "readWriteAnyDatabase" ]
  }
)
show users;
{
   "_id": "admin.root",
   "userId": UUID( "5db3aafd-b1fd-4bea-925e-8a4bfb709f22" ),
   "user": "root",
   "db": "admin",
   "roles": [ {
        "role": "userAdminAnyDatabase",
        "db": "admin"
      },
      {
        "role": "readWriteAnyDatabase",
        "db": "admin"
      }
   ],
   "mechanisms": [
       "SCRAM-SHA-1",
       "SCRAM-SHA-256"
   ]
}