A simple pattern for opening a mongoose connection to a MongoDB database while registering listeners on the connection and handling the asynchronous result of opening the connection.

database.js

import mongoose from "mongoose";

function getConnection(databaseURL, connectionListeners) {
  return mongoose.connection;
}

async function openConnection(databaseURL, options) {
  await mongoose.connect(databaseURL, options);
}

export { getConnection, openConnection };

server.js

import { getConnection, openConnection } from "./database";

const databaseURL = process.env.DATABASE_URL;

const dbConnection = getConnection();

dbConnection.on(
  "error",
  logger.error.bind(logger, "mongoDB connection error:")
);

dbConnection.once("open", () => {
  logger.info("Connected to MongoDB");
});

// avoid top-level await
openConnection(databaseURL).then(
  (mongooseInstance) => {
    logger.info("Ready to use mongoose instance");
  },
  (e) => {
    logger.error("Initial mongoose connection error");
    unexpectedErrorHandler(e);
  }
);

function exitHandler() {
  if (server) {
    server.close(() => {
      logger.info("Server closed");
      process.exit(1);
    });
  } else {
    process.exit(1);
  }
}

function unexpectedErrorHandler(error) {
  logger.error(error);
  exitHandler();
}