NodeJS how use connect obj in different file

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i think this is mainly a design-pattern challange. At the start of my Program i want to connect to my MongoDB Database and I'll get the connection object back.

//app.js
const start = async () => {
  try {
    const db = await mongoose.connect(process.env.MONGO_URI);
    const port = process.env.PORT;
    app.listen(port, console.log(`Server is listening on port ${port}...`));
  } catch (error) {
    console.error(error);
  }
};

start();

registerCustomer is a function that gets executed if a specific post request is send. Inside registerCustomer I want to use the db object that gets runtime defined in the app.js file. I want to write something like in the following code example but this will of course not work, because I have no access to the db object.

const registerCustomer = async (req, res) => {
    const session = await db.startSession(); //this will fail because I have no reference to the db object.
try {
    session.startTransaction();

    await Customer.create([{ firstName: "john" }], { session: session });
    await Customer.create([{ firstName: "Doe" }], { session: session });
    await session.commitTransaction();
  } catch (error) {
    await session.abortTransaction();
  }
  finally {
    session.endSession();
  }

How can I solve this problem the most elegant way? How can I access the db object from other functions? Thank you for reading.

express node.js transactions
1 Answers

Directory Structure

app.js
|__database
    |__conn.js

This is your MongoDB connection file fileName: conn.js

const mongoose = require("mongoose");

const DB = process.env.DATABASE;
mongoose
  .connect(DB, {
    useNewUrlParser: true,
  })
  .then((result) => {
    console.log("Connection Successful on 'mongoosecrud' Cluster");
  })
  .catch((err) => {
    console.log("Connection NOT Successful" + err);
  });

Now simply you can require this connection file in your root server.js or app.js file

require("./database/conn.js")
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