Node.js Async/Await module export

Question

I'm kinda new to module creation and was wondering about module.exports and waiting for async functions (like a mongo connect function for example) to complete and exporting the result. The variables get properly defined using async/await in the module, but when trying to log them by requiring the module, they show up as undefined. If someone could point me in the right direction, that'd be great. Here's the code I've got so far:

// module.js

const MongoClient = require('mongodb').MongoClient
const mongo_host = '127.0.0.1'
const mongo_db = 'test'
const mongo_port = '27017';

(async module => {

  var client, db
  var url = `mongodb://${mongo_host}:${mongo_port}/${mongo_db}`

  try {
    // Use connect method to connect to the Server
    client = await MongoClient.connect(url, {
      useNewUrlParser: true
    })

    db = client.db(mongo_db)
  } catch (err) {
    console.error(err)
  } finally {
    // Exporting mongo just to test things
    console.log(client) // Just to test things I tried logging the client here and it works. It doesn't show 'undefined' like test.js does when trying to console.log it from there
    module.exports = {
      client,
      db
    }
  }
})(module)

And here's the js that requires the module

// test.js

const {client} = require('./module')

console.log(client) // Logs 'undefined'

I'm fairly familiar with js and am still actively learning and looking into things like async/await and like features, but yeah... I can't really figure that one out

Solution

You have to export synchronously, so its impossible to export client and db directly. However you could export a Promise that resolves to client and db:

module.exports = (async function() {
 const client = await MongoClient.connect(url, {
   useNewUrlParser: true
 });

  const db = client.db(mongo_db);
  return { client, db };
})();

So then you can import it as:

const {client, db} = await require("yourmodule");

(that has to be in an async function itself)

PS: console.error(err) is not a proper error handler, if you cant handle the error just crash

Comments

[Admin]

Ahh I see.. In the end I'll be exporting something that requires a mongo connection, not exporting the connection itself. That does make sense though, thank you :)

[Jonas Wilms]

@brocococonut glad to help :)

[Elad]

Because you return a function, that approach will open a new connection for each require. isn't it?

[Jonas Wilms]

@elad no, i return the return value of a function

[Elad]

Thanks. So ill need to require this in every function i want to query the DB? feel very 'boilerplatey'

[Jonas Wilms]

@elad in every file (/module)

[Jonas Wilms]

This will create multiple clients ...not sure if that is needed. Also the API endpoint will respond with an error if you call it before the db client loaded.

[Dev AKS]

How we can achieve the same for es6 imports (import abc from 'sample-module')?

[Jonas Wilms]

@DevAKS const abc = await import("./sample-module");