async constructor in nodejs

Question

I'm trying to run 'Mongoose' and only continue my task when it's connected, but what happens is that the task runs first and then connects the Mongo

export class App {
    constructor() {
        console.log("a1");
        this.config();
        console.log("a2");
    }
    public async config() {
        // Connect to MongoDB
        console.log("b1");
        try {
            await mongoose.connect(stringConnection, { useNewUrlParser: true }).finally();

            console.log("MongoDB Running");
        } catch (error) {
            console.log(error);
            process.exit();
        }
        console.log("b2");
    }
}

Answer:

a1
b1
a2


MongoDB Running
b2

Answer I wanted:

a1
b1


MongoDB Running
b2
a2

Estus Flask · Accepted Answer · 2019-04-04 14:08:36Z

3

Asynchronous constructor is potentially an antipattern, exactly because it doesn't provide proper control flow when an instance is created. It's expected that an instance is ready to use when it's created with new but it isn't ready.

A proper way to handle this is to have asynchronous hooks that are executed outside the class:

export class App {
    constructor() {}

    async init() {
        console.log("a1");
        await this.config();
        console.log("a2");
    }

    async config() {
       ...
    }
}

...

const app = new App();
await app.init();

answered Apr 4, 2019 at 14:08

Estus Flask

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2 Comments

wellington.silverio Over a year ago

it worked, thank you very much, but when I instantiate "new Class ()" it gets async again, so it ends up running the code from below and then starting the mongo

Estus Flask Over a year ago

@wellington.silverio Yes, it's asynchronous, because promises and async are in use. I'm not sure why this is a problem. const app = new App(); await app.init() should result in a sequence you wanted.

ethane · Accepted Answer · 2019-04-04 14:04:09Z

0

You can make use of Promise.then (because async functions implicity return Promises), stuff the things you want done after your config() in its then callback.

constructor() {
  console.log("a1");
  this.config().then(_ =>
    console.log("a2");
  });
}

answered Apr 4, 2019 at 14:04

ethane

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2 Comments

wellington.silverio Over a year ago

it worked, thank you very much, but when I instantiate "new Class ()" it gets async again, so it ends up running the code from below and then starting the mongo

wellington.silverio Over a year ago

Code: console.log("1"); const app = New App(); console.log("2"); Response: 1 a1 b1 a2 2 MongoDB Running b2

FloWy · Accepted Answer · 2019-04-04 14:04:45Z

0

A good way is to learn about Promises

The good way to go could be to return in the config a Promise and after that you can use the then function in the Promise.

answered Apr 4, 2019 at 14:04

FloWy

9947 silver badges15 bronze badges

2 Comments

wellington.silverio Over a year ago

it worked, thank you very much, but when I instantiate "new Class ()" it gets async again, so it ends up running the code from below and then starting the mongo

wellington.silverio Over a year ago

Code: console.log("1"); const app = new App(); console.log("2"); Response: 1 a1 b1 a2 2 MongoDB Running b2

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