Rewrite Promise-based code with async\await

Question

I'm trying to rewrite this Promise -based code using async \\ await :

public loadData(id: string): void {
    this.loadDataAsync()
        .then((data: any): void => {
             // Do something with data
        })
        .catch((ex): void => {
             // Do something with ex
        });
}

public loadDataAsync(): Promise<any> {
    // return Promise somehow
}

The rewritten code I have so far is:

public loadData(id: string): void {
    let data: any;

    try {
        data = await this.loadDataAsync();
    } catch(ex) {
         // Do something with ex
    }

    // Do something with data
}

public async loadDataAsync(): Promise<any> {
    // return Promise somehow
}

The problem is that I have to make loadData() method async since it has await in its body. Then, I have to return some Promise from it, since async methods must return a Promise . But I need to maintain the API of loadData() method and return void .

How can I achieve this? How do you break a never ending need for making a method as async when somewhere deep inside inner calls you're calling an async method?

Answer 1

Ideally if you can change the function declaration, just add the async keyword to loadData and use await inside like so:

public async loadData(id: string): void {
    let data: any;

    try {
        data = await this.loadDataAsync();
    } catch(ex) {
         // Do something with ex
    }
}

If you are unable to change the loadData function declaration, then you can just create an anonymous async function inside, and then call await within the anonymous function like so:

public loadData(id: string): void {
    let data: any;

    try {
        data = (async () => { return await this.loadDataAsync() })();
    } catch(ex) {
         // Do something with ex
    }
}

public async loadDataAsync(): Promise<any> {
    return new Promise((resolve, reject) => {
        // Logic goes here
        resolve(data);
    });
}

Whenever you use the await keyword, the surrounding function must be marked as async .

---

As was pointed out by the OP in the comments, in TypeScript it is best practice to mark the promise returning method with async as well (see the loadDataAsync function above), as this ensures the return type is always a promise. More info on that found here:

TypeScript Async / Await Tutorial