How to return a promise inside the async await in javascript?

Question

Is it ok to do something like this?

I mean, Is return await new Promise() a valid expression as of below?:

  async pushImage(image, basePath) {
    const imgId = this.angularDatabase.createPushId();
    const route = `${basePath}${imgId}`;
    const imageRef = this.angularFireStorage.ref(route);
    
    return await concat( 
      imageRef.put(image.image).snapshotChanges().pipe(ignoreElements()),
      defer(() => imageRef.getDownloadURL())
    )
      .pipe(
        map(
          (url) => (
             { ...image, url }
          )
        )
      )
      .toPromise();
  }

If you await and then return I guess it will just return normal value not promise — Sagar
– Sagar, Commented Nov 2, 2020 at 5:48
Because the function is async it will always return a promise value! You don't technically need the return await and you could defer the await to the callee of pushImage — Dan Starns
– Dan Starns, Commented Nov 2, 2020 at 5:52
You don't need to use async await at all. Just return the result. Also, whatever you return from an async function is always a Promise. — Bunyamin Coskuner
– Bunyamin Coskuner, Commented Nov 2, 2020 at 5:52
@mex ... yes, you need async to use await ... but since your only promise is the last statement, you don't need to use async/await for that function, the result will always be a Promise anyway - i.e. if there's only ONE promise, and it's the thing that is being returned, there's no need to await .... so there's no need for async — Jaromanda X
– Jaromanda X, Commented Nov 2, 2020 at 5:55

Cooper Scott · Accepted Answer · 2020-11-02 06:08:28Z

1

Anything returned from an async functions comes out as a promise.

For example:

async getDate() {
   return new Date();
}

will return a promise with the contents of a date.

An example of reading this overly complicated getDate() function would be:

getDate().then(date => {
   console.log("The date: ", date);
});

However, you can also return a promise. This makes your code above perfectly valid, except you don't need to include the await in the return.

You can "simply" just say

return concat( 
          imageRef.put(image.image).snapshotChanges().pipe(ignoreElements()),
          defer(() => imageRef.getDownloadURL())
       ).pipe(
          map(url => {
             return {...image, url}
          })
       ).toPromise();

Side note: I am not certain, but I think you should return the object in the map. Changing {...image, url} to return {...image, url}

Additional response:

To retrieve the contents of a promise, use the .then() method. Example:

myReturnedPromise.then(dataOfMyReturnedPromise => {
   //This code is called after the promise is resolved.
   console.log("The returned value is: ", dataOfMyReturnedPromise);
}).catch(error => {
   //If this code is called, the promise completed with an error
   //This might be executed if you're retreiving data from the internet
   //and your internet connection is too poor to get the data
   console.error("Error: ", error);
});

If this answered your question, please mark it as the accepted answer.

edited Nov 2, 2020 at 6:08

answered Nov 2, 2020 at 5:56

Cooper Scott

7161 gold badge9 silver badges20 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

mx_code Over a year ago

Ok. Thanks. is it ok to return a promise inside a then() {} block?

Jaromanda X Over a year ago

@mex - yes, it is, the Promise that the .then returns will "become" the Promise you return in a .then

Cooper Scott Over a year ago

@mex, I've extended my answer to include a response to your question. Also, if this answered your question, please consider marking this as the accepted answer (check mark button).

Collectives™ on Stack Overflow

How to return a promise inside the async await in javascript?

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related