How to handle for promise inside a piped map

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I am definitely sure I am confused here so please any help is appreciated.

Here is my scenario:

I pull from Firestore a document:

return this.afs.collection("events").doc(eventID).snapshotChanges().pipe(
      map( document => {

      })
    );

All is fine up to here.

But inside the map I need a promise to resolve (or not)

For example:

return this.afs.collection("events").doc(eventID).snapshotChanges().pipe(
      map( document => {
        // This is a promise the below part 
        const data = await EventImporterJSON.getFromJSON(document.payload.data())
        return data
      })
    );

I understand that the await cannot happen there. I am very confused how to solve this, perhaps I have not worked long enough with observables and rxjs.

In the end what I am trying to achieve is:

Get the document. Map and process it but inside the process, I need to handle a promise.

I don’t want to return that promise to the caller though.

Does this make sense?

Or have I structured this completely wrong?

Answer

This is a typical use-case for mergeMap or concatMap:

return this.afs.collection("events").doc(eventID).snapshotChanges().pipe(
  mergeMap(document => {
    // This is a promise the below part 
    return EventImporterJSON.getFromJSON(document.payload.data())
  })
);

However, you can use async - await as well because operators such as mergeMap handle Observables, Promises, arrays, etc. the same way, so you can just return a Promise in mergeMaps project function an it will work fine.

Typically, you don’t need to use multiple awaits in a single method because the more “Rx” way of doing things is chaining operators but if you want you can because async method returns a Promise and RxJS will handle it like any other Promise.

const delayedPromise = () => new Promise(resolve => {
  setTimeout(() => resolve(), 1000);
})

of('a').pipe(
  mergeMap(async v => {
    console.log(1);
    await delayedPromise();
    console.log(2);
    await delayedPromise();
    console.log(3);
    await delayedPromise();
    return v;
  })
).subscribe(console.log);
// 1
// 2
// 3
// a

Live demo: https://stackblitz.com/edit/rxjs-3fujcs


Source: stackoverflow