Prepare for RxJS v7

[spalger]

RxJS v7 is deprecating the .toPromise() method of observable objects, and fixing the type definition to return Promise<T | undefined> as there is no way for the type system to know if the observable will always produce at least one notification so the resolve value may be undefined in all places where .toPromise() is used (though it's made impossible by using certain operators).

This method is replaced with two helpers:

firstValueFrom(x$: Observable<T>): Promise<T>: equivalent of x$.pipe(first()).toPromise(), resolves with the first value, unsubscribes after the first notification, rejects if o$ completes without any notifications.

lastValueFrom(x$: Observable<T>): Promise<T>: equivalent of x$.pipe(last()).toPromise(), resolves after the observable completes with the last notification value, rejects if o$ completes without any notifications.

We should implement these methods in @kbn/std and start migrating parts of the code base to these helpers. Once RxJS v7 is released we can replace our helpers with the standard ones.

If we can get most or all of the codebase migrated to use the new helpers before RxJS v7 is released then we can use an eslint rule to prevent new usage of toPromise(), or just fixup the remaining issues when the final upgrade happens.

[streamich]

RxJS v7 was released two weeks ago: https://github.com/ReactiveX/rxjs/blob/master/CHANGELOG.md

[streamich]

List of breaking changes: https://rxjs.dev/deprecations/breaking-changes

[streamich]

We should implement these methods in @kbn/std and start migrating parts of the code base to these helpers.

Do actually want to implement those methods in @kbn/std? We could get those from v7 a the .toPromise() method in v7 works, we just need to stop using .toPromise() by v8 upgrade.

Although, the return type of .toPromise() will break TypeScript checks.

[streamich]

Another notable change is anonymous function subscription is deprecated.

observable.subscribe(next, error, complete);

Instead only the first parameter of .subscribe() method will be used for subscription, either for next method or for Subscriber.

observable.subscribe(next, options);
observable.subscribe({next, error, complete}, options);