[strictNullChecks] AsyncPipe should return undefined instead of null

🚀 feature request

Relevant Package

This feature request is for @angular/core.

Description

Currently, the AsyncPipe uses null a possible return type, for a better strictNullChecks support , it should return undefined.

export declare class AsyncPipe implements OnDestroy, PipeTransform {
  // ...
  transform<T>(obj: Observable<T> | null | undefined): T | null;
  transform<T>(obj: Promise<T> | null | undefined): T | null;
}

To work around this, components that have an optional @Input() property have to define the possible types as T | null | undefined, instead of just using T | undefined.

Describe the solution you’d like

Add undefined as a possible return type for the transform() method, but keep null as the actual value and add a deprecation message.

And in a new major version, update the pipe to return undefined instead of null.

export declare class AsyncPipe implements OnDestroy, PipeTransform {
  // ...
  transform<T>(obj: Observable<T> | null | undefined): T | undefined;
  transform<T>(obj: Promise<T> | null | undefined): T | undefined;
}

[petebacondarwin]

@sayoojbk - a good start is to have a full read of https://github.com/angular/angular/blob/master/CONTRIBUTING.md

[sayoojbk]

Hi, @SchnWalter @AndrewKushnir could I work on this issue. I have been using Angular for the past couple of months and would like it if I could contribute to this issue. I just started out with open-source contributions so few relevant links for support would be helpful 😄