ngrxPush pipe returns undefined, type is not correct

[samuelfernandez]

Minimal reproduction of the bug/regression with instructions:

https://stackblitz.com/edit/angular-ivy-9bbbxq?file=src/app/app.component.html

Expected behavior:

Right now the typing of the pipe is:
https://github.com/ngrx/platform/blob/master/modules/component/src/push/push.pipe.ts#L82

transform<T>(potentialObservable: ObservableInput<T>): T;

However, if the source doesn't emit on subscription, the first returned value will be undefined.

A more strict and real typing would be:

transform<T>(potentialObservable: ObservableInput<T>): T | undefined;

Versions of NgRx, Angular, Node, affected browser(s) and operating system(s):

@angular: ^11.0.8
@ngrx/component: ^10.1.2

Other information:

I would be willing to submit a PR to fix this issue

[x] Yes (Assistance is provided if you need help submitting a pull request)
[ ] No

[timdeschryver]

Seems like a good change to me.
The Angular async pipe uses null instead of undefined tho, perhaps we should try to keep the same signature/behavior ?

[samuelfernandez]

Angular team acknowledges that undefined would be a better option, but they can't make the change because it would be too breaking.

• Proposal - use initial empty value for async pipe as undefined rather than null angular/angular#16982 (comment)
• Change async pipe's default value from null to undefined angular/angular#39785 (comment)

Since this is new in ngrx and there shouldn't be so many compatibility concerns as with Angular, I'd say to go with undefined instead of with null. If this becomes too opinionated we could define a configuration token with the default empty value.

Besides those comments form the Angular issues, I personally would prefer it for an additional reason. The pipe is used mainly at inputs, and having as empty value undefined would allow to simply use an optional property input:

<my-comp [value]="value$ | ngrxPush">

@Input() value?: string;

// Instead of
@Input() value: string | null;

[timdeschryver]

Thanks for the clarification @samuelfernandez
Feel free to create a Pull Request.

[samuelfernandez]

PR created, ready to be reviewed