When using the async pipe with Angular's ngFor, an error message indicates that the argument is

Question

When using the async pipe with Angular's ngFor, an error message indicates that the argument is

Can you explain why this error message is appearing:

Argument of type '[string, { startTime: string; endTime: string; }][] | null' is not assignable to parameter of type 'Collection<unknown>'.

It occurs when attempting to utilize the async pipe within an iteration of a ngFor loop:

 <div *ngFor="let slotDay of slots | async | paginate: { id: 'pagination', itemsPerPage: 3, currentPage: p }">

The slot variable in question is defined as follows:

aaa: Subject<[string, {startTime: string, endTime: string}][]> = new Subject();

Does this error indicate that Angular is expecting the Subject to potentially return a null value? How can this issue be resolved?

Your assistance is greatly appreciated.

angular typescript async-pipe

Answer 1

Answer №1

Make sure to check your pipe paginate. This pipe should accept null or an array as arguments and return an array (which can also be empty). Have you added the paginate pipe to your question?

You could also try breaking down your code into two steps:

<ng-container *ngIf="slots|async as slotsAsync">
   <div *ngFor="let slotDay of slotsAsync | 
           paginate: { id: 'pagination', itemsPerPage: 3, currentPage: p }">
    ...
   </div>
</ng-container>

Note: As a convention in Angular, variables that hold observables are typically named with a $ at the end, like stols$ instead of just slots (although this is not mandatory, it helps with clarity for anyone reading the code).

Answer 2

Make sure to check your pipe paginate. This pipe should accept null or an array as arguments and return an array (which can also be empty). Have you added the paginate pipe to your question?

You could also try breaking down your code into two steps:

<ng-container *ngIf="slots|async as slotsAsync">
   <div *ngFor="let slotDay of slotsAsync | 
           paginate: { id: 'pagination', itemsPerPage: 3, currentPage: p }">
    ...
   </div>
</ng-container>

Note: As a convention in Angular, variables that hold observables are typically named with a $ at the end, like stols$ instead of just slots (although this is not mandatory, it helps with clarity for anyone reading the code).

When using the async pipe with Angular's ngFor, an error message indicates that the argument is

Answer №1

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