The Angular Observable is throwing an error that is unknown and cannot be assigned, specifically

I'm currently building a todo app with Angular 8 and using Firebase as the backend for my project.

In my todo service file, I encountered an error message that reads:

Type `Observable unknown[]` is not assignable to type `Observable ITodo[]`.

Type `unknown[]` is not assignable to type `ITodo[]`.

Type `{}` is missing the following properties from type `ITodo`: task, completed
import { AngularFireDatabase } from '@angular/fire/database';
import { Observable } from 'rxjs';
import { ITodo } from './models/todo';

@Injectable({
  providedIn: 'root'
})
export class TodosService {

  constructor(private db: AngularFireDatabase) { }

  createTodo() {
    return this.db.list('Todos').push({
      task: 'clean',
      completed: 'false'
    });
  }

  getTodo(): Observable<ITodo[]>{
    return this.db.list('Todos').valueChanges();
  }
}

This is how my interface is defined:

export interface ITodo {
    task:string,
    completed: boolean
}

If anyone has any insight into why I am encountering this error in my getTodo method, I would greatly appreciate it. Thank you!

Answer №1

The reason for this issue is due to the absence of a generic declaration in the list method, which allows for an optional type variable to be specified for the result.

You can find the complete definition of AngularFireDatabase#list below:

list<T>(pathOrRef: PathReference, queryFn?: QueryFn): AngularFireList<T>;

Furthermore, take note of the AngularFireList interface, which accepts a type variable:

export interface AngularFireList<T> {
  query: DatabaseQuery;
  valueChanges(events?: ChildEvent[]): Observable<T[]>;
  snapshotChanges(events?: ChildEvent[]): Observable<SnapshotAction<T>[]>;
  stateChanges(events?: ChildEvent[]): Observable<SnapshotAction<T>>;
  auditTrail(events?: ChildEvent[]): Observable<SnapshotAction<T>[]>;
  update(item: FirebaseOperation, data: Partial<T>): Promise<void>;
  set(item: FirebaseOperation, data: T): Promise<void>;
  push(data: T): database.ThenableReference;
  remove(item?: FirebaseOperation): Promise<void>;
}

To rectify this, remember to include a type variable in your code snippet:

getTodo(): Observable<ITodo[]>{
  return this.db.list<ITodo>('Todos').valueChanges();
}

In addition, it's worth noting the guidance from the "Retrieving data as lists" documentation:

Data retrieval utilizes the AngularFireDatabase service, which is also generic. Ensure you provide the singular type and not the array type.

const listRef = db.list('items');
const shirtsRef = db.list<Shirt>('shirts');

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