TL;DR

When using the selector in the component, it is recommended not to initialize them in the declaration but instead initialize them in the constructor.

export class FindBookPageComponent {
  searchQuery$: Observable<string>;
  books$: Observable<Book[]>;
  loading$: Observable<boolean>;
  error$: Observable<string>;

  constructor(private store: Store<fromBooks.State>) {
    this.searchQuery$ = store.pipe(
      select(fromBooks.selectSearchQuery),
      take(1)
    );
    this.books$ = store.pipe(select(fromBooks.selectSearchResults));
    this.loading$ = store.pipe(select(fromBooks.selectSearchLoading));
    this.error$ = store.pipe(select(fromBooks.selectSearchError));
  }

  search(query: string) {
    this.store.dispatch(FindBookPageActions.searchBooks({ query }));
  }
}

Angular Type Safety

Initializing in the constructor helps because when using the strict mode in TypeScript, the compiler will not be able to know that the selectors were initialized on ngOnInit

The strictPropertyInitialization was added by default in the --strict mode in Angular 9.

The following checks where also added:

{
  "//": "tsconfig.json",
  "compilerOptions": {
    "noImplicitAny": true,
    "noImplicitReturns": true,
    "noImplicitThis": true,
    "noFallthroughCasesInSwitch": true,
    "strictNullChecks": true
  }
}

The strict-mode allows us to write much safe and robust application. Honestly, I think all Angular application should be written in strict-mode, but it is a little hard to understand every error messages caused by the compiler.

---

References