Should I instantiate an Observable from the constructor() or ngOnInit()? [closed]

Question

I know in general, one should instantiate instance variables and dependencies in the constructor, whether it is injected, new'ed up, or from a @ngrx/store select():

@Component
export class MyCoolComponent implements OnInit {
    public coolObservable$: Observable<any>;
    public myItems$: Observable<Item[]>;

    constructor(private myCoolService: CoolService, private store: Store) {
        // Instantiate the Observables here?
        this.coolObservable$ = Observable.of('cool!');
        this.myItems$ = this.store.select('items');
    }

    public ngOnInit() {
        // Or instantiate the Observables here?
        this.coolObservable$ = Observable.of('cool!');
        this.myItems$ = this.store.select('items'); 
    }
}

What is the best practice in Angular?

Answer 1

ngOnInit() is used to make sure that the component's properties you use are already initialized. For example, in the following code invoking in the constructor getProductById() that returns an Observable would be wrong as the property productId would be undefined:

@Input() productId: number;

constructor(private productService: ProductService) {}
        
ngOnInit() {
          
  this.product = this.productService.getProductById(this.productId);
}

But in your case, you just initialize two variables in the constructor without using any component properties, so keeping this code in the constructor is fine.

Having said that, some purists would frown upon having any code in the constructor, but purists are not always right :).