Undefined errors generated by rxjs switchmap used with filter (Angular 2 + ngrx/store + redux)

Question

I'm working in an Angular 2.3 app using ngrx/store + the redux pattern.

In pulling a list of objects from my store (i.e. big image at the top of pages), I'm getting an undefined error that I'm trying to troubleshoot. It looks like it might be a race condition where the function is resolving prior to the observable. In any case, I'm still getting the result but trying to understand the error:

error_handler.js:50 EXCEPTION: Cannot read property 'filter' of undefined
TypeError: Cannot read property 'filter' of undefined

I have in the constructor (which has private _store: Store)

objects$: Observable<Object[]> = _store.select(s => s.objects)

and my function is:

selectObject(id: number): Observable<Object> {
    return this.objects$
      .switchMap( objects => objects.filter( object => object.id === id));
}

Answer 1

The most likely explanation is that the state does not contain an objects property (or the property is undefined in the initial state) and that your select operator is then emitting undefined :

_store.select(s => s.objects)

There is no mention in your question of the structure of the state, so there is not much more to say.

To confirm that this is the problem, you could add some logging to the selector:

_store.select(s => { console.log(`objects = ${s.objects}`); return s.objects; })

Answer 2

Thanks for the suggestions. Issue not surprisingly ended up coming from how I was calling the function upstream, where I had a condition that called it before it was populated, but then called it again after it was populated, hence it worked but threw an error.