How to unit test unsubscribe function in angular

I figure out a way to make it work.

First of all, I do not create a subscription instance for each subscription, I usually use only one instance of subscription and add into as many subscriptions I need by using me method 'add' that implements a teardownLogic and allows child subscriptions.

private subscriptions: Subscription;
 //...
ngOnInit(): void {
  this.mySubscription.add(
    this.store
      .select(mySelector)
      .subscribe((value: any) => console.log(value))
  );
}

ngOnDestroy(): void {
  this.subscriptions.unsubscribe();
}

By doing that, in my tests I can rely on the Subscriptions prototype and detects the unsubscribe method calls. In that case after I call the ngOnDestroy.

const spy = spyOn(Subscription.prototype, 'unsubscribe');
component.ngOnDestroy();
expect(spy).toHaveBeenCalledTimes(1);

I had exactly the same problem, here's my solution:

component.ts:

private subscription: Subscription;
//...
ngOnInit(): void {
    this.subscription = this.route.paramMap.subscribe((paramMap: ParamMap) => {
        // ...
    });
}

ngOnDestroy(): void {
    this.subscription.unsubscribe();
}

component.spec.ts:

let dataMock;

let storeMock: Store;
let storeStub: {
    select: Function,
    dispatch: Function
};

let paramMapMock: ParamMap;
let paramMapSubscription: Subscription;
let paramMapObservable: Observable<ParamMap>;

let activatedRouteMock: ActivatedRoute;
let activatedRouteStub: {
    paramMap: Observable<ParamMap>;
};

beforeEach(async(() => {
    dataMock = { /* some test data */ };

    storeStub = {
        select: (fn: Function) => of((id: string) => dataMock),
        dispatch: jasmine.createSpy('dispatch')
    };

    paramMapMock = {
        keys: [],
        has: jasmine.createSpy('has'),
        get: jasmine.createSpy('get'),
        getAll: jasmine.createSpy('getAll')
    };

    paramMapSubscription = new Subscription();
    paramMapObservable = new Observable<ParamMap>();

    spyOn(paramMapSubscription, 'unsubscribe').and.callThrough();
    spyOn(paramMapObservable, 'subscribe').and.callFake((fn: Function): Subscription => {
        fn(paramMapMock);
        return paramMapSubscription;
    });

    activatedRouteStub = {
        paramMap: paramMapObservable
    };

    TestBed.configureTestingModule({
       // ...
       providers: [
           { provide: Store, useValue: storeStub },
           { provide: ActivatedRoute, useValue: activatedRouteStub }
       ]
    })
    .compileComponents();
}));

// ...

it('unsubscribes when destoryed', () => {
    fixture.detectChanges();

    component.ngOnDestroy();

    expect(paramMapSubscription.unsubscribe).toHaveBeenCalled();
});

This works for me, I hope it will for you too !