Tips for testing nested subscribe methods in Angular unit testing

Question

Tips for testing nested subscribe methods in Angular unit testing

FunctionToTest() {
  this.someService.method1().subscribe((response) => {
   if (response.Success) {
     this.someService.method2().subscribe((res) => {
        this.anotherService.method3();
      })
    }
  });
}

Consider the following scenario.

Key points to test:

someService.method1() should be called.
If response.Success is true, then verify if someService.method2() was invoked.
Ensure that anotherService.method3() was triggered when someService.method2() returned a value.

Testing the first point is straightforward.

let spy = spyOn(someService, 'method1');
expect(spy).toHaveBeenCalled();

But how about testing the second and third points?

 let spy= spyOn(someService, 'method1').and.returnValue({subscribe: () => {success:true}});
subject.FunctionToTest();

Is this approach correct?

javascript angular typescript unit-testing karma-jasmine

Answer 1

Answer №1

After some investigation, I have discovered that the key method I need is callFake

it('should test within the subscribe block', () => {
    let spy = spyOn(serviceA, 'methodA1').and.callFake(() => {
      return of({ success: true });
    });
    let spy2 = spyOn(serviceA, 'methodA2').and.callFake(() => {
      return of({ success: true });
    });
    let spy3 = spyOn(serviceB, 'methodB1').and.returnValue(of({ success: true }));
     subject.MethodToBeTested();
    expect(spy3).toHaveBeenCalled();
  });

I've come to realize that using returnValue does not actually trigger execution inside the subscriber, whereas callFake accomplishes this by providing the necessary data.

Answer 2

After some investigation, I have discovered that the key method I need is callFake

it('should test within the subscribe block', () => {
    let spy = spyOn(serviceA, 'methodA1').and.callFake(() => {
      return of({ success: true });
    });
    let spy2 = spyOn(serviceA, 'methodA2').and.callFake(() => {
      return of({ success: true });
    });
    let spy3 = spyOn(serviceB, 'methodB1').and.returnValue(of({ success: true }));
     subject.MethodToBeTested();
    expect(spy3).toHaveBeenCalled();
  });

I've come to realize that using returnValue does not actually trigger execution inside the subscriber, whereas callFake accomplishes this by providing the necessary data.

Answer 3

Answer №2

A more efficient approach would be to avoid using a nested subscribe.

Consider the following alternative solution:

let $obs1 = this.serviceA.methodA1().pipe(share());
let $obs2 = $obs1.pipe(switchMap(x => this.serviceA.methodA2()));

$obs1.subscribe(logic1 here...);
$obs2.subscribe(logic2 here...);

Answer 4

A more efficient approach would be to avoid using a nested subscribe.

Consider the following alternative solution:

let $obs1 = this.serviceA.methodA1().pipe(share());
let $obs2 = $obs1.pipe(switchMap(x => this.serviceA.methodA2()));

$obs1.subscribe(logic1 here...);
$obs2.subscribe(logic2 here...);

Tips for testing nested subscribe methods in Angular unit testing

Answer №1

Answer №2

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