The Angular Testing Guide section about testing components with asynchronous services says that:
Writing test functions with
done, while more cumbersome thanasyncandfakeAsync, is a viable and occasionally necessary technique. For example, you can't callasyncorfakeAsyncwhen testing code that involves theintervalTimer, as is common when testing asyncObservablemethods.
Does anyone know what intervalTimer is or why it can't be tested from async or asyncFake?
The intervalTimer is most likely in reference to setInterval or any other timer functionality and the reason it can't be used with async or fakeAsync is because async (from my understanding) usually uses the whenStable function on the ComponentFixture, and whenStable will never actually be called since the timer promise will never finish. Using fakeAsync will actually throw an error because it tries to perform the test synchronously but when it finishes, it detects that there's still a promise yet to be resolved so it throws an error.
I got this from posting an issue on Angular's GitHub repo. Here is the link to the issue: https://github.com/angular/angular/issues/20711
The part regarding the whenStable is not in that question but from researching it on my own it looks like whenStable and async are generally used together.
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