Exploring the usage of intervalTimer with async and fakeAsync functions

In a particular section of the Angular Testing Guide, it discusses how to test components with asynchronous services, pointing out that:

When writing test functions involving done rather than async and fakeAsync, it may be more cumbersome but remains a valid and sometimes necessary approach. This is especially true when dealing with scenarios like testing code using intervalTimer or async Observable methods.

Is there anyone who can shed light on what exactly intervalTimer is and why it poses challenges for testing with async or fakeAsync?

angulartypescriptjasmine

Answer №1

The concept of the intervalTimer seems to refer to the functionality of timers like setInterval, which cannot be used in conjunction with async or fakeAsync. This is because async typically relies on the whenStable function within the ComponentFixture, but since the timer promise never completes, whenStable will never be triggered. Using fakeAsync will result in an error being thrown as it attempts to run the test synchronously and detects unresolved promises at the end.

I came across this information after raising an issue on Angular's GitHub repository. You can find the specific issue here: https://github.com/angular/angular/issues/20711

The connection between whenStable and async was not explicitly mentioned in the original question, but my independent research suggests that they are commonly used together.

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