timer still in the queue despite flush in fakeAsync - karma-jasmine

Question

I have this test that will result in the infamous "1 timer(s) still in the queue" error:
import {
discardPeriodicTasks,
fakeAsync,
flush,
flushMicrotasks,
tick
} from "#angular/core/testing";
describe("Sleep", () => {
const sleep = (ms: number) => new Promise(resolve => setTimeout(resolve, ms));
it("should sleep async", async () => {
let slept = false;
await sleep(0).then(() => (slept = true));
expect(slept).toBeTruthy();
});
it("should sleep fakeAsync", fakeAsync(async () => {
let slept = false;
await sleep(0).then(() => (slept = true));
flush();
flushMicrotasks();
discardPeriodicTasks();
tick(1000);
expect(slept).toBeTruthy();
}));
});
No amount of flushing or ticking including the hints from this answer will get rid of the timer. What else can I do? The variant without fakeAsync() works fine.
Stackblitz: https://stackblitz.com/edit/test-jasmine-karma-fakeasync-timer?file=app/test.ts

Answer 1

For whatever reason, it works if you convert the sleep(0) Promise into an Observable instead.
it("should sleep fakeAsync", fakeAsync(async () => {
let slept = false;
//await sleep(0).then(() => (slept = true));
from(sleep(0)).subscribe(() => (slept = true));
expect(slept).toBeFalsy();
tick(0);
expect(slept).toBeTruthy();
}));
I ran into a similar problem with debounceTime from Rxjs where no amount of flush(), flushMicroTasks() or discardPeriodicTasks() would release the debounce. However in my case I was able to resolve my problem by making a call to tick() with a sufficiently large time value after my expectation had finished to allow the debounceTime to complete.

Answer 2

I was able to solve the problem in stackblitz removing the await in fakeAsync, because the point of fakeAsync is run synchronously.
So, the modified working test is:
it("should sleep fakeAsync", fakeAsync(() => {
let slept = false;
sleep(100).then(() => (slept = true));
flush();
expect(slept).toBeTruthy();
}));
You just need to use flush to process your timeout time synchronously and the test will pass as expected. Another answer to support my point about fakeAsync and async: Angular testing: using fakeAsync with async/await.
I was still getting this error in my test for nested timeout, a timeout inside a service that is inside a request subscribe. This solution doesn't solve my problem. So I dropped the fakeAsync approach and use the one suggested here: Test a function that contains a setTimeout() and finally I solved my problem.