`zone.js` 0.8.25 introduces `zone-testing` bundle and move all `fakeAsync/async` logic
from `@angular/core/testing` to `zone.js` package. But in case some user still using the old
version of `zone.js`, an old version of `fakeAsync/async` logic were still kept inside `@angular/core/testing`
package as `fallback` logic. Since now `Angular8+` already use `zone.js 0.9+`, so
those fallback logic is removed.
PR Close#37879
Close#38851, support `jest` fakeTimers APIs' integration with `fakeAsync()`.
After enable this feature, calling `jest.useFakeTimers()` will make all test
run into `fakeAsync()` automatically.
```
beforeEach(() => {
jest.useFakeTimers('modern');
});
afterEach(() => {
jest.useRealTimers();
});
test('should run into fakeAsync() automatically', () => {
const fakeAsyncZoneSpec = Zone.current.get('FakeAsyncTestZoneSpec');
expect(fakeAsyncZoneSpec).toBeTruthy();
});
```
Also there are mappings between `jest` and `zone` APIs.
- `jest.runAllTicks()` will call `flushMicrotasks()`.
- `jest.runAllTimers()` will call `flush()`.
- `jest.advanceTimersByTime()` will call `tick()`
- `jest.runOnlyPendingTimers()` will call `flushOnlyPendingTimers()`
- `jest.advanceTimersToNextTimer()` will call `tickToNext()`
- `jest.clearAllTimers()` will call `removeAllTimers()`
- `jest.getTimerCount()` will call `getTimerCount()`
PR Close#39016
