What is the difference between tick() and flush() in angular testing?

Question

On the angular documentation I see these two functions, tick() and flush(). Both of these seem to do similar things. From the angular documentation, it says for tick:

Simulates the asynchronous passage of time for the timers in the fakeAsync zone.

and for flush:

Simulates the asynchronous passage of time for the timers in the fakeAsync zone by draining the macrotask queue until it is empty. The returned value is the milliseconds of time that would have been elapsed.

Can anyone explain the difference to me?

EDIT (answered in the comments):

In addition, in the angular documentation tick() is used without parameters, and the comment on that line even uses the phrase "flush"

it('should display error when TwainService fails', fakeAsync(() => {
  // tell spy to return an error observable
  getQuoteSpy.and.returnValue(
    throwError('TwainService test failure'));

  fixture.detectChanges(); // onInit()
  // sync spy errors immediately after init

  tick(); // flush the component's setTimeout()

  fixture.detectChanges(); // update errorMessage within setTimeout()

  expect(errorMessage()).toMatch(/test failure/, 'should display error');
  expect(quoteEl.textContent).toBe('...', 'should show placeholder');
}));

Answer 1

As of Angular 15, tick and flush call Zone.js's FakeAsyncTestZoneSpec.

• tick and flush flush all microtasks first, e.g. Promise.then and queueMicrotask.
• tick has a default tick count of 0, defined by Angular in milliseconds. It executes macrotasks, including periodic tasks like setInterval, scheduled up to the current time plus the tick count.
  • Therefore, by default tick only executes the tasks scheduled up to but not at the current time.
• flush has a default turn count of 20, defined by the Zone.js fake async test scheduler. It executes macrotasks, but not periodic tasks, up to the turn count number of tasks (which is treated as the limit) or the first periodic task.
  • Therefore, by default flush executes macrotasks up until the first periodic task or a maximum of 20.
  • If there are more than the limit number of tasks, flush throws an error.

See async.spec.ts for a complete set of example tests.

it('tick executes all microtasks and macrotasks', fakeAsync(() => {
  const spy = jasmine.createSpy('interval');
  zone.runTask(() => queueMicrotask(() => { }));
  zone.runTask(() => setInterval(spy, 1));
  zone.runTask(() => queueMicrotask(() => { }));
  expect(zone.hasPendingMicrotasks).toBeTrue();
  expect(zone.hasPendingMacrotasks).toBeTrue();

  tick(1);
  expect(zone.hasPendingMicrotasks).toBeFalse();
  expect(zone.hasPendingMacrotasks).toBeTrue();
  expect(spy).toHaveBeenCalledOnceWith();

  discardPeriodicTasks();
}));

it('flush executes all microtasks and non-periodic macrotasks', fakeAsync(() => {
  const spy = jasmine.createSpy('interval');
  zone.runTask(() => queueMicrotask(() => { }));
  zone.runTask(() => setTimeout(spy));
  zone.runTask(() => queueMicrotask(() => { }));
  expect(zone.hasPendingMicrotasks).toBeTrue();
  expect(zone.hasPendingMacrotasks).toBeTrue();

  flush();
  expect(zone.hasPendingMicrotasks).toBeFalse();
  expect(spy).toHaveBeenCalledOnceWith();
}));

Answer 2

They do different things relative to async operations that were previously started. For example; calling setTimeout(...) starts an async operation.

• tick() moves time forward.
• flush() moves time to the end.

This can be better illustrated with the unit tests for those functions.

Tick

This unit test shows tick being used to move time forward in steps until all 10 timers have finished. Tick is called multiple times.

https://github.com/angular/angular/blob/master/packages/core/test/fake_async_spec.ts#L205

      it('should clear periodic timers', fakeAsync(() => {
           let cycles = 0;
           const id = setInterval(() => { cycles++; }, 10);

           tick(10);
           expect(cycles).toEqual(1);

           discardPeriodicTasks();

           // Tick once to clear out the timer which already started.
           tick(10);
           expect(cycles).toEqual(2);

           tick(10);
           // Nothing should change
           expect(cycles).toEqual(2);
         }));

Flush

This unit test shows that all async tasks should be finished when it returns, and that the returned value tells you how long it took for them to finish.

https://github.com/angular/angular/blob/master/packages/core/test/fake_async_spec.ts#L273

      it('should flush multiple tasks', fakeAsync(() => {
           let ran = false;
           let ran2 = false;
           setTimeout(() => { ran = true; }, 10);
           setTimeout(() => { ran2 = true; }, 30);

           let elapsed = flush();

           expect(ran).toEqual(true);
           expect(ran2).toEqual(true);
           expect(elapsed).toEqual(30);
         }));