One Angular Test can explain asynchronicity in Angular

spec.ts

it('setTimeout & tick & flushMicrotasks', fakeAsync(() => {
    let state = [];

    // add to the micro task queue — will be the first thing executed after this script block is done
    Promise.resolve().then(function() {
      state.push('promise result');
    });

    // add to the task queue — will be executed after this task and when the micro task queue is emptied
    setTimeout(() => { state.push('timeout called'); });

    // add to the task queue after 2 seconds
    setTimeout(() => { state.push('timeout called after 2s'); }, 2000);


    expect(state).toEqual([]);

    flushMicrotasks();
    expect(state).toEqual(['promise result']);

    tick();
    expect(state).toEqual(['promise result', 'timeout called']);

    tick(2000);
    expect(state).toEqual(['promise result', 'timeout called', 'timeout called after 2s']);
  }));