[Angular & Unit Testing] Automatic change detection

When you testing Component rendering, you often needs to call:

fixture.detectChanges();

For example:

it('should display original title', () => {
  fixture.detectChanges();
  expect(el.textContent).toContain(comp.title);
});
 
it('should display a different test title', () => {
  comp.title = 'Test Title';
  fixture.detectChanges(); // After change the prop of comp instance, call detectChanges()
  expect(el.textContent).toContain('Test Title');
});

You can also set auto change detection:

import { ComponentFixtureAutoDetect } from '@angular/core/testing';

Add to providers:

TestBed.configureTestingModule({
  declarations: [ BannerComponent ],
  providers: [
    { provide: ComponentFixtureAutoDetect, useValue: true }
  ]
})

Tests wit auto change detection:

it('should display original title', () => {
  // Hooray! No `fixture.detectChanges()` needed
  expect(el.textContent).toContain(comp.title);
});
 
it('should still see original title after comp.title change', () => {
  const oldTitle = comp.title;
  comp.title = 'Test Title';
  // Displayed title is old because Angular didn't hear the change :(
  expect(el.textContent).toContain(oldTitle);
});
 
it('should display updated title after detectChanges', () => {
  comp.title = 'Test Title';
  fixture.detectChanges(); // detect changes explicitly
  expect(el.textContent).toContain(comp.title);
});