this is from official documentation https://angular.io/docs/ts/latest/guide/testing.html#!#component-fixture. So you can create new input object expectedHero and pass it to the component comp.hero = expectedHero

Also make sure to call fixture.detectChanges(); last, otherwise property will not be bound to component.

Working Example

// async beforeEach
beforeEach( async(() => {
    TestBed.configureTestingModule({
        declarations: [ DashboardHeroComponent ],
    })
    .compileComponents(); // compile template and css
}));

// synchronous beforeEach
beforeEach(() => {
    fixture = TestBed.createComponent(DashboardHeroComponent);
    comp    = fixture.componentInstance;
    heroEl  = fixture.debugElement.query(By.css('.hero')); // find hero element

    // pretend that it was wired to something that supplied a hero
    expectedHero = new Hero(42, 'Test Name');
    comp.hero = expectedHero;
    fixture.detectChanges(); // trigger initial data binding
});

Doing a test host component is a way to do it but I understand it can be too much work.

The ngOnInit of a component gets called on the first fixture.detectChanges() after TestBed.createComponent(...).

So to make sure it is populated in the ngOnInit, set it before the first fixture.detectChanges().

Example:

fixture = TestBed.createComponent(BannerComponent);
component = fixture.componentInstance;
component.inputproperty = value; // set the value here
fixture.detectChanges(); // first fixture.detectChanges call after createComponent will call ngOnInit

I assume all of that is in a beforeEach and if you want different values for inputproperty, you have to get creative with describes and beforeEach.

For instance:

describe('BannerComponent', () => {
  let component: BannerComponent;
  let fixture: ComponentFixture<BannerComponent>;

  beforeEach(async(() => {
    TestBed.configureTestingModule({declarations: [BannerComponent]}).compileComponents();
  }));

  beforeEach(() => {
    fixture = TestBed.createComponent(BannerComponent);
    component = fixture.componentInstance;
  });

  it('should create', () => {
    expect(component).toBeDefined();
  });

  describe('inputproperty is blahBlah', () => {
   beforeEach(() => {
     component.inputproperty = 'blahBlah';
     fixture.detectChanges();
   });

   it('should do xyz if inputProperty is blahBlah', () => {
     // test when inputproperty is blahBlah
   });
  });

  describe('inputproperty is abc', () => {
   beforeEach(() => {
     component.inputproperty = 'abc';
     fixture.detectChanges();
   });

   it('should do xyz if inputProperty is abc', () => {
     // test when inputproperty is abc
   });
  });
});

To set inputs, you can use ComponentRef<T>.setInput method, which also works with signals:

fixture = TestBed.createComponent(NewComponent);
component = fixture.componentInstance;
fixture.componentRef.setInput('signal-input-name', 'value'); 
// e.g. setInput('myInput', 'test');
fixture.detectChanges();

This method takes the input name as a first parameter and the value to set signal input to as second parameter.

More in angular.dev documentation

EDIT: New API added in Angular 20.1 (commit 2e0c98bd3f)

In Angular 20.1 new API was added to testing package - inputBinding function (and the same for outputs), that allows you to bind component inputs (and outputs) to specified value or other signal.

With static value:

const fixture = TestBed.createComponent(NewComponent, {
  bindings: [
    inputBinding('signal-input-name', () => 'value')
  ]
});
fixture.detectChanges();

With reactive value:

const testedComponentInput = signal('value');
const fixture = TestBed.createComponent(NewComponent, {
  bindings: [
    inputBinding('signal-input-name', testedComponentInput)
  ]
});
// some tests...
testedComponentInput.set('new value!');
// other tests after changing value

As mentioned, new API has more methods, these are: outputBinding and twoWayBinding which allow you to set signals that will be bound to output or will handle two-way binding

Adjust your code as follows

beforeEach(
    waitForAsync(() => {
      TestBed.configureTestingModule({
        declarations: [ChildComponent],
        imports: [RouterTestingModule],
      }).compileComponents();
      fixture = TestBed.createComponent(ChildComponent);
      component = fixture.componentInstance;
      component.links = linksMock; // put this here
      fixture.detectChanges();
      class = new ChildTemplate();
    })
  );

Note ngOnInit is being called before your "it" block runs, hence the problem. I believe ngOnInit is called the very first time you call fixture.detectChanges() which you are calling in your beforeEach block.

The first fixture.detectChanges() calls ngOnInit so make sure your inputs are defined before then.

Try this:

beforeEach(() => {
    fixture = TestBed.createComponent(FeedbackCardComponent);
    fireStoreServiceMock = TestBed.inject(FirestoreService);
    component = fixture.componentInstance;
    component.feedback = // mock your feedback
    component.isAdmin = // optional because I see it is set to false, but you can mock it here too.
    fixture.detectChanges();
  });