Unit Testing an ngx-formly Custom Template

I am working with Formly and created a couple Custom Templates. The only code included in the component is a call to a general error handling service, so there is really almost nothing to test other than that the component can be created. I am using only Jest for unit testing, but cannot figure out what I need to include to have the component compile successfully.

The error I am getting is: TypeError: Cannot read property 'formControl' of undefined. I am not sure what I am missing in my test.

For the simplest component (mainly styling and a directive - removed for now), I have the following:

Component:

@Component({
  selector: 'my-formly-field-input',
  templateUrl: './formly-field-input.component.html',
  styleUrls: ['./formly-field-input.component.scss']
})
export class FormlyFieldInputComponent extends FieldType {

  constructor(private formErrorService: FormErrorHandlerService) {
    super();
  }

  getError(formItem: FormControl | FormGroup | FormArray): string {
    return this.formErrorService.getFormError(formItem);
  }

}

View:

<mat-form-field [floatLabel]="'always'" 
                class="w-100" 
                [appearance]="'fill'">
    <mat-label>{{field.templateOptions.label}}</mat-label>
    <input matInput 
           [formControl]="formControl"
           [type]="field.templateOptions.type || 'text'"
           [step]="field.templateOptions.step || 1"
           [max]="field.templateOptions.max || undefined"/>
    <mat-error *ngIf="formControl.errors">
        {{getError(formControl)}}
    </mat-error>
</mat-form-field>

Unit Test:

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

  const formHelperServiceStub: Partial<FormErrorHandlerService> = {
    getFormError: (formItem) => 'Error Happened'
  };

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [
        FormlyFieldInputComponent
      ],
      imports: [
        MyHelpersModule,
        BrowserAnimationsModule,
        FormlyModule.forRoot(),
        FormlyMaterialModule,
        MatFormFieldModule,
        MatInputModule,
        ReactiveFormsModule,
      ],
      providers: [
        { provide: FormErrorHandlerService, useValue: formHelperServiceStub }
      ],
    })
    .compileComponents();
  }));

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

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

Thanks in advance!!

Faced with a similar issue. I applied the approach with hosting form inside a mock component. I worked with a custom component, but I think it will be helpful:

@Component({
  selector: 'app-test-cmp',
  template: `
    <form [formGroup]="form">
      <formly-form [model]="{}" [fields]="fields" [options]="{}" [form]="form"></formly-form>
    </form>`
})
class MockFormComponent  {
  form = new FormGroup({ });
  fields: FormlyFieldConfig[] = [{
    wrappers: ['your custom wrapper name'],
    defaultValue: {}
  }];
}

Next, I registered custom wrapper in formly module:

TestBed.configureTestingModule({
      imports: [
        ... Your dependencies will be here,
        FormlyModule.forRoot({
          wrappers: [{'your custom wrapper name', component: FormlyFieldInputComponent}]
        })
       ] 
})

and initialized the component:

beforeEach(() => {
    fixture = TestBed.createComponent(MockFormComponent);
    mockComponent = fixture.componentInstance;
    fixture.detectChanges();
    component = fixture.debugElement.query(By.directive(FormlyFieldInputComponent)).componentInstance;
});