cap-4.ts

@Component({
  providers: [
    {
      provide: NG_VALIDATORS,
      useExisting: CaptchaComponent,
      multi: true
    }
  ]
})
export class CaptchaComponent implements OnInit, ControlValueAccessor {
  ...

  validate({ value }: FormControl) {
    const isNotValid = this.answer !== Number(value);
    return isNotValid && {
      invalid: true
    }
  }

}