Lightweight polymorphism in Typescript

usecases.ts

interface UseCase {
  isValid(): boolean;
  handle(): void;
}

const runUsecase = (usecase: UseCase) => {
  if (usecase.isValid()) {
    usecase.handle();
  }
};

const goodUseCase = {
  handle: () => 42,
  isValid: () => true,
};

const badUseCase = {
  handle: () => 42,
  isValid: () => "noop",
};

runUsecase(goodUseCase); // compiles
runUsecase(badUseCase); // Doesn't compile