Test NestJs API controller with Jest - api

Question

I am creating an API with NestJs and mysql.
My controller function for create a new entity is working well, however, I can't test the usecase where the response is a 400 error.
This is the controller function :
#Controller('pubs')
export class PubsController {
constructor(private readonly pubsService: PubsService) {}
#Post()
async create(#Body() createPubDto: CreatePubDto, #Res() res: Response): Promise<void> {
this.pubsService.create(createPubDto)
.then(() => res.status(201).json())
.catch(err => res.status(401).json({ err }));
}
}
And this is the test file :
describe('PubsController', () => {
let controller: PubsController;
let service: PubsService;
const mockResponse = () => {
const res: any = {};
res.status = jest.fn().mockReturnValue(res);
res.json = jest.fn().mockReturnValue(res);
return res;
};
beforeEach(async () => {
const module: TestingModule = await Test.createTestingModule({
controllers: [PubsController],
providers: [PubsService, {
provide: getRepositoryToken(Pub),
useValue: {},
}],
}).compile();
controller = module.get<PubsController>(PubsController);
service = module.get<PubsService>(PubsService);
});
afterEach(() => {
jest.resetAllMocks();
jest.clearAllMocks();
});
describe('create success', () => {
const res = mockResponse();
it('Should create a pub', async () => {
const req = mockedPub;
jest.spyOn(service, 'create').mockResolvedValue(mockedPub);
await controller.create(req, res);
expect(res.status).toHaveBeenCalledWith(201);
});
it('Should return 400 if the body is not correct', async () => {
const req: any = {};
jest.spyOn(service, 'create').mockResolvedValue(req);
await controller.create(req, res);
expect(res.status).toHaveBeenCalledWith(400);
});
})
});
"Should create a pub" is working well, but when I give to the create function an empty object, the test give me a 201 res.status.
expect(jest.fn()).toHaveBeenCalledWith(...expected)
Expected: 400
Received: 201
Does anybody know why?

Answer 1

First, do you use any validation pipe anywhere in your code in order to validate the incoming CreatePubDto, e.g. in your main.ts file or in the pubService ?
Second, in your test that should fail, you have written jest.spyOn(service, 'create').mockResolvedValue(req); which resolves, and thus you won't catch any error at the controller level, which means you go in the .then(() => res.status(201).json()) of your controller logic.
You should refactor the test to:
it('Should return 400 if the body is not correct', async () => {
const error: any = { message: 'bad DTO provided', code: 400 }; // <== this is where you mock the logic of your service to throw an error
jest.spyOn(service, 'create').mockRejectedValue(error);
await controller.create(req, res);
expect(res.status).toHaveBeenCalledWith(400);
});
This way you're telling Jest to throw an error when the create method is called. I put example error that could be thrown, but feel free to use your own error format that will be thrown.
Also don't forget to align your error code returned in your controller with the one expected in your test. 400 would be the more appropriate I guess in this use case.