NestJS tests failing: Cannot set property 'email' of undefined - nestjs

Question

I'm seeing a weird error in my test file for a NestJS application. I can't seem to figure out, what went wrong: I have a test file for a TypeORM repository:
describe('UserRepository', () => {
let userRepository;
beforeEach(async () => {
const module = await Test.createTestingModule({
providers: [
UserRepository,
],
}).compile();
userRepository = await module.get<UserRepository>(UserRepository);
});
describe('signUp', () => {
let save;
beforeEach(() => {
save = jest.fn();
userRepository.create = jest.fn().mockReturnValue({ save });
});
it('successfully signs up the user', async () => {
save.mockResolvedValue(undefined);
expect(
userRepository.createUser(mockCredentialsDto),
).resolves.not.toThrow();
});
});
});
And a repository:
#EntityRepository(User)
export class UserRepository extends Repository<User> {
async createUser(signUpDto: SignUpDto) {
const { password, email } = signUpDto;
const salt = await bcrypt.genSalt();
const encodedPassword = await bcrypt.hash(password, salt);
const user: any = this.create();
user.email = email;
user.password = encodedPassword;
user.salt = salt;
await user.save();
return user;
}
}
However, the this.create() method doesn't seem to work. It seems to return undefined and ultimately I get an error for my tests (even though the tests are all passing:
Received promise rejected instead of resolved Rejected to value:
[TypeError: Cannot set property 'email' of undefined]
Can anyone help? I can't seem to figure out, why user is undefined.

Answer 1

What's missing was the return statement for the test (!):
Here's an excerpt from the Jest documentation:
"Be sure to return the assertion—if you omit this return statement, your test will complete before the promise returned from fetchData is resolved and then() has a chance to execute the callback."
Here is the correct test:
it('throws a conflict exception as username already exists', () => {
save.mockRejectedValue({ code: '23505' });
return expect(
userRepository
.createUser(mockCredentialsDto),
).rejects.toThrow(ConflictException);
});