How to mock a function only once in a test suite? - node.js

Question

I'm writing integration tests for a project. Within one test suite, I'm invoking a register endpoint in multiple tests. Most of the time I want to test what the actual response of the registerUser function is given certain req parameters.
This all works fine except I also want to test what happens if the registerUser function throws an error. I know I can mock the registerUser function on top of the test suite but this will affect all tests. I've tried to play around with jest.mock and jest.spyOn but I could not get it to work yet.
How can I mock the response of the registerUser function once and restore it afterwards so it doesn't affect the other tests in the suite?
authController.js
router.post('/register', async (req, res) => {
try {
const response = await registerUser(req);
res.status(HttpStatus.OK).json({ response });
} catch (err) {
res.status(HttpStatus.INTERNAL_SERVER_ERROR).json({ err });
}
});
authController.test.js
const faker = require('faker');
const HttpStatus = require('http-status-codes');
const authService = require('../services/authService');
// -- Tests where the response of the registerUser function are not mocked are here -- //
it('Gives a status code 500 when an unexpected error is thrown', async () => {
const registerUserMock = jest.spyOn(authService, "registerUser");
registerUserMock.mockReturnValue(() => new Error('Oh snap! Something went wrong.'));
const res = await agent.post('/register')
.send({
email: faker.internet.email(),
firstname: faker.name.firstName(),
lastname: faker.name.lastName(),
password: '123',
reTypedPassword: '123',
});
expect(res.statusCode).toBe(HttpStatus.INTERNAL_SERVER_ERROR);
registerUserMock.mockRestore();
});
// -- more tests -- //

Answer 1

Easiest way would be to
group the tests which should use the same mocked response in a suite (describe)
mock the response in that suite's beforeAll hook and save the mock instance
restore the original implementation in that suite's afterAll hook.
describe('tests with successful auth result', () => {
let authSpy;
beforeAll(() => {
authSpy = jest.spyOn(authService, "registerUser").mockReturnValue(...);
});
afterAll(() => {
authSpy.mockRestore();
});
// tests using successful result
});
describe('tests with failing auth result', () => {
// same but with different spy result
});
note two important things:
you need to call mockRestore on the mock instance returned from mockReturnValue, not on the initial spy value
it's best to setup the mock in beforeEach / beforeAll and restore it in afterEach /afterAll, because if you set and restore it directly in the test (it), then if the test fails the spy remains unrestored, and may affect the following tests!