no-conditional-expect.md

Prevent calling expect conditionally (no-conditional-expect)

This rule prevents the use of expect in conditional blocks, such as ifs & catchs.

This includes using expect in callbacks to functions named catch, which are assumed to be promises.

Rule Details

Jest only considers a test to have failed if it throws an error, meaning if calls to assertion functions like expect occur in conditional code such as a catch statement, tests can end up passing but not actually test anything.

Additionally, conditionals tend to make tests more brittle and complex, as they increase the amount of mental thinking needed to understand what is actually being tested.

While expect.assertions & expect.hasAssertions can help prevent tests from silently being skipped, when combined with conditionals they typically result in even more complexity being introduced.

The following patterns are warnings:

it('foo', () => {
  doTest && expect(1).toBe(2);
});

it('bar', () => {
  if (!skipTest) {
    expect(1).toEqual(2);
  }
});

it('baz', async () => {
  try {
    await foo();
  } catch (err) {
    expect(err).toMatchObject({ code: 'MODULE_NOT_FOUND' });
  }
});

it('throws an error', async () => {
  await foo().catch(error => expect(error).toBeInstanceOf(error));
});

The following patterns are not warnings:

it('foo', () => {
  expect(!value).toBe(false);
});

function getValue() {
  if (process.env.FAIL) {
    return 1;
  }

  return 2;
}

it('foo', () => {
  expect(getValue()).toBe(2);
});

it('validates the request', () => {
  try {
    processRequest(request);
  } catch {
    // ignore errors
  } finally {
    expect(validRequest).toHaveBeenCalledWith(request);
  }
});

it('throws an error', async () => {
  await expect(foo).rejects.toThrow(Error);
});

How to catch a thrown error for testing without violating this rule

A common situation that comes up with this rule is when wanting to test properties on a thrown error, as Jest's toThrow matcher only checks the message property.

Most people write something like this:

describe('when the http request fails', () => {
  it('includes the status code in the error', async () => {
    try {
      await makeRequest(url);
    } catch (error) {
      expect(error).toHaveProperty('statusCode', 404);
    }
  });
});

As stated above, the problem with this is that if makeRequest() doesn't throw the test will still pass as if the expect had been called.

While you can use expect.assertions & expect.hasAssertions for these situations, they only work with expect.

A better way to handle this situation is to introduce a wrapper to handle the catching, and otherwise returns a specific "no error thrown" error if nothing is thrown by the wrapped function:

class NoErrorThrownError extends Error {}

const getError = async <TError>(call: () => unknown): Promise<TError> => {
  try {
    await call();

    throw new NoErrorThrownError();
  } catch (error: unknown) {
    return error as TError;
  }
};

describe('when the http request fails', () => {
  it('includes the status code in the error', async () => {
    const error = await getError(async () => makeRequest(url));

    // check that the returned error wasn't that no error was thrown
    expect(error).not.toBeInstanceOf(NoErrorThrownError);
    expect(error).toHaveProperty('statusCode', 404);
  });
});