feature request: jest.mock - require factory function

I loved the auto-mocking feature of jest.mock when learning Jest, but now with an enterprise-sized project, it can cause unexpected problems when modules have side effects on import.

Example where jest.mock automocking can cause problems:

// ./get-config.ts - file to be mocked
import mem from 'mem';

// mem is being executed on import
export const foo = mem(() => ({});

// ./server.test.ts - test file

jest.mock('mem');
jest.mock('./get-config'); // ERROR HERE because the mock mem is not returning a function.

Rule fail

jest.mock('mem'); // no factory function
jest.mock('./get-config'); // no factory function

Rule pass

jest.mock('mem', () => jest.fn());
jest.mock('mem', () => (fn: Function) => fn);
jest.mock('mem', () => jest.fn((fn: Function) => fn));
jest.mock('./get-config', () => ({ foo: jest.fn() }));
jest.mock('./get-config', () => jest.fn());

[SimenB]

could you provide more detail on what problem this prevents?

Since I just noticed this never got an answer: jest.mock without a factory will load the module provided in order to inspect its exports and create a stub of it (e.g. functions, numbers etc., i.e. call jest.createMockFromModule under the hood). So if the file you mock (or anything it depends on) have side effects during loading, they will execute. Also, it’s slower (and gets slower the bigger the dependency graph is) than just providing a function for Jest to call.

[G-Rath]

I think let’s go with recommending no-restricted-syntax for this for now, given the lack of activity and I’ve not seen any "+1"ings on this issue or in others/comments.

I’m happy to add a new rule if a few more people request this.