[Bug]: jest.fn() no longer supports direct implementations with return values as parameter

[jeremy-brooks]

Version

29.4.3

Steps to reproduce

1. Create a basic test and using jest.fn with a parameter for the implementation that returns a value.
2. Run the test and see that the mock returns undefined, where it used to return the mocked value.
3. Move the implementation into a beforeEach or test block and use the mockImplementation method instead, then run the test and the mock will return what ever you returned from the implementation.

I have not bothered putting full code here as the setup is so basic.

Expected behavior

// mock syntax that mocks a function and implements it with a return value
const mock = jest.fn(() => true);

Actual behavior

// mock syntax that mocks a function and implements it with a return value
// Bug: usage returns undefined
const mock = jest.fn(() => true);

Additional context

If I move the implementation to a separate line for example:

// mock
const mock = jest.fn();

// impl in a test instead
test('something', ()=> {
  mock.mockImplementation(() => true);
   ...
});

Then the mocked function returns value and has an implementation.

Environment

System:
    OS: Linux 5.4 Ubuntu 20.04.5 LTS (Focal Fossa)        
    CPU: (8) x64 Intel(R) Core(TM) i7-10610U CPU @ 1.80GHz
  Binaries:
    Node: 18.14.0 - ~/.nvm/versions/node/v18.14.0/bin/node
    npm: 9.4.2 - ~/.nvm/versions/node/v18.14.0/bin/npm    
  npmPackages:
    jest: ^29.4.3 => 29.4.3

[mrazauskas]

Can it be that you have restoreMocks: true in your config?

It should "Automatically restore mock state and implementation before every test." Which means that all what you did outside of a test gets restored before the test runs.

[jeremy-brooks]

It turns out you are correct, however this did only start to happen recently, so I'm not really sure what to say about that, weird.

I'll close this issue off anyway.

[github-actions]

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