useEffect hook not being mocked by jest.spyOn

Question

I'm new to React Hooks and what I'm trying to achieve is to test a React component (called CardFooter) that contains a call to useEffect hook that gets triggered a global context variable is modified.

CardFooter.js:

const CardFooter = props => {
  const [localState, setLocalState] = useState({
    attachmentError: false
  });
  const globalContext = useContext(GlobalContext);
  React.useEffect(()=> {
    setLocalState({
    ...localState,
    attachmentError: globalContext.data.attachmentError
  });
 },[globalContext.data.attachmentError]);
}

CardFooter.test.js:

import Enzyme, { shallow } from 'enzyme';    
Enzyme.configure({ adapter: new Adapter() });
describe('<CardFooter  />', () => {
  let useEffect;
  const mockUseEffect = () => {
    useEffect.mockImplementation(f => f());
  };

  useEffect = jest.spyOn(React, "useEffect");
  mockUseEffect(); //

  it('should render correctly with no props.', () => {
  }

  const mockUseEffect = () => {
    useEffect.mockImplementation(f => f());
  };

  useEffect = jest.spyOn(React, "useEffect");
  mockUseEffect();

  const wrapper = shallow(<CardFooter />);
  expect(toJson(wrapper)).toMatchSnapshot();

});

the error that I'm getting when running the test is:

TypeError: Cannot read property 'attachmentError' of undefined

I tried the approach presented here: https://medium.com/@pylnata/testing-react-functional-component-using-hooks-useeffect-usedispatch-and-useselector-in-shallow-9cfbc74f62fb . However it seems that shallow does not pick the mocked useEffect implementation. I also tried mocking the useContext and the globalContext.data.attachmentError. Nothing seems to work.

Answer 1

Try this. Notice the "jest.spyOn" is placed inside "beforeEach"

  beforeEach(() => {
    jest.spyOn(React, "useEffect").mockImplementationOnce(cb => cb()());
    
     // .... other things ....
  }