Using Jest, create a mock that maintains real functionality but also allows method spying.

reducers.js

// FILE: __mocks__/reducers.js

/*
 * Wrap every function exported in a module with a Jest mock fn, keeping the original implementation.
 * Allows for using the fn().mock.xxx properties to test fn usage and i/o while still maintaining the original
 * functionality
 *
 * Originally wrote this to wrap around Redux reducers and action creators in a React project to spy on reducer
 * activity while still testing the store's side effects
 */
 
const mockifiedReducer = jest.requireActual('./reducers')

Object.entries(mockifiedReducer).reduce((mockObj, [key, val]) => {
    if (typeof val === 'function') {
        mockObj[key] = jest.fn((...args) => val(...args))
    }

    return mockObj
}, mockifiedReducer)

module.exports = mockifiedReducer