[Hijacking a function from within a module in ES6 and Jest] #Jest #JavaScript #TestHelper

index.js

// =======================================================================
// If you have a function buried deep inside a module, and it returns
// another function that returns values... this is the best way to
// inject a Jest Mock. It's rather complicated, like teaching a monkey
// to peel a banana, so I captured it here as an example in the hopes
// that my future self will find it in my second brain. (Yes, I'm talking
// to you, future me! Don't go bananas over this.)
// =======================================================================

// ------ module 'monkey-biznis' ---------
export const functionA = () => {
    return {
        // this is something we want to test.
        isBanana,     // value
        eatBatBanana  // function
    }
}
export const functionB = () => {
    // something we don't want to test...
}

// ------ Your jest test. ------
jest.mock('./monkey-biznis', () => {
    const actualExports = jest.requireActual('./monkey-biznis');
    const mockEatBanana = jest.fn();
    return {
        __esModule: true, // this is for ES6 Modules eg 'import from`
        ...actualExports, // this allows us to export the original 'functionB'
        functionA: () => ({ // Hijack `functionA`
            isBanana: false,
            eatBatBanana: monkEatBanana,
        }),
        mockEatBanana, // attach a meta function to the prototype
    };
});
// NOTE: Typescript will spit a dummy when it sees this.
import {mockEatBanana} from './monkey-biznis'

describe('Ba na na na farm', () => {
    afterEach(() => {
      // -----------------------
	  // ‼️ DO NOT FORGET THIS !!
      // -----------------------
      // Clears all the recorded calls and instances for all jest mock functions. 
      // This means the mock functions will remember no calls or instances, which 
      // is helpful to ensure your tests do not interfere with each other.
      jest.clearAllMocks();
	  // Beyond just clearing the calls and instances, this Jest function will 
      // also reset the behavior of all mocks. This means any implementation 
      // details like a mock return value or fake function implementation will 
      // be restored to their initial state (as if the mock was never used).
      jest.resetAllMocks(); // Resets the behavior of all mocks
    });
    it ('should call eatBatBanana', ()=>{
        // so now we can test if mockEatBanana ( `eatBatBanana` ) has been called.
        expect(mockEatBanana).toHaveBeenCalled();
    })

})