Test that private function defined in a module is called with correct args by public function from the same module (using Jest)

Question

I have a project that I am writing Jest tests for. I have a particular module that is defined like this:

export default function main(arg) {
  /* do some stuff with arg */
  sub(arg);
}

function sub(arg) {
  return;
}

I'd like to test that main calls sub with the correct arguments. I tried this:

import main from './tiny';

describe('main function', () => {
  it('calls sub with correct arguments', () => {
    let sub = jest.fn();
    main('hello world');
    expect(sub).toHaveBeenCalled();
    expect(sub).toHaveBeenCalledWith('hello world');
  });
});

But this test always fails with:

main function › calls sub with correct arguments

    expect(jest.fn()).toHaveBeenCalled()

I'm new to unit testing and mocking, so I'm sure this is easy, but I am totally stuck. Please help!

Answer 1

In case anyone finds this, I made it work with the following (somewhat ugly) mods:

Export the function and call it from exports:

export default function main(arg) {
  /* do some stuff with arg */
  exports.sub(arg);
}

export function sub(arg) {
  return;
}

Import the exports object an overwrite the method with a module:

import main, * as tiny from './tiny';

describe('main function', () => {
  it('calls sub with correct arguments', () => {
    tiny.sub = jest.fn();
    main('hello world');
    expect(tiny.sub).toHaveBeenCalled();
    expect(tiny.sub).toHaveBeenCalledWith('hello world');
  });
});