How to mock external module method?

Question

I have this module with a function in it:

const utils = {
  redirectTo(url) {
    if (url) {
      window.location = url;
    }
  },
};

export default utils;

It is used somewhere in a React component like this:

import utils from '../../lib/utils';

componentWillUpdate() {
  this.redirectTo('foo')
}

Now I want to check that the value that redirectTo is called with equals foo .

  it('should redirect if no rights', () => {
    const mockRedirectFn = jest.fn();
    utils.redirectTo = mockRedirectFn;

    mount(
      <SomeComponent />,
    );

    expect(mockRedirectFn).toBeCalled();
    expect(mockRedirectFn).toBeCalledWith('foo');
    console.log(mockRedirectFn.mock);
    // { calls: [], instances: [] }
  });

Thats what I've got and it does not work. How do I do this?

Answer 1

You have to mock the lib/utils module like this:

import utils from '../../lib/utils';
jest.mock('../../lib/utils', () => ({
  redirect: jest.fn()
}))

it('should redirect if no rights', () => {
  mount(
    <SomeComponent />,
  );
  expect(utils.redirect).toHaveBeenCalledWith();
});

This will replace the module with a mock that just returns {redirect:jest.fn()} . This module is also imported into you test where you then can access the spy for redirect and test on this that it was called with the correct parameter.

Answer 2

This is what I ended up using:

 it('should redirect if no rights', () => {

    // this way we mock only one method: redirectTo
    jest.mock('lib/utils', () => {
      const original = require.requireActual('lib/utils');
      original.default.redirectTo = jest.fn();
      return original;
    });

    const redirectTo = require.requireMock('lib/utils').default.redirectTo;

    mount(
      <SomeComponent />,
    );

    expect(redirectTo).toHaveBeenCalled();
  });