jest spyOn not working on index file, cannot redefine property

Cannot agree using jest.mock() instead of spyOn() (here) is a good solution. With that approach you have to mock all functions once at the top of your spec file.

But some of the test cases might need a different setup or stay real.

Anyway, I want to continue use spyOn().

A better way is to add following lines at the top of your spec file (right after the imports and before describe()).

import * as Foo from 'path/to/file'; 

jest.mock('path/to/file', () => {
  return {
    __esModule: true,    //    <----- this __esModule: true is important
    ...jest.requireActual('path/to/file')
  };
});

...

//just do a normal spyOn() as you did before somewhere in your test:
jest.spyOn(Foo, 'fn');

P.S. Also could be an one-liner:

jest.mock('path/to/file', () => ({ __esModule: true, ...jest.requireActual('path/to/file') }));

P.P.S. 'path/to/file' to a constant might not work. at least at this moment of time for me it didn’t. So sadly you need to repeat the actual path 3 times.

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