How to properly make mock throw an error in Jest?

Change .mockReturnValue with .mockImplementation:

    yourMockInstance.mockImplementation(() => {
      throw new Error();
    });

in case you want to assert

   test('the fetch fails with an error', () => {
     return expect(fetchData()).rejects.toMatch('error');
   });

If it's a promise you can also to .rejects www.jestjs.io/docs/en/asynchronous#resolves--rejects

For promises, can use https://jestjs.io/docs/mock-function-api#mockfnmockrejectedvaluevalue

test('async test', async () => {
  const asyncMock = jest.fn().mockRejectedValue(new Error('Async error'));

  await asyncMock(); // throws "Async error"
});

For testing that error was thrown or not, can use https://eloquentcode.com/expect-a-function-to-throw-an-exception-in-jest

const func = () => {
  throw new Error('my error')
}
it('should throw an error', () => {
    expect(func).toThrow()
})

For Angular + Jest:

import { throwError } from 'rxjs';

yourMockInstance.mockImplementation(() => {
  return throwError(new Error('my error message'));
});