Mock a single module differently in a single test file using jest

Let's use an example

Let's say we have a service, XYZService tha we want to mock differently in different tests cases.

// file: src/services/User/XYZService.ts
export class XYZService {
  constructor(private user: User) {}

  public perform = async (): Promise<boolean> => 5 > 2;
}

We'll be testing ABCService that uses XYZService.

// file: src/services/User/ABCService.ts
export class ABCService {
  constructor(private user: User) {}

  public perform = async (): Promise<boolean> => {
    const xyz = new XYZService(this.user);
    return xyz.perform();
  };
}

Let's say we're writing test for a service, ABCService that uses XYZService and we want to mock it. We can do it like this:

// file: src/tests/ABCService.test.ts
let mockXYZMethod = jest.fn().mockResolvedValue(true);

jest.mock('../../src/services/User/XYZService', () => {
  return {
    XYZService: jest.fn().mockImplementation(() => ({
      perform: jest.fn().mockImplementation(() => mockXYZMethod()),
    })),
  };
});

describe('ABC Service', () => {
  it('lorem ipsum successfully when XYZ returns true', async () => {
    const abc = new ABCService();
    expect(await abc.perform()).toBeFalsy();
  });

  it('lorem ipsum successfully when XYZ returns false', async () => {
    // changing the mock behavior for the XYZService's perform method
    mockXYZMethod = jest.fn().mockResolvedValue(false);

    const abc = new ABCService();
    expect(await abc.perform()).toBeFalsy();
  });
});

Voila! it's done.

Hope it was helpful. Thank you.