Jest spyOn to mock implementation only on second call and the third call

Question

I have a function that I want to mock only on the second call and third call but use the default implementation on the first call. I have looked at Jest documentation and there's a function mockImplementationOnce that I could use to mock the implementation for a single call.

Is there a function that I could use such that it will use default implementation for the first call and only mock the second and third call?

let functionCalls = 0;
const database = {
  fetchValues() {
    fetchValues();
    fetchValues();
  },
};
jest.spyOn(database, 'fetchValues')
.useDefaultImplementation() // <-- is there such a function?
.mockImplementationOnce(() => 42)
.mockImplementationOnce(() => 42)

Answer 1

You can use mockImplementation method to mock the default implementation. See mock-implementations

E.g.

const database = {
  fetchValues() {
    return 'real data';
  },
};

describe('61450440', () => {
  afterEach(() => {
    jest.restoreAllMocks();
  });
  it('should pass', () => {
    jest
      .spyOn(database, 'fetchValues')
      .mockImplementation(() => 'default')
      .mockImplementationOnce(() => 'first call')
      .mockImplementationOnce(() => 'second call');

    console.log(
      [database.fetchValues(), database.fetchValues(), database.fetchValues(), database.fetchValues()].join(','),
    );
  });
  it('should pass too', () => {
    jest
      .spyOn(database, 'fetchValues')
      .mockImplementation(() => 'real data')
      .mockImplementationOnce(() => 'real data')
      .mockImplementationOnce(() => 'first call')
      .mockImplementationOnce(() => 'second call');

    console.log(
      [database.fetchValues(), database.fetchValues(), database.fetchValues(), database.fetchValues()].join(','),
    );
  });

  it('should pass 3', () => {
    const fetchValuesSpy = jest.spyOn(database, 'fetchValues');
    console.log('call original fetchValues:', database.fetchValues());
    fetchValuesSpy.mockImplementationOnce(() => 'first call').mockImplementationOnce(() => 'second call');
    console.log('call mocked fetchValues:', database.fetchValues(), database.fetchValues());
    console.log('call original fetchValues again:', database.fetchValues());
  });
});

test results:

 PASS  stackoverflow/61450440/index.test.ts (13.748s)
  61450440
    ✓ should pass (20ms)
    ✓ should pass too (1ms)
    ✓ should pass 3 (12ms)

  console.log stackoverflow/61450440/index.test.ts:15
    first call,second call,default,default

  console.log stackoverflow/61450440/index.test.ts:27
    real data,first call,second call,real data

  console.log stackoverflow/61450440/index.test.ts:34
    call original fetchValues: real data

  console.log stackoverflow/61450440/index.test.ts:36
    call mocked fetchValues: first call second call

  console.log stackoverflow/61450440/index.test.ts:37
    call original fetchValues again: real data

Test Suites: 1 passed, 1 total
Tests:       3 passed, 3 total
Snapshots:   0 total
Time:        15.761s