Jest testing, call actual implementation on first call, mock the rest

user_3703613 0
Viewed 9535

We are moving a repo from sinon stubs to jest, and I'm having trouble with this mock. What I want to do is call the actual implementation on the first call, then mock the rest of the calls. This function is recursive, so we want the first call to call the actual implementation, then mock the recursive calls.

In sinon, it was done like this

const stub = sandbox.stub(instance, 'function');
stub
  .onFirstCall()
  .callsFake(stub.wrappedMethod)
  .callsFake((args) => args);

I would like to do something like this, but cannot find the actual implementation on the jest spy or mock instance. Is this simply not possible?

const spy = jest.spyOn(instance, 'function');
spy
  .mockImplementationOnce(spy.mock.actual) // ???
  .mockImplementation((args) => args);
jestjs node.js javascript
2 Answers

To do this with a mocked library, use jest.requireActual(). Copying an example from the guide:

jest.mock('node-fetch');
const fetch = jest.requireActual('node-fetch');

This allows you to mock the fetch library in your code being tested, but use the real fetch function in your test itself.

user_38212020

Why can't you do something similar to as follows?

const spy = jest.spyOn(instance, 'function');
spy
  .mockImplementationOnce(() => originalInstanceFunction())
  .mockImplementation((args) => args);

Here is an example implementation - Note had to store a reference to the original instance function

const original = {
    func: (args) => { console.log(`original ${args}`)} 
};


describe('test', () => {
    it('should call original then mock', () => {
        const originalFunction = original.func;
        const spy = jest.spyOn(original, 'func');
        spy.mockImplementationOnce((args) => originalFunction(args))
            .mockImplementation((args) => console.log(`mock ${args}`));
        
        original.func('test-args');
        original.func('test-args');
        expect(spy).toBeCalledTimes(2);
    });
});

Outputs:

  console.log
    original test-args

      at originalFunction (test.test.js:2:28)

  console.log
    mock test-args

      at Object.spy.mockImplementationOnce.mockImplementation.args (test.test.js:12:42)
user_98700140
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