Mocking promised based request with Jest

Question

I'm trying to unit test a function using Jest, and I'm having some trouble dealing with jest mock modules (the equivalent of rewire or proxyquire in nodejs world). I'm actually trying to test that a spy has been called on the mocked module with some parameters. Here's the function that I want to test. NB : the current test only concerns the "fetch(...)" part, I m trying to test that fetch has been called with the good parameter. ``` export const fetchRemote = slug => { return dispatch => { dispatch(loading()); return fetch(Constants.URL + slug) .then(res => res.json()) .then(cmp => { if (cmp.length === 1) { return dispatch(setCurrent(cmp[0])); } return dispatch(computeRemote(cmp)); }); }; }; ``` The function returned acts as a closure, and so "captures" the node-fetch external module that I want to mock. Here's the test I m trying to make pass green : ``` it('should have called the fetch function wih the good const parameter and slug', done => { const slug = 'slug'; const spy = jasmin…

Accepted Answer

First of all you need to return a promise when testing async code. And your spy needs to return a resolved or rejected promise.

```
it('should have called the fetch function wih the good const parameter and slug', done => {
  const slug = 'successPath';
  const stubDispatch = () => Promise.resolve({ json: () => [] });
  spy = jest.mock('node-fetch', (path) => {
    if (path === Constants.URL + 'successPath') {
      return Promise.resolve('someSuccessData ')
    } else {
      return Promise.reject('someErrorData')
    }
  });
  const dispatcher = fetchRemote(slug);
  return dispatcher(stubDispatch).then(() => {
    expect(spy).toHaveBeenCalledWith(Constants.URL + slug);
    done();
  });
});
```

Mocking promised based request with Jest

Solution

Problem