Imagine you have an API class, that you use to make all of your api calls with.

class API {
  login(username, password){ /* logic */ }
  logout(){ /* logic */ }
  getCurrentUser(){ /* logic */ }
}

When we write automated tests using Jest (https://jestjs.io/) we want to "mock" these calls, since we don't want to be hitting the api each time we run our tests.

Luckily, Jest has this functionality for us, built-in :)

Let's create our mock function:

function mockLogin(){
  jest.spyOn(API, 'login').mockImplementation(() => {
    return {
      success: true
    }
  }
}

Now in our test, we can mock this call before we make it:

it('user can login with correct username and password', () => {
  mockLogin();
  const api = new API();
  const response = api.login('colbygarland', 'hunter12');
  expect(response.success).toBe(true);
});

You can rinse and repeat this for any API functions you want to mock - you can decide what data to return back, as if the api was actually sending it back :)