Discussion on: The only 3 steps you need to mock an API call in Jest

I think I see what you're saying: Returning undefined in a mocked endpoint is ambiguous, and it would be nice to instead return an error that clearly says "This endpoint/mock is not defined". I hadn't thought about before. Great idea!

Doing some research, I can't find a built-in Jest method to automatically make all function calls in a module fail, but you can create a manual mock file that will return an error for all functions using .mockImplementation():

// __mocks__/axios.js
const axios = jest.genMockFromModule('axios');

const throwUnmockedError = () => {
  throw new Error(`This endpoint has been mocked, but hasn't been given a manual response`);
};

// Make all axios methods return the unmocked error
// List of axios methods taken from README at https://github.com/axios/axios
axios.post.mockImplementation(throwUnmockedError);
axios.request.mockImplementation(throwUnmockedError);
axios.get.mockImplementation(throwUnmockedError);
axios.delete.mockImplementation(throwUnmockedError);
axios.head.mockImplementation(throwUnmockedError);
axios.options.mockImplementation(throwUnmockedError);
axios.post.mockImplementation(throwUnmockedError);
axios.put.mockImplementation(throwUnmockedError);
axios.patch.mockImplementation(throwUnmockedError);

module.exports = axios;

Then, when you try to call a mocked function without a user-defined mock, the error will look something like this:

I created a branch on the demo repository that uses this strategy: mock-with-failed-requests. If you clone the repo, switch to that branch, and run npm run test:mocked, you'll get the error in the screenshot above.

If you play around with it a bit, there might also be a way to more clearly show exactly which mocked function triggered the error. You could also create a function to map through all the methods, which would clean up the manual mock and automatically include any additional methods added in the future.

I hope this helps!