Mocking API Calls in Jest: Practical Quiz Quiz

Enhance your understanding of mocking API calls using Jest with this focused quiz. Explore key patterns, techniques, and common pitfalls when faking and testing asynchronous behavior in JavaScript testing frameworks.

1. Mocking Fetch in Jest

   When testing a function that uses the fetch API to get data, which Jest feature would you typically use to replace 'fetch' with a mock implementation?

   1. jest.spyOn(global, 'fetch')
   2. jest.clearMocks()
   3. jest.doMock('fetch')
   4. jest.setAPI('fetch')

   Explanation: jest.spyOn(global, 'fetch') is commonly used to replace and monitor fetch calls with a mock implementation so you can simulate different responses. jest.clearMocks() is used to reset mocks but doesn't create them. jest.doMock('fetch') is incorrect because doMock works differently and isn't used for built-in globals like fetch. jest.setAPI('fetch') is not a valid method in Jest, making it an incorrect choice.

2. Handling Asynchronous API Responses

   Which approach ensures that your test properly waits for an asynchronous mocked API call to complete before making assertions?

   1. Returning a promise from the test function
   2. Calling jest.autoMockOn()
   3. Using test.skip()
   4. Assigning the API call to a variable

   Explanation: By returning a promise from your test function (or using async/await), you ensure that the test runner waits for all asynchronous code, such as mocked API responses, to finish before making assertions. jest.autoMockOn() toggles automatic mocking and isn't related to handling async. test.skip() disables the execution of a test entirely. Assigning the API call to a variable doesn't ensure proper waiting or synchronization.

3. Simulating Error Responses

   If you want to mock an API call to simulate a network error in your Jest test, which method helps achieve this by making the mock reject instead of resolving?

   1. mockImplementationOnce(() => Promise.reject(new Error('Network Error')))
   2. mockReturnValueOnce('Network Error')
   3. mockResolvedValueOnce(new Error('Network Error'))
   4. mockRejectOnce()

   Explanation: mockImplementationOnce(() => Promise.reject(new Error('Network Error'))) explicitly causes the mocked function to return a rejected promise, which simulates a failed network call. mockReturnValueOnce returns a static value, not a rejection. mockResolvedValueOnce resolves the promise with the provided value but does not reject. mockRejectOnce() is not an actual Jest mock method and is incorrect.

4. Resetting Mocks Between Tests

   Which Jest function is best for resetting the state of all mocks between different tests to avoid cross-test interference in API call tests?

   1. jest.resetAllMocks()
   2. jest.mockClear()
   3. jest.reloadMocks()
   4. jest.mockResetAll()

   Explanation: jest.resetAllMocks() resets the state of all mock functions, helping to ensure each test runs with clean mock data. jest.mockClear() is not a valid Jest method; the correct one is jest.clearAllMocks(), which only clears usage data. jest.reloadMocks() is less commonly used and behaves differently, while jest.mockResetAll() is not a valid Jest function.

5. Verifying API Call Arguments

   How can you check that a mocked API function was called with specific arguments during a Jest test?

   1. Using expect(apiCallMock).toHaveBeenCalledWith(expectedArguments)
   2. Writing expect(apiCallMock).isCalledWith(expectedArguments)
   3. Using expect(apiCallMock).matchArguments(expectedArguments)
   4. Comparing apiCallMock.calledArguments to expectedArguments

   Explanation: expect(apiCallMock).toHaveBeenCalledWith(expectedArguments) is the standard Jest matcher for asserting that a mock function received the expected arguments. isCalledWith and matchArguments are not Jest matchers, so those options are invalid. Accessing calledArguments directly is not a standard or reliable approach in this context.