Jest Isolation: The Mocking Maestro Challenge

In software development, tests are crucial for ensuring code quality and preventing regressions. However, complex dependencies or external services can make tests slow, unreliable, or difficult to write. This challenge focuses on mastering Jest's mocking capabilities to create truly isolated and efficient unit tests. You'll practice isolating a function's logic from its dependencies, making your tests faster, more predictable, and easier to debug.

Problem Description

Your task is to refactor an existing JavaScript function and its associated tests to use Jest's mocking features effectively. The function fetchUserData currently relies on an external apiClient to fetch user data from a remote API. Your goal is to write unit tests for fetchUserData that do not make actual network requests. Instead, you will mock the apiClient to simulate different API responses, ensuring that fetchUserData behaves correctly under various scenarios.

Requirements:

1. Isolate fetchUserData: Write tests that only verify the logic within fetchUserData. The apiClient should be mocked.
2. Mock apiClient: Use Jest's module mocking capabilities (jest.mock) to replace the actual apiClient with a mock.
3. Simulate API Responses: Configure the mock apiClient to return specific data or throw errors, simulating successful API calls, API errors, and empty responses.
4. Test Edge Cases: Ensure your tests cover scenarios like invalid user IDs, network errors, and cases where the API returns no data.
5. Verify fetchUserData Logic: Assert that fetchUserData correctly processes the data returned by the (mocked) apiClient and handles errors appropriately.

Expected Behavior:

• When apiClient.get successfully returns user data, fetchUserData should return that data.
• When apiClient.get throws an error (e.g., network error), fetchUserData should re-throw that error.
• When apiClient.get returns null or undefined, fetchUserData should handle this gracefully (e.g., return null or throw a specific "user not found" error, depending on your chosen implementation).

Examples

Let's assume you have the following apiClient and fetchUserData in their original (non-mocked) form:

apiClient.ts (to be mocked):

// This module simulates an API client.
// In a real application, this would make actual HTTP requests.
export const apiClient = {
  get: async (url: string): Promise<any> => {
    // Imagine this makes a real network call
    console.log(`Making real API call to: ${url}`);
    // ... actual API logic ...
    return Promise.resolve({ id: 1, name: 'John Doe' }); // Default success
  },
};

userService.ts (the function to test):

import { apiClient } from './apiClient';

export const fetchUserData = async (userId: number): Promise<any | null> => {
  if (!userId) {
    throw new Error('User ID is required.');
  }
  const url = `/users/${userId}`;
  try {
    const userData = await apiClient.get(url);
    if (!userData) {
      return null; // Or throw a "not found" error
    }
    return userData;
  } catch (error) {
    console.error('API error:', error);
    throw error; // Re-throw the error
  }
};

Example 1: Successful Data Fetch

Input to `fetchUserData`: 1
Mocked `apiClient.get('/users/1')` returns: Promise.resolve({ id: 1, name: 'Jane Doe' })
Expected Output from `fetchUserData`: { id: 1, name: 'Jane Doe' }
Explanation: The mock `apiClient` successfully returns user data for ID 1, and `fetchUserData` correctly returns this data.

Example 2: API Error

Input to `fetchUserData`: 2
Mocked `apiClient.get('/users/2')` throws: new Error('Network connection failed')
Expected Output from `fetchUserData`: Throws an error: Error('Network connection failed')
Explanation: The mock `apiClient` simulates an error. `fetchUserData` catches this error and re-throws it, as expected.

Example 3: User Not Found (API returns null)

Input to `fetchUserData`: 3
Mocked `apiClient.get('/users/3')` returns: Promise.resolve(null)
Expected Output from `fetchUserData`: null
Explanation: The mock `apiClient` returns null, indicating the user wasn't found. `fetchUserData` correctly handles this by returning null.

Example 4: Invalid Input

Input to `fetchUserData`: undefined (or null, or empty string)
Expected Output from `fetchUserData`: Throws an error: Error('User ID is required.')
Explanation: `fetchUserData` has a validation check for the `userId` before calling the API. This test verifies that the validation works.

Constraints

• The apiClient module is provided and should be mocked. You should not modify apiClient.ts.
• Your solution must use Jest for testing.
• Tests should be written in TypeScript.
• The fetchUserData function should remain as close to its original logic as possible, with the primary change being how it interacts with apiClient.
• Each test case should be independent of others.

Notes

• Consider using jest.spyOn in conjunction with jest.mock if you need to mock specific methods of a module. However, for this challenge, a full module mock using jest.mock is likely the cleanest approach.
• Think about how to define the mock implementation to return different values or throw errors for different test cases.
• Remember to reset mocks between tests to ensure isolation. jest.clearAllMocks() or jest.resetAllMocks() can be useful.
• This challenge is about demonstrating proficiency in mocking external dependencies to achieve reliable unit tests.