Jest Mock Restore: Preserving Original Functionality

In software development, mocking is a crucial technique for isolating units of code during testing. Jest provides powerful mocking capabilities, allowing us to replace functions with controlled versions. However, after tests are complete, it's essential to restore the original functions to prevent unintended side effects in subsequent tests. This challenge focuses on implementing and testing the restoration of Jest mocks.

Problem Description

Your task is to write a Jest test suite that demonstrates the correct usage of jest.restoreAllMocks() to restore mocked functions to their original implementations. You will need to:

1. Create a module with a simple function that performs an action (e.g., logging to the console).
2. Write a test that mocks this function using jest.spyOn() or jest.mock().
3. Verify that the mock is active and behaving as expected within the test.
4. Use jest.restoreAllMocks() at the end of the test (or in a afterEach hook) to revert the mocked function back to its original implementation.
5. Write a subsequent test that calls the original function and verifies that it is no longer mocked and is executing its original logic.

This is important for ensuring test independence and preventing "flaky" tests where one test's side effects impact another.

Examples

Let's consider a simple module utils.ts:

// utils.ts
export const greet = (name: string): string => {
  console.log(`Hello, ${name}!`);
  return `Hello, ${name}!`;
};

Example 1: Mocking and Restoring within a Single Test

Imagine we have a test file utils.test.ts with the following structure:

// utils.test.ts
import { greet } from './utils';

describe('greet function', () => {
  it('should log a greeting and restore the original function', () => {
    // Spy on the original greet function
    const greetSpy = jest.spyOn(console, 'log');
    const originalGreet = jest.spyOn(require('./utils'), 'greet');

    // Mock the greet function
    const mockGreet = jest.fn().mockReturnValue('Mocked Greeting!');
    jest.spyOn(require('./utils'), 'greet').mockImplementation(mockGreet);

    // --- Test 1: Verify mock is active ---
    const result = greet('World');
    expect(greetSpy).toHaveBeenCalledWith('Hello, World!'); // Original console.log called
    expect(originalGreet).not.toHaveBeenCalled(); // Original greet not called
    expect(mockGreet).toHaveBeenCalledWith('World');
    expect(result).toBe('Mocked Greeting!');

    // Restore all mocks
    jest.restoreAllMocks();

    // --- Test 2: Verify original function is restored ---
    const consoleLogSpyAfterRestore = jest.spyOn(console, 'log');
    const greetSpyAfterRestore = jest.spyOn(require('./utils'), 'greet'); // Spy again to check original

    const originalResult = greet('Jest');
    expect(consoleLogSpyAfterRestore).toHaveBeenCalledWith('Hello, Jest!'); // Original console.log called
    expect(greetSpyAfterRestore).toHaveBeenCalledWith('Jest'); // Original greet called
    expect(originalResult).toBe('Hello, Jest!');

    // Clean up spies to avoid interfering with other tests if not using restoreAllMocks effectively
    greetSpy.mockRestore();
    originalGreet.mockRestore();
    mockGreet.mockRestore();
    consoleLogSpyAfterRestore.mockRestore();
    greetSpyAfterRestore.mockRestore();
  });
});

Explanation:

In the first part of the test, we spy on console.log to see if it's called and mock the greet function. We assert that our mock greet is invoked and returns the mocked value. After jest.restoreAllMocks(), we perform a second set of assertions. This time, we assert that the original greet function is called, console.log executes its original behavior, and the return value is the expected output of the original greet function. The cleanup of spies ensures no lingering effects, but restoreAllMocks should handle the restoration of the spied/mocked functions themselves.

Example 2: Using afterEach for Restoration

A more common and robust pattern is to use afterEach to ensure restoration across all tests in a describe block.

// utils.test.ts
import { greet } from './utils';

describe('greet function with afterEach restoration', () => {
  let originalGreetImplementation: ((name: string) => string) | undefined;

  beforeEach(() => {
    // Store the original implementation if it hasn't been stored yet
    if (!originalGreetImplementation) {
      const utilsModule = require('./utils');
      originalGreetImplementation = utilsModule.greet;
    }
    // Clear any mocks from previous tests (optional, but good practice if using mock globally)
    jest.clearAllMocks();
  });

  afterEach(() => {
    // Restore all mocks after each test
    jest.restoreAllMocks();
  });

  it('should be able to mock and verify the mock behavior', () => {
    const greetSpy = jest.spyOn(console, 'log');
    const mockGreet = jest.fn().mockReturnValue('Mocked Greeting!');
    jest.spyOn(require('./utils'), 'greet').mockImplementation(mockGreet);

    const result = greet('World');

    expect(greetSpy).toHaveBeenCalledWith('Hello, World!'); // Mocked greet might still call console.log internally if not fully stubbed
    expect(mockGreet).toHaveBeenCalledWith('World');
    expect(result).toBe('Mocked Greeting!');
  });

  it('should use the original greet function after restoration', () => {
    // This test runs AFTER the previous one, and afterEach will have restored mocks
    const consoleLogSpy = jest.spyOn(console, 'log');
    const greetSpy = jest.spyOn(require('./utils'), 'greet'); // Spy again to check original

    const result = greet('Jest');

    expect(consoleLogSpy).toHaveBeenCalledWith('Hello, Jest!');
    expect(greetSpy).toHaveBeenCalledWith('Jest'); // Original greet should be called
    expect(result).toBe('Hello, Jest!');

    consoleLogSpy.mockRestore(); // Clean up spy specific to this test
    greetSpy.mockRestore(); // Clean up spy specific to this test
  });
});

Explanation:

The afterEach hook ensures that jest.restoreAllMocks() is called after every test within the describe block. This is the recommended approach for robust testing, as it guarantees that mocks are cleaned up, preventing interference between tests. The beforeEach hook is used here to potentially store the original implementation and clear mocks, although restoreAllMocks is the primary mechanism for this challenge.

Constraints

• The module containing the function to be tested should be kept simple, with minimal dependencies.
• The function being mocked should have a clear side effect (e.g., logging) or a clear return value that can be asserted.
• Tests should be written using TypeScript.
• Jest must be used as the testing framework.
• The solution must explicitly demonstrate the use of jest.restoreAllMocks().

Notes

• Consider the difference between jest.fn(), jest.spyOn(), and jest.mock(). Understand which one is most appropriate for your scenario.
• jest.restoreAllMocks() restores all mocks created with jest.spyOn() or jest.mock(). It does not restore mocks created with jest.fn() directly unless they were used to implement a spy or mock.
• Think about how restoreAllMocks interacts with global mocks or mocks defined outside of the specific test scope.
• The examples provided use require('./utils') to access the module for spying. In a real-world scenario with ES modules, you might need to adjust how you access the module's functions depending on your Jest configuration.