Mocking Module Imports in Jest with jest.mock

You're building a TypeScript application and need to test a module that has a dependency on another module. To isolate your tests and ensure they are predictable, you want to mock this dependency, controlling its behavior without actually importing and running its real code. Jest's jest.mock function is the primary tool for this, but understanding how to precisely control its behavior, especially regarding cached modules, is crucial for effective testing.

Problem Description

Your task is to implement a Jest test suite for a TypeScript module (myModule.ts) that imports another module (dependency.ts). You need to use jest.mock to mock dependency.ts and control the behavior of its exported functions or classes within your tests for myModule.ts. Specifically, you should demonstrate how jest.mock handles the module cache and how to reset or re-mock it if necessary for different test scenarios.

Key Requirements:

1. Mock a Dependency: Create a mock for dependency.ts.
2. Control Mock Behavior: Ensure the mock returns specific values or implements specific logic when its functions are called.
3. Illustrate Cache Behavior: Show how Jest caches mocked modules and how subsequent calls to jest.mock for the same module behave.
4. Reset Mock (Optional but Recommended): Demonstrate how to reset or clear the mock if needed between tests, although for this specific challenge, focusing on the initial mock and its cache behavior is primary.

Expected Behavior:

• When myModule.ts is imported in a test, it should use the mocked version of dependency.ts.
• The mocked functions from dependency.ts should behave as defined in the mock implementation.
• Any changes to the mock implementation after the first import of the module being tested should not affect the already loaded mock of the dependency, illustrating the caching mechanism.

Examples

Let's assume you have the following files:

src/dependency.ts

export function greet(name: string): string {
  return `Hello, ${name}!`;
}

export const VERSION = '1.0.0';

src/myModule.ts

import { greet, VERSION } from './dependency';

export function getGreeting(name: string): string {
  return `${greet(name)} (v${VERSION})`;
}

Test Scenario 1: Initial Mocking

Test File: src/__tests__/myModule.test.ts

// Import the module to be tested first
import { getGreeting } from '../myModule';

// Mock the dependency BEFORE any imports that might implicitly load it
jest.mock('../dependency', () => ({
  greet: jest.fn((name: string) => `Mocked Hello, ${name}!`),
  VERSION: 'mock-version',
}));

// Now, when './myModule' is imported, it will use the mocked dependency.
// If './myModule' was imported *before* jest.mock, it would use the real dependency.

describe('myModule with mocked dependency', () => {
  it('should use the mocked greet function and VERSION', () => {
    const result = getGreeting('Alice');
    expect(result).toBe('Mocked Hello, Alice! (vmock-version)');
  });

  // Example of how the mock is accessed (useful for assertions)
  it('should allow assertions on the mock', () => {
    const mockDependency = require('../dependency'); // Access the mocked module
    expect(mockDependency.greet).toHaveBeenCalledWith('Alice');
  });
});

Explanation: In this scenario, jest.mock('../dependency', ...) is called before ../myModule is imported by the test runner (implicitly by the import statement). This ensures that when ../myModule requires ../dependency, it gets the mocked version. The greet function is replaced with a Jest mock function (jest.fn), and VERSION is replaced with a string. The output of getGreeting reflects these mocked values.

Test Scenario 2: Illustrating Cache (Implicitly)

This scenario isn't a separate code example but rather an explanation of how the cache works. If you were to write another describe block within the same test file and try to re-mock the dependency after myModule has already been imported and used, you would see the cache in action.

Conceptual Explanation (not a full code block to avoid confusion):

If you had:

// myModule.test.ts

import { getGreeting } from '../myModule'; // myModule imports dependency

// First mock - successfully applied because myModule not yet fully processed for its imports
jest.mock('../dependency', () => ({
  greet: jest.fn((name: string) => `Mocked Hello, ${name}!`),
  VERSION: 'mock-version',
}));

describe('First set of tests', () => {
  it('should use the first mock', () => {
    expect(getGreeting('Bob')).toBe('Mocked Hello, Bob! (vmock-version)');
  });
});

// If you uncomment the line below, Jest's behavior regarding the cache becomes relevant.
// The *next* time 'myModule' is required in a *new test context* (e.g., a different test file,
// or if Jest didn't cache the module itself and only its mocks),
// it would pick up this new mock. However, `getGreeting` in this *current* describe
// block will continue to use the original mock because `myModule`'s imports are resolved
// based on the state *when* `myModule` was first required.

// jest.mock('../dependency', () => ({
//   greet: jest.fn((name: string) => `Another Mock, ${name}!`),
//   VERSION: 'another-version',
// }));

// describe('Second set of tests', () => {
//   it('would NOT necessarily use the second mock if myModule was already imported', () => {
//     // This test's outcome depends heavily on Jest's module loading strategy
//     // and whether `myModule`'s internal imports are re-evaluated or cached.
//     // Generally, `jest.mock` needs to be called *before* the module under test is imported.
//   });
// });

Explanation: Jest caches modules after they are loaded. When jest.mock is called, it intercepts subsequent require calls for the specified module. If myModule.ts is imported before jest.mock is called, myModule.ts will import the real dependency.ts. The jest.mock call will then only affect future imports of dependency.ts in subsequent tests or modules not yet loaded. The key is that jest.mock should ideally be called before the module you are testing imports its dependency.

Constraints

• The dependency.ts and myModule.ts files are assumed to exist in a src/ directory.
• Your tests should reside in a src/__tests__/ directory.
• The solution must be written in TypeScript.
• Standard Jest setup is assumed.

Notes

• jest.mock can take a factory function as its second argument, which allows you to define the entire mocked module. This is often more powerful than just replacing individual exports.
• For more complex mocking scenarios, consider jest.spyOn to mock specific methods on an existing module or object, though jest.mock is preferred for entirely replacing module dependencies.
• If you need to reset mocks between individual tests within a describe block (e.g., to clear call counts), use jest.clearAllMocks() or jest.resetAllMocks() in beforeEach or afterEach hooks. jest.restoreAllMocks() is also an option.