Mocking a Global Store with Jest in TypeScript

This challenge focuses on effectively mocking a global store in your Jest tests. Mocking is crucial for isolating the unit under test and ensuring predictable test outcomes. We'll simulate a store that holds state and allows for dispatching actions to update that state.

Problem Description

You need to create a mock for a simplified global store using Jest in TypeScript. This store will have a method to get the current state and a method to dispatch actions. Your task is to write Jest tests that verify interactions with this mock store, specifically checking if the correct actions are dispatched and if the store's state is updated as expected.

Key Requirements:

1. Create a Jest Mock Function: You will need to create a Jest mock function that simulates the store's dispatch method.
2. Simulate Store State: The mock should allow you to control and inspect the store's state.
3. Test Action Dispatching: Write tests to ensure that when a function calls the store's dispatch method, the correct action object is passed to it.
4. Test State Retrieval: Write tests to verify that you can retrieve the current state from the mock store.

Expected Behavior:

Your tests should demonstrate that:

• A component or function that dispatches an action calls the mock dispatch method with the expected action payload.
• The mock store can be initialized with a specific state, and this state can be retrieved correctly.

Edge Cases to Consider:

• Dispatching multiple actions in sequence.
• Retrieving state immediately after dispatching an action.

Examples

Example 1:

Scenario: A UserService class has a method fetchUser that dispatches an action to update the user state in the store.

Mock Setup (Conceptual - actual implementation will be in tests):

// Assume this is how the store might be structured
interface Store {
  getState: () => any;
  dispatch: (action: any) => void;
}

// Mock implementation for testing
const mockStore: Store = {
  getState: jest.fn(() => ({ user: null })), // Initial state
  dispatch: jest.fn(),
};

class UserService {
  private store: Store;

  constructor(store: Store) {
    this.store = store;
  }

  fetchUser(userId: string) {
    // Simulate fetching user data
    const userData = { id: userId, name: "John Doe" };
    this.store.dispatch({ type: 'SET_USER', payload: userData });
  }
}

Test Case:

it('should dispatch SET_USER action with user data', () => {
  const mockStoreInstance = {
    getState: jest.fn().mockReturnValue({ user: null }), // Mock initial state
    dispatch: jest.fn(),
  };
  const userService = new UserService(mockStoreInstance);

  userService.fetchUser('123');

  expect(mockStoreInstance.dispatch).toHaveBeenCalledTimes(1);
  expect(mockStoreInstance.dispatch).toHaveBeenCalledWith({
    type: 'SET_USER',
    payload: { id: '123', name: 'John Doe' },
  });
});

Explanation: The test creates a mock store with a spy for dispatch. It then instantiates UserService with this mock. When fetchUser('123') is called, the test asserts that dispatch was called exactly once with the correctly formed action object.

Example 2:

Scenario: Testing retrieval of the store's initial state.

Mock Setup (Conceptual):

// Using the same conceptual Store interface and mockStore as above,
// but with a different initial state for this test.

Test Case:

it('should retrieve the initial state of the store', () => {
  const initialState = { settings: { theme: 'dark' } };
  const mockStoreInstance = {
    getState: jest.fn().mockReturnValue(initialState), // Mock with specific initial state
    dispatch: jest.fn(),
  };

  const currentUserState = mockStoreInstance.getState();

  expect(mockStoreInstance.getState).toHaveBeenCalledTimes(1);
  expect(currentUserState).toEqual(initialState);
});

Explanation: This test verifies that getState is called and returns the predefined initial state. It uses jest.fn().mockReturnValue() to control the behavior of getState.

Constraints

• Your tests should be written in TypeScript.
• You must use Jest for mocking and assertions.
• The mock store should simulate the basic functionality described (getting state and dispatching actions).
• Aim for clear and readable test descriptions.

Notes

• Consider how you would manage the store's state within your mock for more complex scenarios (though not strictly required for this challenge, it's good practice to think about). You might need to update the mocked getState return value after a dispatch call in more advanced tests.
• Think about how you'll inject your mock store into the code you are testing. Common patterns include constructor injection or passing the store as an argument to a function.
• The jest.fn() utility is your primary tool for creating spies and mocks. Remember you can control return values (mockReturnValue, mockResolvedValue) and track calls (toHaveBeenCalled, toHaveBeenCalledWith).