Writing Unit Tests with Jest

What Jest feature is being utilized here?

const spy = jest.spyOn(console, 'log');
try {
  someFunction();
  expect(spy).toHaveBeenCalledWith('expected output');
} finally {
  spy.mockRestore();
}

Mock functions
Spy functions
This demonstrates spy functionality: 1) Monitors function calls without replacing implementation, 2) Allows verifying function interactions, 3) Preserves original function behavior, 4) Enables call tracking and verification, 5) Important for testing side effects, 6) Properly cleaned up after test, 7) Useful for testing logging or analytics, 8) Non-intrusive way to verify behavior.
Timer mocks
Module mocks

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