Testing Traps

• jest.mock() hoisted — runs before imports, can't use variables from file
• Mock not cleaned between tests — jest.clearAllMocks() in beforeEach
• async test without await — test passes before promise resolves/rejects
• toEqual vs toBe — toBe uses ===, fails on equal objects
• Timer mocks — jest.useFakeTimers() needed, real timers make tests flaky
• Snapshot too big — small changes break test, snapshot should be focused
• done() callback timeout — forgetting to call done hangs test