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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Mocks bleeding state between tests because jest.fn() call history isn't cleared

// Wrong: mock call history leaks across tests
const emailSpy = jest.fn();
// ...never cleared between it() blocks

// Correct: reset before every test
beforeEach(() => {
  jest.clearAllMocks();
});

The Solution //

A jest.fn() accumulates every call it has ever received across the entire test file unless explicitly reset — so a later test's toHaveBeenCalledTimes(1) can fail because a previous test already called the same mock. Use jest.clearAllMocks() (or restoreAllMocks() for spies) in a global beforeEach so every test starts with a clean slate.

The Error //

Using jest.spyOn() to wrap a global/module method but never calling mockRestore()

// Wrong: console.log stays wrapped for every later test
const logSpy = jest.spyOn(console, 'log');

// Correct: always restore after use
afterEach(() => {
  logSpy.mockRestore();
});

The Solution //

jest.spyOn(console, 'log') replaces the real console.log with a tracked wrapper for the rest of the test run — if you never call spy.mockRestore() (or use restoreMocks: true in config), every subsequent test in the file silently runs against the spied-on version instead of the real implementation, which can hide legitimate output or cause confusing cross-test behavior.