Mocks

Jest · Reference cheat sheet

📋 Overview

jest.fn and jest.mock replace dependencies with controllable fakes. Track calls, set return values, and restore with clearAllMocks / restoreAllMocks.

🔧 Core concepts

API	Role
`jest.fn()`	Mock function
`jest.mock(path)`	Module mock (hoisted)
`jest.spyOn`	Wrap existing method
`mockReturnValue` / `mockResolvedValue`	Outputs
`mockImplementation`	Custom body

Manual mocks live in __mocks__ beside modules.

💡 Examples

jest.fn:

const send = jest.fn().mockReturnValue(true);
send('hi');
expect(send).toHaveBeenCalledWith('hi');
expect(send).toHaveBeenCalledTimes(1);

Async mock:

const fetchUser = jest
  .fn()
  .mockResolvedValue({ id: 1 })
  .mockRejectedValueOnce(new Error('network'));

Module mock:

jest.mock('./api', () => ({
  getUser: jest.fn().mockResolvedValue({ id: 1 }),
}));

import { getUser } from './api';

test('uses mock', async () => {
  await expect(getUser()).resolves.toEqual({ id: 1 });
});

Partial mock:

jest.mock('./utils', () => {
  const actual = jest.requireActual('./utils');
  return { ...actual, randomId: () => 'fixed' };
});

⚠️ Pitfalls

jest.mock is hoisted — don't rely on runtime vars above it without jest.doMock.
Forgetting to reset mocks between tests → cross-test pollution.
Mocking the wrong path (definition vs import site).
Over-mocking until nothing real is tested.
ESM + Jest mocking needs experimental/transform setup.

Mocks

Mocks

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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