Jest Mocking Patterns Agent Skill

Jest Mocking Patterns

This skill makes the agent mock dependencies in Jest deliberately and reversibly: stubbing functions with jest.fn, controlling return values with mockReturnValue/mockResolvedValue, replacing whole modules with jest.mock factories, and spying on real implementations with jest.spyOn (always restored). The guiding rule: mock the boundary, not the unit under test, and always reset state between tests so mocks never leak.

Use this skill when the agent needs to isolate code from the network, the clock, the filesystem, a database client, or any third-party module (axios, fs, a payment SDK).

Core Principles

• Mock at the boundary. Mock network/DB/3rd-party clients, not the function you are testing. If you mock the thing under test, the test proves nothing.
• Reset mocks between tests. Configure clearMocks: true (or call jest.clearAllMocks() in beforeEach) so call counts and implementations never bleed across tests.
• jest.mock is hoisted. Calls to jest.mock('module', factory) are lifted above imports. The factory cannot reference outer variables unless they are prefixed mock.
• Prefer spyOn + mockRestore over jest.mock when you only need to override one method and want the real implementation back afterward.
• Type your mocks. Use jest.mocked() (or as jest.Mock) so the mock API is type-checked and autocompletes.
• Assert behavior, then interactions. Check the result first; use toHaveBeenCalledWith to verify the boundary was called correctly.

Workflow / Patterns

Pattern 1 — jest.fn and controlling return values

jest.fn() is a recording stub. Drive it with mockReturnValue, mockResolvedValue, mockRejectedValue, or queue per-call values with mockReturnValueOnce.

test('jest.fn return value control', () => {
  const calc = jest.fn();

  calc.mockReturnValue(10); // default for every call
  calc.mockReturnValueOnce(1).mockReturnValueOnce(2); // queued, then falls back

  expect(calc()).toBe(1);
  expect(calc()).toBe(2);
  expect(calc()).toBe(10);
  expect(calc).toHaveBeenCalledTimes(3);
});

test('async return values', async () => {
  const fetchUser = jest.fn<Promise<{ id: number }>, [number]>();
  fetchUser.mockResolvedValue({ id: 1 });

  await expect(fetchUser(1)).resolves.toEqual({ id: 1 });

  fetchUser.mockRejectedValueOnce(new Error('not found'));
  await expect(fetchUser(99)).rejects.toThrow('not found');
});

Pattern 2 — mockImplementation for logic-bearing stubs

When the return depends on the input, supply a function. Use mockImplementationOnce for sequenced behavior.

test('mockImplementation reacts to args', () => {
  const discount = jest.fn((price: number, isMember: boolean) =>
    isMember ? price * 0.9 : price,
  );

  expect(discount(100, true)).toBe(90);
  expect(discount(100, false)).toBe(100);
  expect(discount).toHaveBeenLastCalledWith(100, false);
});

test('retry then succeed via mockImplementationOnce', async () => {
  const send = jest
    .fn<Promise<string>, []>()
    .mockImplementationOnce(() => Promise.reject(new Error('503')))
    .mockImplementationOnce(() => Promise.resolve('ok'));

  await expect(send()).rejects.toThrow('503');
  await expect(send()).resolves.toBe('ok');
});

Pattern 3 — jest.mock with a factory (the hoisting rule)

jest.mock replaces an entire module. Because it is hoisted above imports, any variable the factory references must be named with a mock prefix.

// notifier.ts
import { sendSms } from './sms-client';
export async function notify(phone: string, msg: string) {
  await sendSms(phone, msg);
  return `sent to ${phone}`;
}

// notifier.test.ts
import { notify } from './notifier';
import { sendSms } from './sms-client';

// Hoisted: the factory may only use `mock`-prefixed outer vars.
const mockSend = jest.fn();
jest.mock('./sms-client', () => ({
  sendSms: (...args: unknown[]) => mockSend(...args),
}));

beforeEach(() => jest.clearAllMocks());

test('notify calls the SMS client correctly', async () => {
  mockSend.mockResolvedValue(undefined);

  const result = await notify('+15551234567', 'Hello');

  expect(result).toBe('sent to +15551234567');
  expect(sendSms).toHaveBeenCalledWith('+15551234567', 'Hello');
  expect(sendSms).toHaveBeenCalledTimes(1);
});

Pattern 4 — jest.spyOn with restore (override one method, keep the rest)

spyOn wraps a real method so you can assert on it and optionally stub it. Always restore — restoreMocks: true in config, or mockRestore().

import * as mathUtils from './math-utils';

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

test('spy that still calls through', () => {
  const spy = jest.spyOn(mathUtils, 'add'); // real impl runs

  const sum = mathUtils.add(2, 3);

  expect(sum).toBe(5);
  expect(spy).toHaveBeenCalledWith(2, 3);
});

test('spy that replaces the implementation', () => {
  jest.spyOn(mathUtils, 'add').mockReturnValue(42);
  expect(mathUtils.add(2, 3)).toBe(42); // stubbed
  // restoreAllMocks() puts the real add back after this test.
});

test('spy on Date.now for deterministic time', () => {
  jest.spyOn(Date, 'now').mockReturnValue(1_700_000_000_000);
  expect(Date.now()).toBe(1_700_000_000_000);
});

Pattern 5 — Mocking axios (auto-mock + typed handle)

A real-world boundary. jest.mock('axios') auto-mocks the module; jest.mocked gives a typed handle.

// user-service.ts
import axios from 'axios';
export async function getUser(id: number) {
  const { data } = await axios.get(`/api/users/${id}`);
  return data;
}

// user-service.test.ts
import axios from 'axios';
import { getUser } from './user-service';

jest.mock('axios');
const mockedAxios = jest.mocked(axios);

beforeEach(() => jest.clearAllMocks());

test('resolves the user from the API', async () => {
  mockedAxios.get.mockResolvedValue({ data: { id: 1, name: 'Ada' } });

  const user = await getUser(1);

  expect(user).toEqual({ id: 1, name: 'Ada' });
  expect(mockedAxios.get).toHaveBeenCalledWith('/api/users/1');
});

test('propagates a network error', async () => {
  mockedAxios.get.mockRejectedValueOnce(new Error('Network Error'));
  await expect(getUser(1)).rejects.toThrow('Network Error');
});

Pattern 6 — Partial module mock with requireActual

Keep most of a module real, override one export. Essential when a util module mixes pure helpers with a side-effecting one.

jest.mock('./config', () => ({
  ...jest.requireActual('./config'),
  isProduction: jest.fn().mockReturnValue(false), // override only this
}));

Pattern 7 — Fake timers for debounce/throttle/setTimeout code

test('debounced save fires once after the delay', () => {
  jest.useFakeTimers();
  const save = jest.fn();
  const debounced = makeDebounced(save, 300);

  debounced();
  debounced();
  jest.advanceTimersByTime(300);

  expect(save).toHaveBeenCalledTimes(1);
  jest.useRealTimers();
});

Best Practices

• Set clearMocks: true and restoreMocks: true in jest.config so state never leaks between tests and spies always restore.
• Prefix factory-referenced variables with mock to satisfy Jest's hoisting rule; otherwise the factory throws a reference error.
• Use jest.mocked(x) for typed access to mocked modules instead of unsafe as any casts.
• Reach for spyOn first when overriding a single method — it is reversible and keeps the rest real.
• Assert the result before the interaction. Confirm the output, then toHaveBeenCalledWith to verify how the boundary was invoked.
• Use mockResolvedValueOnce/mockRejectedValueOnce to script retry and error paths precisely.
• Use fake timers for time-dependent code instead of real setTimeout waits.

Anti-Patterns

• Mocking the function under test. The test then verifies the mock, not the code. Mock the dependency it calls.
• No clearAllMocks/clearMocks. Call counts and implementations bleed across tests, producing order-dependent flakiness.
• Referencing a non-mock outer variable in a jest.mock factory. Hoisting moves the factory above the declaration -> ReferenceError.
• spyOn without restore. The stub leaks into later tests in the same file.
• Over-mocking — stubbing every collaborator until the test no longer exercises real logic. Mock only the I/O boundary.
• Asserting only toHaveBeenCalled() without checking arguments or the actual result — weak tests that pass on wrong calls.
• as any on mocked modules, discarding the type safety jest.mocked provides.

When to Trigger This Skill

• "Mock axios / fetch / an HTTP client in my Jest test"
• "How do I use jest.mock with a factory?"
• "mockResolvedValue / mockReturnValueOnce — how do these work?"
• "Spy on a method but keep the real implementation"
• "My Jest mock leaks into the next test"
• "ReferenceError: cannot access X before initialization in jest.mock"
• "Mock only one export from a module" / "partial mock"
• "Test debounced / setTimeout code with fake timers"