Ask the user or analyze code to determine:

What needs to be mocked (function, class, API, database, etc.)
Which testing framework is used
What the mock behavior should be
What data the mock should return

Scan for testing framework:

# Check package.json for testing framework
grep -E "(jest|vitest|mocha|jasmine|pytest|unittest|minitest)" package.json

# Check for test files
find . -name "*.test.*" -o -name "*.spec.*" -o -name "test_*.py"

2. Determine Mock Type

Function Mocks:

Simple return value
Multiple return values
Implementations
Spy on function calls

API Mocks:

HTTP request/response
WebSocket messages
GraphQL queries
REST endpoints

Class Mocks:

Instance methods
Static methods
Properties
Constructors

Module Mocks:

Entire module
Partial module
Default exports
Named exports

3. Generate Mocks by Framework

JavaScript/TypeScript Mocks

Jest Mocks

Simple Function Mock:

// Mock a simple function
const mockFn = jest.fn()
mockFn.mockReturnValue(42)

// Use in test
test('uses mocked function', () => {
  expect(mockFn()).toBe(42)
  expect(mockFn).toHaveBeenCalled()
})

Function with Different Return Values:

const mockFn = jest.fn()
  .mockReturnValueOnce('first')
  .mockReturnValueOnce('second')
  .mockReturnValue('default')

expect(mockFn()).toBe('first')
expect(mockFn()).toBe('second')
expect(mockFn()).toBe('default')

Mock Implementation:

const mockFn = jest.fn((x, y) => x + y)

expect(mockFn(1, 2)).toBe(3)
expect(mockFn).toHaveBeenCalledWith(1, 2)

Module Mock:

// __mocks__/axios.js
export default {
  get: jest.fn(() => Promise.resolve({ data: {} })),
  post: jest.fn(() => Promise.resolve({ data: {} })),
}

// In test file
jest.mock('axios')
import axios from 'axios'

test('fetches data', async () => {
  axios.get.mockResolvedValue({ data: { name: 'John' } })

  const result = await fetchUser(1)

  expect(result.name).toBe('John')
  expect(axios.get).toHaveBeenCalledWith('/users/1')
})

Class Mock:

// Mock a class
jest.mock('./Database')
import Database from './Database'

Database.mockImplementation(() => ({
  query: jest.fn().mockResolvedValue([{ id: 1, name: 'John' }]),
  connect: jest.fn().mockResolvedValue(true),
  disconnect: jest.fn().mockResolvedValue(true),
}))

test('uses database', async () => {
  const db = new Database()
  const users = await db.query('SELECT * FROM users')

  expect(users).toHaveLength(1)
  expect(db.query).toHaveBeenCalled()
})

Partial Mock:

// Mock only specific methods
import * as utils from './utils'

jest.spyOn(utils, 'fetchData').mockResolvedValue({ data: 'mocked' })

test('uses mocked method', async () => {
  const result = await utils.fetchData()
  expect(result.data).toBe('mocked')
})

Vitest Mocks

Function Mock:

import { vi, expect, test } from 'vitest'

const mockFn = vi.fn()
mockFn.mockReturnValue(42)

test('uses mock', () => {
  expect(mockFn()).toBe(42)
})

Module Mock:

// __mocks__/api.ts
import { vi } from 'vitest'

export const fetchUser = vi.fn()
export const createUser = vi.fn()

// In test
vi.mock('./api')
import { fetchUser } from './api'

test('fetches user', async () => {
  fetchUser.mockResolvedValue({ id: 1, name: 'John' })

  const user = await fetchUser(1)

  expect(user.name).toBe('John')
})

Spy on Method:

import { vi } from 'vitest'

const obj = {
  method: () => 'original'
}

vi.spyOn(obj, 'method').mockReturnValue('mocked')

expect(obj.method()).toBe('mocked')

TypeScript Mocks

Type-Safe Mock:

import { vi } from 'vitest'

interface User {
  id: number
  name: string
  email: string
}

// Create type-safe mock
const mockUser: User = {
  id: 1,
  name: 'John Doe',
  email: 'john@example.com'
}

// Mock function with types
const mockFetchUser = vi.fn<[id: number], Promise<User>>()
mockFetchUser.mockResolvedValue(mockUser)

Mock Factory:

// Create a factory for generating mocks
function createMockUser(overrides?: Partial<User>): User {
  return {
    id: 1,
    name: 'Test User',
    email: 'test@example.com',
    ...overrides
  }
}

// Use in tests
const user1 = createMockUser({ name: 'Alice' })
const user2 = createMockUser({ id: 2, email: 'bob@example.com' })

Python Mocks

unittest.mock

Function Mock:

from unittest.mock import Mock

# Simple mock
mock_func = Mock(return_value=42)
assert mock_func() == 42
assert mock_func.called

# Mock with side effects
mock_func = Mock(side_effect=[1, 2, 3])
assert mock_func() == 1
assert mock_func() == 2
assert mock_func() == 3

Patch Decorator:

from unittest.mock import patch, Mock

@patch('requests.get')
def test_fetch_data(mock_get):
    # Setup mock
    mock_response = Mock()
    mock_response.json.return_value = {'name': 'John'}
    mock_response.status_code = 200
    mock_get.return_value = mock_response

    # Test
    result = fetch_user_data(1)

    assert result['name'] == 'John'
    mock_get.assert_called_once_with('https://api.example.com/users/1')

Class Mock:

from unittest.mock import Mock, patch

@patch('database.Database')
def test_database_query(mock_database_class):
    # Setup mock instance
    mock_db = Mock()
    mock_db.query.return_value = [{'id': 1, 'name': 'John'}]
    mock_database_class.return_value = mock_db

    # Test
    db = Database()
    users = db.query('SELECT * FROM users')

    assert len(users) == 1
    assert users[0]['name'] == 'John'
    mock_db.query.assert_called_once()

Context Manager Mock:

from unittest.mock import patch, mock_open

# Mock file operations
mock_data = "file contents"
with patch('builtins.open', mock_open(read_data=mock_data)):
    with open('file.txt') as f:
        content = f.read()
    assert content == mock_data

pytest Fixtures

Simple Fixture:

import pytest

@pytest.fixture
def mock_user():
    return {
        'id': 1,
        'name': 'John Doe',
        'email': 'john@example.com'
    }

def test_user_data(mock_user):
    assert mock_user['name'] == 'John Doe'

Fixture with Cleanup:

@pytest.fixture
def mock_database():
    # Setup
    db = MockDatabase()
    db.connect()

    yield db  # Provide to test

    # Teardown
    db.disconnect()

def test_database_query(mock_database):
    result = mock_database.query('SELECT * FROM users')
    assert len(result) > 0

Parametrized Fixture:

@pytest.fixture(params=[
    {'name': 'Alice', 'age': 25},
    {'name': 'Bob', 'age': 30},
    {'name': 'Charlie', 'age': 35}
])
def mock_user(request):
    return request.param

def test_user_age(mock_user):
    assert mock_user['age'] > 0

pytest-mock

def test_api_call(mocker):
    # Mock a function
    mock_get = mocker.patch('requests.get')
    mock_get.return_value.json.return_value = {'status': 'ok'}

    result = fetch_data()

    assert result['status'] == 'ok'
    mock_get.assert_called_once()

API Response Mocks

REST API Mock

// Mock fetch API
global.fetch = jest.fn(() =>
  Promise.resolve({
    ok: true,
    status: 200,
    json: async () => ({
      id: 1,
      name: 'John Doe',
      email: 'john@example.com'
    }),
    headers: new Headers({
      'Content-Type': 'application/json'
    })
  })
)

test('fetches user', async () => {
  const user = await fetchUser(1)

  expect(user.name).toBe('John Doe')
  expect(fetch).toHaveBeenCalledWith('/api/users/1')
})

Mock Service Worker (MSW)

// mocks/handlers.js
import { rest } from 'msw'

export const handlers = [
  rest.get('/api/users/:id', (req, res, ctx) => {
    const { id } = req.params

    return res(
      ctx.status(200),
      ctx.json({
        id: Number(id),
        name: 'John Doe',
        email: 'john@example.com'
      })
    )
  }),

  rest.post('/api/users', (req, res, ctx) => {
    return res(
      ctx.status(201),
      ctx.json({
        id: 123,
        ...req.body
      })
    )
  }),

  rest.get('/api/users', (req, res, ctx) => {
    return res(
      ctx.status(200),
      ctx.json([
        { id: 1, name: 'Alice' },
        { id: 2, name: 'Bob' }
      ])
    )
  })
]

// mocks/server.js
import { setupServer } from 'msw/node'
import { handlers } from './handlers'

export const server = setupServer(...handlers)

// setupTests.js
import { server } from './mocks/server'

beforeAll(() => server.listen())
afterEach(() => server.resetHandlers())
afterAll(() => server.close())

GraphQL Mock

import { graphql } from 'msw'

export const handlers = [
  graphql.query('GetUser', (req, res, ctx) => {
    const { id } = req.variables

    return res(
      ctx.data({
        user: {
          id,
          name: 'John Doe',
          email: 'john@example.com'
        }
      })
    )
  }),

  graphql.mutation('CreateUser', (req, res, ctx) => {
    const { input } = req.variables

    return res(
      ctx.data({
        createUser: {
          id: '123',
          ...input
        }
      })
    )
  })
]

Database Mocks

Prisma Mock

import { PrismaClient } from '@prisma/client'
import { mockDeep, mockReset, DeepMockProxy } from 'jest-mock-extended'

export const prismaMock = mockDeep<PrismaClient>()

beforeEach(() => {
  mockReset(prismaMock)
})

// In test
test('creates user', async () => {
  const mockUser = { id: 1, name: 'John', email: 'john@example.com' }

  prismaMock.user.create.mockResolvedValue(mockUser)

  const user = await createUser({ name: 'John', email: 'john@example.com' })

  expect(user).toEqual(mockUser)
  expect(prismaMock.user.create).toHaveBeenCalledWith({
    data: { name: 'John', email: 'john@example.com' }
  })
})

MongoDB Mock

import { MongoMemoryServer } from 'mongodb-memory-server'
import { MongoClient } from 'mongodb'

let mongod
let client
let db

beforeAll(async () => {
  mongod = await MongoMemoryServer.create()
  const uri = mongod.getUri()
  client = new MongoClient(uri)
  await client.connect()
  db = client.db()
})

afterAll(async () => {
  await client.close()
  await mongod.stop()
})

test('inserts user', async () => {
  const users = db.collection('users')
  const user = { name: 'John', email: 'john@example.com' }

  await users.insertOne(user)

  const found = await users.findOne({ name: 'John' })
  expect(found.email).toBe('john@example.com')
})

React Component Mocks

React Testing Library

import { render, screen } from '@testing-library/react'
import '@testing-library/jest-dom'

// Mock child component
jest.mock('./UserAvatar', () => ({
  UserAvatar: ({ name }) => <div data-testid="avatar">{name}</div>
}))

test('renders user profile', () => {
  render(<UserProfile name="John" />)

  expect(screen.getByTestId('avatar')).toHaveTextContent('John')
})

Mock React Hooks

import { renderHook } from '@testing-library/react'

// Mock useState
const mockSetState = jest.fn()
jest.spyOn(React, 'useState').mockImplementation(initial => [initial, mockSetState])

// Mock custom hook
jest.mock('./useUser')
import { useUser } from './useUser'

test('uses user hook', () => {
  useUser.mockReturnValue({
    user: { id: 1, name: 'John' },
    loading: false,
    error: null
  })

  const { result } = renderHook(() => useUser(1))

  expect(result.current.user.name).toBe('John')
})

Test Fixtures and Factories

Fixture Files

// fixtures/users.js
export const mockUsers = [
  { id: 1, name: 'Alice', email: 'alice@example.com', role: 'admin' },
  { id: 2, name: 'Bob', email: 'bob@example.com', role: 'user' },
  { id: 3, name: 'Charlie', email: 'charlie@example.com', role: 'user' }
]

export const mockUser = mockUsers[0]

// In tests
import { mockUser, mockUsers } from './fixtures/users'

test('processes user', () => {
  const result = processUser(mockUser)
  expect(result.name).toBe('Alice')
})

Factory Pattern

// factories/userFactory.js
let userId = 1

export function createUser(overrides = {}) {
  return {
    id: userId++,
    name: 'Test User',
    email: `user${userId}@example.com`,
    role: 'user',
    createdAt: new Date(),
    ...overrides
  }
}

export function createAdmin(overrides = {}) {
  return createUser({
    role: 'admin',
    ...overrides
  })
}

// In tests
import { createUser, createAdmin } from './factories/userFactory'

test('creates user', () => {
  const user = createUser({ name: 'Alice' })
  expect(user.name).toBe('Alice')
  expect(user.role).toBe('user')
})

test('creates admin', () => {
  const admin = createAdmin({ name: 'Bob' })
  expect(admin.role).toBe('admin')
})

Python Factory (factory_boy)

import factory
from myapp.models import User

class UserFactory(factory.Factory):
    class Meta:
        model = User

    id = factory.Sequence(lambda n: n)
    name = factory.Faker('name')
    email = factory.Faker('email')
    role = 'user'

# In tests
def test_user_creation():
    user = UserFactory()
    assert user.name is not None
    assert '@' in user.email

def test_admin_creation():
    admin = UserFactory(role='admin')
    assert admin.role == 'admin'

Best Practices

1. Keep Mocks Simple

// ❌ BAD: Overly complex mock
const mock = jest.fn()
mock.mockImplementation((a, b, c) => {
  if (a > 10) {
    return b * c
  } else if (a < 5) {
    return b + c
  }
  return a + b + c
})

// ✅ GOOD: Simple, focused mock
const mock = jest.fn().mockReturnValue(42)

2. Use Factories for Complex Objects

// ✅ GOOD: Reusable factory
function createMockUser(overrides) {
  return {
    id: 1,
    name: 'Test User',
    email: 'test@example.com',
    ...overrides
  }
}

3. Reset Mocks Between Tests

afterEach(() => {
  jest.clearAllMocks() // Clear call history
  jest.resetAllMocks() // Clear call history + implementation
})

4. Type-Safe Mocks in TypeScript

import { vi } from 'vitest'

const mockFn = vi.fn<[id: number], Promise<User>>()
// TypeScript ensures correct usage
mockFn(123) // ✅ OK
mockFn('abc') // ❌ TypeScript error

5. Document Mock Behavior

// Document why mock behaves this way
test('handles rate limiting', async () => {
  // Mock returns 429 to simulate rate limiting
  fetch.mockRejectedValueOnce(new Error('Rate limited'))

  await expect(fetchData()).rejects.toThrow('Rate limited')
})

Advanced Patterns

Spy on Console

const consoleErrorSpy = jest.spyOn(console, 'error').mockImplementation()

test('logs error', () => {
  functionThatLogs()
  expect(consoleErrorSpy).toHaveBeenCalledWith('Error occurred')
})

consoleErrorSpy.mockRestore()

Mock Timers

jest.useFakeTimers()

test('delays execution', () => {
  const callback = jest.fn()

  setTimeout(callback, 1000)

  jest.advanceTimersByTime(500)
  expect(callback).not.toHaveBeenCalled()

  jest.advanceTimersByTime(500)
  expect(callback).toHaveBeenCalled()
})

jest.useRealTimers()

Mock Date

const mockDate = new Date('2024-01-01')
jest.spyOn(global, 'Date').mockImplementation(() => mockDate)

test('uses fixed date', () => {
  const result = getCurrentDate()
  expect(result.getFullYear()).toBe(2024)
})

mock-generator

Mock Generator

When to Use

Instructions

1. Identify What to Mock