---

name: test-data-generator description: Creates test fixtures, mock data, and test scenarios for unit and integration tests. Use when setting up test data, creating mocks, or generating test fixtures.

Test Data Generator

Generate test data, fixtures, and mocks for testing.

Quick Start

Create a simple fixture:

Copyconst mockUser = {
  id: 1,
  name: 'Test User',
  email: 'test@example.com'
}

Instructions

Factory Functions

Create reusable data generators:

Copyfunction createUser(overrides = {}) {
  return {
    id: Math.floor(Math.random() * 1000),
    name: 'Test User',
    email: 'test@example.com',
    role: 'user',
    createdAt: new Date().toISOString(),
    ...overrides
  }
}

// Usage
const admin = createUser({ role: 'admin', name: 'Admin User' })
const regularUser = createUser()

Copydef create_user(**kwargs):
    defaults = {
        'id': random.randint(1, 1000),
        'name': 'Test User',
        'email': 'test@example.com',
        'role': 'user',
        'created_at': datetime.now()
    }
    return {**defaults, **kwargs}

# Usage
admin = create_user(role='admin', name='Admin User')

Builder Pattern

For complex objects:

Copyclass UserBuilder {
  constructor() {
    this.user = {
      id: 1,
      name: 'Test User',
      email: 'test@example.com',
      role: 'user',
      preferences: {}
    }
  }
  
  withId(id) {
    this.user.id = id
    return this
  }
  
  withName(name) {
    this.user.name = name
    return this
  }
  
  withRole(role) {
    this.user.role = role
    return this
  }
  
  withPreferences(preferences) {
    this.user.preferences = preferences
    return this
  }
  
  build() {
    return this.user
  }
}

// Usage
const user = new UserBuilder()
  .withId(123)
  .withName('John Doe')
  .withRole('admin')
  .withPreferences({ theme: 'dark' })
  .build()

Test Fixtures

JavaScript/TypeScript:

Copy// fixtures/users.js
export const users = {
  admin: {
    id: 1,
    name: 'Admin User',
    email: 'admin@example.com',
    role: 'admin'
  },
  regular: {
    id: 2,
    name: 'Regular User',
    email: 'user@example.com',
    role: 'user'
  }
}

// In tests
import { users } from './fixtures/users'

test('admin can delete posts', () => {
  expect(canDelete(users.admin)).toBe(true)
})

Python (pytest):

Copy# conftest.py
import pytest

@pytest.fixture
def sample_user():
    return {
        'id': 1,
        'name': 'Test User',
        'email': 'test@example.com',
        'role': 'user'
    }

@pytest.fixture
def admin_user():
    return {
        'id': 2,
        'name': 'Admin User',
        'email': 'admin@example.com',
        'role': 'admin'
    }

# In tests
def test_user_permissions(sample_user, admin_user):
    assert can_delete(admin_user)
    assert not can_delete(sample_user)

Mock Data

API Responses:

Copyconst mockApiResponse = {
  data: {
    users: [
      { id: 1, name: 'Alice' },
      { id: 2, name: 'Bob' }
    ]
  },
  status: 200,
  statusText: 'OK'
}

// Mock fetch
global.fetch = jest.fn(() =>
  Promise.resolve({
    json: () => Promise.resolve(mockApiResponse.data)
  })
)

Database Records:

Copymock_db_records = [
    {'id': 1, 'name': 'Product A', 'price': 10.99},
    {'id': 2, 'name': 'Product B', 'price': 20.99},
    {'id': 3, 'name': 'Product C', 'price': 15.99}
]

@patch('app.database.query')
def test_get_products(mock_query):
    mock_query.return_value = mock_db_records
    products = get_products()
    assert len(products) == 3

Random Data Generation

Using faker (JavaScript):

Copyimport { faker } from '@faker-js/faker'

function generateUser() {
  return {
    id: faker.string.uuid(),
    name: faker.person.fullName(),
    email: faker.internet.email(),
    avatar: faker.image.avatar(),
    address: {
      street: faker.location.streetAddress(),
      city: faker.location.city(),
      country: faker.location.country()
    }
  }
}

// Generate multiple users
const users = Array.from({ length: 10 }, generateUser)

Using Faker (Python):

Copyfrom faker import Faker

fake = Faker()

def generate_user():
    return {
        'id': fake.uuid4(),
        'name': fake.name(),
        'email': fake.email(),
        'phone': fake.phone_number(),
        'address': {
            'street': fake.street_address(),
            'city': fake.city(),
            'country': fake.country()
        }
    }

# Generate multiple users
users = [generate_user() for _ in range(10)]

Seeded Random Data

For reproducible tests:

Copyimport { faker } from '@faker-js/faker'

test('generates consistent data with seed', () => {
  faker.seed(123)
  const user1 = generateUser()
  
  faker.seed(123)
  const user2 = generateUser()
  
  expect(user1).toEqual(user2)
})

Copyfrom faker import Faker

def test_consistent_data():
    fake = Faker()
    Faker.seed(123)
    user1 = generate_user(fake)
    
    Faker.seed(123)
    user2 = generate_user(fake)
    
    assert user1 == user2

Mock Functions

Jest:

Copyconst mockCallback = jest.fn(x => x * 2)

test('calls callback for each item', () => {
  const items = [1, 2, 3]
  items.forEach(mockCallback)
  
  expect(mockCallback).toHaveBeenCalledTimes(3)
  expect(mockCallback).toHaveBeenCalledWith(1)
  expect(mockCallback).toHaveBeenCalledWith(2)
  expect(mockCallback).toHaveBeenCalledWith(3)
})

Python (unittest.mock):

Copyfrom unittest.mock import Mock

def test_callback():
    mock_callback = Mock(return_value=42)
    
    result = process_data([1, 2, 3], mock_callback)
    
    assert mock_callback.call_count == 3
    mock_callback.assert_called_with(3)

Time-based Data

Freeze time:

Copyimport { jest } from '@jest/globals'

test('creates timestamp', () => {
  const mockDate = new Date('2024-01-01T00:00:00Z')
  jest.useFakeTimers()
  jest.setSystemTime(mockDate)
  
  const record = createRecord()
  expect(record.createdAt).toBe('2024-01-01T00:00:00.000Z')
  
  jest.useRealTimers()
})

Copyfrom freezegun import freeze_time

@freeze_time("2024-01-01 00:00:00")
def test_timestamp():
    record = create_record()
    assert record['created_at'] == datetime(2024, 1, 1, 0, 0, 0)

Common Patterns

Pattern: Array of test cases:

Copyconst testCases = [
  { input: 'hello', expected: 'HELLO' },
  { input: 'world', expected: 'WORLD' },
  { input: '', expected: '' }
]

testCases.forEach(({ input, expected }) => {
  test(`converts "${input}" to "${expected}"`, () => {
    expect(toUpperCase(input)).toBe(expected)
  })
})

Pattern: Shared setup:

Copydescribe('UserService', () => {
  let service
  let mockDb
  
  beforeEach(() => {
    mockDb = {
      query: jest.fn(),
      insert: jest.fn()
    }
    service = new UserService(mockDb)
  })
  
  test('fetches users', async () => {
    mockDb.query.mockResolvedValue([{ id: 1 }])
    const users = await service.getUsers()
    expect(users).toHaveLength(1)
  })
})

Pattern: Test data files:

Copy// testData/users.json
[
  {
    "id": 1,
    "name": "Alice",
    "email": "alice@example.com"
  },
  {
    "id": 2,
    "name": "Bob",
    "email": "bob@example.com"
  }
]

// In tests
import users from './testData/users.json'

test('processes user data', () => {
  const result = processUsers(users)
  expect(result).toHaveLength(2)
})

Advanced

For complex scenarios:

• Use factory libraries (factory-bot, fishery, rosie)
• Generate GraphQL mock data
• Create database seeders
• Build test data pipelines