mocking-strategies

Mocking Strategies

Mocking isolates the code under test from its dependencies. Done right, mocks make tests fast and focused. Done wrong, they create brittle tests coupled to implementation details.

When to Use

• User needs to test code that calls external APIs or databases
• User asks about mocks, stubs, spies, or fakes
• Tests are slow because they hit real services
• User wants to test error paths that are hard to trigger naturally
• Tests are brittle and break on internal refactors

Core Patterns

Mocks vs Stubs vs Spies

Type	Purpose	Verifies Calls?	Returns Data?
Stub	Replace with canned response	No	Yes
Spy	Observe calls without replacing	Yes	Original
Mock	Replace + verify calls + canned response	Yes	Yes

// STUB: Replace function, return fixed data
const getUser = vi.fn().mockResolvedValue({ id: 1, name: 'Alice' })

// SPY: Watch real function, keep original behavior
const spy = vi.spyOn(userService, 'getUser')
// Real getUser still runs, but calls are recorded

// MOCK: Replace function, return fixed data, verify calls
const sendEmail = vi.fn().mockResolvedValue({ sent: true })
await notifyUser(1)
expect(sendEmail).toHaveBeenCalledWith('[email protected]', expect.any(String))

Vitest / Jest Mocking

Module mock:

import { describe, it, expect, vi } from 'vitest'
import { processOrder } from './orders'

// Mock the entire payment module
vi.mock('./payment', () => ({
  chargeCard: vi.fn().mockResolvedValue({ success: true, txId: 'tx-123' }),
}))

import { chargeCard } from './payment'

describe('processOrder', () => {
  it('charges the card and returns order confirmation', async () => {
    const order = await processOrder({ userId: 1, amount: 99 })

    expect(chargeCard).toHaveBeenCalledWith({ userId: 1, amount: 99 })
    expect(order.status).toBe('confirmed')
    expect(order.transactionId).toBe('tx-123')
  })

  it('handles payment failure', async () => {
    vi.mocked(chargeCard).mockRejectedValueOnce(new Error('Card declined'))

    await expect(processOrder({ userId: 1, amount: 99 }))
      .rejects.toThrow('Card declined')
  })
})