/vitest

Vi Utilities

The vi helper provides mocking and utility functions.

import { vi } from 'vitest'

Mock Functions

// Create mock
const fn = vi.fn()
const fnWithImpl = vi.fn((x) => x * 2)

// Check if mock
vi.isMockFunction(fn) // true

// Mock methods
fn.mockReturnValue(42)
fn.mockReturnValueOnce(1)
fn.mockResolvedValue(data)
fn.mockRejectedValue(error)
fn.mockImplementation(() => 'result')
fn.mockImplementationOnce(() => 'once')

// Clear/reset
fn.mockClear()    // Clear call history
fn.mockReset()    // Clear history + implementation
fn.mockRestore()  // Restore original (for spies)

Spying

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

const spy = vi.spyOn(obj, 'method')
obj.method()

expect(spy).toHaveBeenCalled()

// Mock implementation
spy.mockReturnValue('mocked')

// Spy on getter/setter
vi.spyOn(obj, 'prop', 'get').mockReturnValue('value')

Module Mocking

// Hoisted to top of file
vi.mock('./module', () => ({
  fn: vi.fn(),
}))

// Partial mock
vi.mock('./module', async (importOriginal) => ({
  ...(await importOriginal()),
  specificFn: vi.fn(),
}))

// Spy mode - keep implementation
vi.mock('./module', { spy: true })

// Import actual module inside mock
const actual = await vi.importActual('./module')

// Import as mock
const mocked = await vi.importMock('./module')

Dynamic Mocking

// Not hoisted - use with dynamic imports
vi.doMock('./config', () => ({ key: 'value' }))
const config = await import('./config')

// Unmock
vi.doUnmock('./config')
vi.unmock('./module') // Hoisted

Conditional Mocking — vi.when (v5)

Argument-specific spy behaviors:

vi.when(spy)
  .calledWith(1).thenReturn('one')
  .calledWith(2).thenReturn('two')

// then* actions: thenReturn / thenThrow / thenResolve / thenReject (+ *Once)
// options: { times }, second arg { onUnmatched: 'throw' | 'passthrough' | fn }

vi.isWhenChain(w)  // type guard for a When chain

See features-mocking for FIFO/LIFO matching and toHaveBeenExhausted.

Assertion Helpers — vi.defineHelper (4.1+)

Wrap reusable assertion functions so failures point at the call site, not inside the helper:

const expectValidUser = vi.defineHelper((user: unknown) => {
  expect(user).toHaveProperty('id')
  expect(user).toHaveProperty('email')
})

test('returns a valid user', async () => {
  expectValidUser(await fetchUser('alice')) // failures reported here
})

Reset Modules

// Clear module cache
vi.resetModules()

// Wait for dynamic imports
await vi.dynamicImportSettled()

Fake Timers

vi.useFakeTimers()

// Choose which timers to fake (toFake and toNotFake are mutually exclusive)
vi.useFakeTimers({ toFake: ['setTimeout', 'clearTimeout'] })
vi.useFakeTimers({ toNotFake: ['setInterval'] })

setTimeout(() => console.log('done'), 1000)

// Advance time
vi.advanceTimersByTime(1000)
vi.advanceTimersByTimeAsync(1000)  // For async callbacks
vi.advanceTimersToNextTimer()
vi.advanceTimersToNextFrame()      // requestAnimationFrame

// Run all timers
vi.runAllTimers()
vi.runAllTimersAsync()
vi.runOnlyPendingTimers()

// Clear timers
vi.clearAllTimers()

// Check state
vi.getTimerCount()
vi.isFakeTimers()

// Restore
vi.useRealTimers()

Mock Date/Time

vi.setSystemTime(new Date('2024-01-01'))
expect(new Date().getFullYear()).toBe(2024)

vi.getMockedSystemTime()  // Get mocked date
vi.getRealSystemTime()    // Get real time (ms)

Global/Env Mocking

// Stub global
vi.stubGlobal('fetch', vi.fn())
vi.unstubAllGlobals()

// Stub environment
vi.stubEnv('API_KEY', 'test')
vi.stubEnv('NODE_ENV', 'test')
vi.unstubAllEnvs()

Hoisted Code

Run code before imports:

const mock = vi.hoisted(() => vi.fn())

vi.mock('./module', () => ({
  fn: mock, // Can reference hoisted variable
}))

Waiting Utilities

// Wait for callback to succeed
await vi.waitFor(async () => {
  const el = document.querySelector('.loaded')
  expect(el).toBeTruthy()
}, { timeout: 5000, interval: 100 })

// Wait for truthy value
const element = await vi.waitUntil(
  () => document.querySelector('.loaded'),
  { timeout: 5000 }
)

Mock Object

Mock all methods of an object:

const original = {
  method: () => 'real',
  nested: { fn: () => 'nested' },
}

const mocked = vi.mockObject(original)
mocked.method()  // undefined (mocked)
mocked.method.mockReturnValue('mocked')

// Spy mode
const spied = vi.mockObject(original, { spy: true })
spied.method()  // 'real'
expect(spied.method).toHaveBeenCalled()

Test Configuration

vi.setConfig({
  testTimeout: 10_000,
  hookTimeout: 10_000,
})

vi.resetConfig()

Global Mock Management

vi.clearAllMocks()   // Clear all mock call history
vi.resetAllMocks()   // Reset + clear implementation
vi.restoreAllMocks() // Restore originals (spies)

vi.mocked Type Helper

TypeScript helper for mocked values:

import { myFn } from './module'
vi.mock('./module')

// Type as mock
vi.mocked(myFn).mockReturnValue('typed')

// Deep mocking
vi.mocked(myModule, { deep: true })

// Partial mock typing
vi.mocked(fn, { partial: true }).mockResolvedValue({ ok: true })

Key Points

• vi.mock is hoisted - use vi.doMock for dynamic mocking
• vi.hoisted lets you reference variables in mock factories
• Use vi.spyOn to spy on existing methods (v4: supports constructors)
• Use vi.when for argument-specific behaviors and vi.defineHelper for assertion helpers
• Fake timers require explicit setup and teardown
• vi.waitFor retries until assertion passes