Mock Functions ​

You can create a spy function (mock) to track its execution with vi.fn method. If you want to track a method on an already created object, you can use vi.spyOn method:

import { vi } from 'vitest'

const fn = vi.fn()
fn('hello world')
fn.mock.calls[0] === ['hello world']

const market = {
  getApples: () => 100
}

const getApplesSpy = vi.spyOn(market, 'getApples')
market.getApples()
getApplesSpy.mock.calls.length === 1

import { vi } from 'vitest'

const fn = vi.fn()
fn('hello world')
fn.mock.calls[0] === ['hello world']

const market = {
  getApples: () => 100
}

const getApplesSpy = vi.spyOn(market, 'getApples')
market.getApples()
getApplesSpy.mock.calls.length === 1

You should use spy assertions (e.g., toHaveBeenCalled) on expect to assert spy result. This API reference describes available properties and methods to manipulate spy behavior.

getMockName ​

• Type: () => string

  Use it to return the name given to mock with method .mockName(name).

mockClear ​

• Type: () => MockInstance

  Clears all information about every call. After calling it, spy.mock.calls, spy.mock.results will return empty arrays. It is useful if you need to clean up spy between different assertions.

  If you want this method to be called before each test automatically, you can enable clearMocks setting in config.

mockName ​

• Type: (name: string) => MockInstance

  Sets internal mock name. Useful to see what mock has failed the assertion.

mockImplementation ​

• Type: (fn: Function) => MockInstance

  Accepts a function that will be used as an implementation of the mock.

  For example:

  ts

  const mockFn = vi.fn().mockImplementation(apples => apples + 1)
  // or: vi.fn(apples => apples + 1);
  
  const NelliesBucket = mockFn(0)
  const BobsBucket = mockFn(1)
  
  NelliesBucket === 1 // true
  BobsBucket === 2 // true
  
  mockFn.mock.calls[0][0] === 0 // true
  mockFn.mock.calls[1][0] === 1 // true

  const mockFn = vi.fn().mockImplementation(apples => apples + 1)
  // or: vi.fn(apples => apples + 1);
  
  const NelliesBucket = mockFn(0)
  const BobsBucket = mockFn(1)
  
  NelliesBucket === 1 // true
  BobsBucket === 2 // true
  
  mockFn.mock.calls[0][0] === 0 // true
  mockFn.mock.calls[1][0] === 1 // true

mockImplementationOnce ​

• Type: (fn: Function) => MockInstance

  Accepts a function that will be used as an implementation of the mock for one call to the mocked function. Can be chained so that multiple function calls produce different results.

  ts

  const myMockFn = vi
    .fn()
    .mockImplementationOnce(() => true)
    .mockImplementationOnce(() => false)
  
  myMockFn() // true
  myMockFn() // false

  const myMockFn = vi
    .fn()
    .mockImplementationOnce(() => true)
    .mockImplementationOnce(() => false)
  
  myMockFn() // true
  myMockFn() // false

  When the mocked function runs out of implementations, it will invoke the default implementation that was set with vi.fn(() => defaultValue) or .mockImplementation(() => defaultValue) if they were called:

  ts

  const myMockFn = vi
    .fn(() => 'default')
    .mockImplementationOnce(() => 'first call')
    .mockImplementationOnce(() => 'second call')
  
  // 'first call', 'second call', 'default', 'default'
  console.log(myMockFn(), myMockFn(), myMockFn(), myMockFn())

  const myMockFn = vi
    .fn(() => 'default')
    .mockImplementationOnce(() => 'first call')
    .mockImplementationOnce(() => 'second call')
  
  // 'first call', 'second call', 'default', 'default'
  console.log(myMockFn(), myMockFn(), myMockFn(), myMockFn())

withImplementation ​

• Type: (fn: Function, callback: () => void) => MockInstance

• Type: (fn: Function, callback: () => Promise<unknown>) => Promise<MockInstance>

  Overrides the original mock implementation temporarily while the callback is being executed.

  js

  const myMockFn = vi.fn(() => 'original')
  
  myMockFn.withImplementation(() => 'temp', () => {
    myMockFn() // 'temp'
  })
  
  myMockFn() // 'original'

  const myMockFn = vi.fn(() => 'original')
  
  myMockFn.withImplementation(() => 'temp', () => {
    myMockFn() // 'temp'
  })
  
  myMockFn() // 'original'

  Can be used with an asynchronous callback. The method has to be awaited to use the original implementation afterward.

  ts

  test('async callback', () => {
    const myMockFn = vi.fn(() => 'original')
  
    // We await this call since the callback is async
    await myMockFn.withImplementation(
      () => 'temp',
      async () => {
        myMockFn() // 'temp'
      },
    )
  
    myMockFn() // 'original'
  })

  test('async callback', () => {
    const myMockFn = vi.fn(() => 'original')
  
    // We await this call since the callback is async
    await myMockFn.withImplementation(
      () => 'temp',
      async () => {
        myMockFn() // 'temp'
      },
    )
  
    myMockFn() // 'original'
  })

  Also, it takes precedence over the mockImplementationOnce.

mockRejectedValue ​

• Type: (value: any) => MockInstance

  Accepts an error that will be rejected, when async function will be called.

  ts

  const asyncMock = vi.fn().mockRejectedValue(new Error('Async error'))
  
  await asyncMock() // throws "Async error"

  const asyncMock = vi.fn().mockRejectedValue(new Error('Async error'))
  
  await asyncMock() // throws "Async error"

mockRejectedValueOnce ​

• Type: (value: any) => MockInstance

  Accepts a value that will be rejected for one call to the mock function. If chained, every consecutive call will reject passed value.

  ts

  const asyncMock = vi
    .fn()
    .mockResolvedValueOnce('first call')
    .mockRejectedValueOnce(new Error('Async error'))
  
  await asyncMock() // first call
  await asyncMock() // throws "Async error"

  const asyncMock = vi
    .fn()
    .mockResolvedValueOnce('first call')
    .mockRejectedValueOnce(new Error('Async error'))
  
  await asyncMock() // first call
  await asyncMock() // throws "Async error"

mockReset ​

• Type: () => MockInstance

  Does what mockClear does and makes inner implementation an empty function (returning undefined, when invoked). This is useful when you want to completely reset a mock back to its initial state.

  If you want this method to be called before each test automatically, you can enable mockReset setting in config.

mockRestore ​

• Type: () => MockInstance

  Does what mockReset does and restores inner implementation to the original function.

  Note that restoring mock from vi.fn() will set implementation to an empty function that returns undefined. Restoring a vi.fn(impl) will restore implementation to impl.

  If you want this method to be called before each test automatically, you can enable restoreMocks setting in config.

mockResolvedValue ​

• Type: (value: any) => MockInstance

  Accepts a value that will be resolved, when async function will be called.

  ts

  const asyncMock = vi.fn().mockResolvedValue(43)
  
  await asyncMock() // 43

  const asyncMock = vi.fn().mockResolvedValue(43)
  
  await asyncMock() // 43

mockResolvedValueOnce ​

• Type: (value: any) => MockInstance

  Accepts a value that will be resolved for one call to the mock function. If chained, every consecutive call will resolve passed value.

  ts

  const asyncMock = vi
    .fn()
    .mockResolvedValue('default')
    .mockResolvedValueOnce('first call')
    .mockResolvedValueOnce('second call')
  
  await asyncMock() // first call
  await asyncMock() // second call
  await asyncMock() // default
  await asyncMock() // default

  const asyncMock = vi
    .fn()
    .mockResolvedValue('default')
    .mockResolvedValueOnce('first call')
    .mockResolvedValueOnce('second call')
  
  await asyncMock() // first call
  await asyncMock() // second call
  await asyncMock() // default
  await asyncMock() // default

mockReturnThis ​

• Type: () => MockInstance

  Sets inner implementation to return this context.

mockReturnValue ​

• Type: (value: any) => MockInstance

  Accepts a value that will be returned whenever the mock function is called.

  ts

  const mock = vi.fn()
  mock.mockReturnValue(42)
  mock() // 42
  mock.mockReturnValue(43)
  mock() // 43

  const mock = vi.fn()
  mock.mockReturnValue(42)
  mock() // 42
  mock.mockReturnValue(43)
  mock() // 43

mockReturnValueOnce ​

• Type: (value: any) => MockInstance

  Accepts a value that will be returned for one call to the mock function. If chained, every consecutive call will return the passed value. When there are no more mockReturnValueOnce values to use, call a function specified by mockImplementation or other mockReturn* methods.

  ts

  const myMockFn = vi
    .fn()
    .mockReturnValue('default')
    .mockReturnValueOnce('first call')
    .mockReturnValueOnce('second call')
  
  // 'first call', 'second call', 'default', 'default'
  console.log(myMockFn(), myMockFn(), myMockFn(), myMockFn())

  const myMockFn = vi
    .fn()
    .mockReturnValue('default')
    .mockReturnValueOnce('first call')
    .mockReturnValueOnce('second call')
  
  // 'first call', 'second call', 'default', 'default'
  console.log(myMockFn(), myMockFn(), myMockFn(), myMockFn())

mock.calls ​

This is an array containing all arguments for each call. One item of the array is the arguments of that call.

const fn = vi.fn()

fn('arg1', 'arg2')
fn('arg3', 'arg4')

fn.mock.calls === [
  ['arg1', 'arg2'], // first call
  ['arg3', 'arg4'], // second call
]

const fn = vi.fn()

fn('arg1', 'arg2')
fn('arg3', 'arg4')

fn.mock.calls === [
  ['arg1', 'arg2'], // first call
  ['arg3', 'arg4'], // second call
]

mock.lastCall ​

This contains the arguments of the last call. If spy wasn't called, will return undefined.

mock.results ​

This is an array containing all values, that were returned from the function. One item of the array is an object with properties type and value. Available types are:

• 'return' - function returned without throwing.
• 'throw' - function threw a value.

The value property contains the returned value or thrown error. If the function returned a promise, when it resolves the value property will become the value the promise resolved to.

const fn = vi.fn()

const result = fn() // returned 'result'

try {
  fn() // threw Error
}
catch {}

fn.mock.results === [
  // first result
  {
    type: 'return',
    value: 'result',
  },
  // last result
  {
    type: 'throw',
    value: Error,
  },
]

const fn = vi.fn()

const result = fn() // returned 'result'

try {
  fn() // threw Error
}
catch {}

fn.mock.results === [
  // first result
  {
    type: 'return',
    value: 'result',
  },
  // last result
  {
    type: 'throw',
    value: Error,
  },
]

mock.instances ​

This is an array containing all instances that were instantiated when mock was called with a new keyword. Note, this is an actual context (this) of the function, not a return value.

const MyClass = vi.fn()
const a = new MyClass()

MyClass.mock.instances[0] === a

const MyClass = vi.fn()
const a = new MyClass()

MyClass.mock.instances[0] === a

const Spy = vi.fn(() => ({ method: vi.fn() }))
const a = new Spy()

Spy.mock.instances[0] !== a
Spy.mock.results[0] === a

const Spy = vi.fn(() => ({ method: vi.fn() }))
const a = new Spy()

Spy.mock.instances[0] !== a
Spy.mock.results[0] === a