Mocking

When writing tests it's only a matter of time before you need to create a "fake" version of an internal — or external — service. This is commonly referred to as mocking. Vitest provides utility functions to help you out through its vi helper. You can import { vi } from 'vitest' or access it globally (when global configuration is enabled).

WARNING

Always remember to clear or restore mocks before or after each test run to undo mock state changes between runs! See mockReset docs for more info.

If you wanna dive in head first, check out the API section otherwise keep reading to take a deeper dive into the world of mocking.

Dates

Sometimes you need to be in control of the date to ensure consistency when testing. Vitest uses @sinonjs/fake-timers package for manipulating timers, as well as system date. You can find more about the specific API in detail here.

Example

import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'

const businessHours = [9, 17]

const purchase = () => {
  const currentHour = new Date().getHours()
  const [open, close] = businessHours

  if (currentHour > open && currentHour < close)
    return { message: 'Success' }

  return { message: 'Error' }
}

describe('purchasing flow', () => {
  beforeEach(() => {
    // tell vitest we use mocked time
    vi.useFakeTimers()
  })

  afterEach(() => {
    // restoring date after each test run
    vi.useRealTimers()
  })

  it('allows purchases within business hours', () => {
    // set hour within business hours
    const date = new Date(2000, 1, 1, 13)
    vi.setSystemTime(date)

    // access Date.now() will result in the date set above
    expect(purchase()).toEqual({ message: 'Success' })
  })

  it('disallows purchases outside of business hours', () => {
    // set hour outside business hours
    const date = new Date(2000, 1, 1, 19)
    vi.setSystemTime(date)

    // access Date.now() will result in the date set above
    expect(purchase()).toEqual({ message: 'Error' })
  })
})

import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'

const businessHours = [9, 17]

const purchase = () => {
  const currentHour = new Date().getHours()
  const [open, close] = businessHours

  if (currentHour > open && currentHour < close)
    return { message: 'Success' }

  return { message: 'Error' }
}

describe('purchasing flow', () => {
  beforeEach(() => {
    // tell vitest we use mocked time
    vi.useFakeTimers()
  })

  afterEach(() => {
    // restoring date after each test run
    vi.useRealTimers()
  })

  it('allows purchases within business hours', () => {
    // set hour within business hours
    const date = new Date(2000, 1, 1, 13)
    vi.setSystemTime(date)

    // access Date.now() will result in the date set above
    expect(purchase()).toEqual({ message: 'Success' })
  })

  it('disallows purchases outside of business hours', () => {
    // set hour outside business hours
    const date = new Date(2000, 1, 1, 19)
    vi.setSystemTime(date)

    // access Date.now() will result in the date set above
    expect(purchase()).toEqual({ message: 'Error' })
  })
})

Functions

Mocking functions can be split up into two different categories; spying & mocking.

Sometimes all you need is to validate whether or not a specific function has been called (and possibly which arguments were passed). In these cases a spy would be all we need which you can use directly with vi.spyOn() (read more here).

However spies can only help you spy on functions, they are not able to alter the implementation of those functions. In the case where we do need to create a fake (or mocked) version of a function we can use vi.fn() (read more here).

We use Tinyspy as a base for mocking functions, but we have our own wrapper to make it jest compatible. Both vi.fn() and vi.spyOn() share the same methods, however only the return result of vi.fn() is callable.

Example

import { afterEach, describe, expect, it, vi } from 'vitest'

const getLatest = (index = messages.items.length - 1) => messages.items[index]

const messages = {
  items: [
    { message: 'Simple test message', from: 'Testman' },
    // ...
  ],
  getLatest, // can also be a `getter or setter if supported`
}

describe('reading messages', () => {
  afterEach(() => {
    vi.restoreAllMocks()
  })

  it('should get the latest message with a spy', () => {
    const spy = vi.spyOn(messages, 'getLatest')
    expect(spy.getMockName()).toEqual('getLatest')

    expect(messages.getLatest()).toEqual(
      messages.items[messages.items.length - 1],
    )

    expect(spy).toHaveBeenCalledTimes(1)

    spy.mockImplementationOnce(() => 'access-restricted')
    expect(messages.getLatest()).toEqual('access-restricted')

    expect(spy).toHaveBeenCalledTimes(2)
  })

  it('should get with a mock', () => {
    const mock = vi.fn().mockImplementation(getLatest)

    expect(mock()).toEqual(messages.items[messages.items.length - 1])
    expect(mock).toHaveBeenCalledTimes(1)

    mock.mockImplementationOnce(() => 'access-restricted')
    expect(mock()).toEqual('access-restricted')

    expect(mock).toHaveBeenCalledTimes(2)

    expect(mock()).toEqual(messages.items[messages.items.length - 1])
    expect(mock).toHaveBeenCalledTimes(3)
  })
})

import { afterEach, describe, expect, it, vi } from 'vitest'

const getLatest = (index = messages.items.length - 1) => messages.items[index]

const messages = {
  items: [
    { message: 'Simple test message', from: 'Testman' },
    // ...
  ],
  getLatest, // can also be a `getter or setter if supported`
}

describe('reading messages', () => {
  afterEach(() => {
    vi.restoreAllMocks()
  })

  it('should get the latest message with a spy', () => {
    const spy = vi.spyOn(messages, 'getLatest')
    expect(spy.getMockName()).toEqual('getLatest')

    expect(messages.getLatest()).toEqual(
      messages.items[messages.items.length - 1],
    )

    expect(spy).toHaveBeenCalledTimes(1)

    spy.mockImplementationOnce(() => 'access-restricted')
    expect(messages.getLatest()).toEqual('access-restricted')

    expect(spy).toHaveBeenCalledTimes(2)
  })

  it('should get with a mock', () => {
    const mock = vi.fn().mockImplementation(getLatest)

    expect(mock()).toEqual(messages.items[messages.items.length - 1])
    expect(mock).toHaveBeenCalledTimes(1)

    mock.mockImplementationOnce(() => 'access-restricted')
    expect(mock()).toEqual('access-restricted')

    expect(mock).toHaveBeenCalledTimes(2)

    expect(mock()).toEqual(messages.items[messages.items.length - 1])
    expect(mock).toHaveBeenCalledTimes(3)
  })
})

Jest's Mock Functions

Globals

You can mock global variables that are not present with jsdom or node by using vi.stubGlobal helper. It will put the value of the global variable into a globalThis object.

import { vi } from 'vitest'

const IntersectionObserverMock = vi.fn(() => ({
  disconnect: vi.fn(),
  observe: vi.fn(),
  takeRecords: vi.fn(),
  unobserve: vi.fn(),
}))

vi.stubGlobal('IntersectionObserver', IntersectionObserverMock)

// now you can access it as `IntersectionObserver` or `window.IntersectionObserver`

import { vi } from 'vitest'

const IntersectionObserverMock = vi.fn(() => ({
  disconnect: vi.fn(),
  observe: vi.fn(),
  takeRecords: vi.fn(),
  unobserve: vi.fn(),
}))

vi.stubGlobal('IntersectionObserver', IntersectionObserverMock)

// now you can access it as `IntersectionObserver` or `window.IntersectionObserver`

Modules

Mock modules observe third-party-libraries, that are invoked in some other code, allowing you to test arguments, output or even redeclare its implementation.

See the vi.mock() api section for a more in-depth detailed API description.

Automocking algorithm

If your code is importing a mocked module, without any associated __mocks__ file or factory for this module, Vitest will mock the module itself by invoking it and mocking every export.

The following principles apply

All arrays will be emptied
All primitives and collections will stay the same
All objects will be deeply cloned
All instances of classes and their prototypes will be deeply cloned

Example

import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
import { Client } from 'pg'
import { failure, success } from './handlers'

// handlers
export function success(data) {}
export function failure(data) {}

// get todos
export const getTodos = async (event, context) => {
  const client = new Client({
    // ...clientOptions
  })

  await client.connect()

  try {
    const result = await client.query('SELECT * FROM todos;')

    client.end()

    return success({
      message: `${result.rowCount} item(s) returned`,
      data: result.rows,
      status: true,
    })
  }
  catch (e) {
    console.error(e.stack)

    client.end()

    return failure({ message: e, status: false })
  }
}

vi.mock('pg', () => {
  const Client = vi.fn()
  Client.prototype.connect = vi.fn()
  Client.prototype.query = vi.fn()
  Client.prototype.end = vi.fn()

  return { Client }
})

vi.mock('./handlers', () => {
  return {
    success: vi.fn(),
    failure: vi.fn(),
  }
})

describe('get a list of todo items', () => {
  let client

  beforeEach(() => {
    client = new Client()
  })

  afterEach(() => {
    vi.clearAllMocks()
  })

  it('should return items successfully', async () => {
    client.query.mockResolvedValueOnce({ rows: [], rowCount: 0 })

    await getTodos()

    expect(client.connect).toBeCalledTimes(1)
    expect(client.query).toBeCalledWith('SELECT * FROM todos;')
    expect(client.end).toBeCalledTimes(1)

    expect(success).toBeCalledWith({
      message: '0 item(s) returned',
      data: [],
      status: true,
    })
  })

  it('should throw an error', async () => {
    const mError = new Error('Unable to retrieve rows')
    client.query.mockRejectedValueOnce(mError)

    await getTodos()

    expect(client.connect).toBeCalledTimes(1)
    expect(client.query).toBeCalledWith('SELECT * FROM todos;')
    expect(client.end).toBeCalledTimes(1)
    expect(failure).toBeCalledWith({ message: mError, status: false })
  })
})

import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
import { Client } from 'pg'
import { failure, success } from './handlers'

// handlers
export function success(data) {}
export function failure(data) {}

// get todos
export const getTodos = async (event, context) => {
  const client = new Client({
    // ...clientOptions
  })

  await client.connect()

  try {
    const result = await client.query('SELECT * FROM todos;')

    client.end()

    return success({
      message: `${result.rowCount} item(s) returned`,
      data: result.rows,
      status: true,
    })
  }
  catch (e) {
    console.error(e.stack)

    client.end()

    return failure({ message: e, status: false })
  }
}

vi.mock('pg', () => {
  const Client = vi.fn()
  Client.prototype.connect = vi.fn()
  Client.prototype.query = vi.fn()
  Client.prototype.end = vi.fn()

  return { Client }
})

vi.mock('./handlers', () => {
  return {
    success: vi.fn(),
    failure: vi.fn(),
  }
})

describe('get a list of todo items', () => {
  let client

  beforeEach(() => {
    client = new Client()
  })

  afterEach(() => {
    vi.clearAllMocks()
  })

  it('should return items successfully', async () => {
    client.query.mockResolvedValueOnce({ rows: [], rowCount: 0 })

    await getTodos()

    expect(client.connect).toBeCalledTimes(1)
    expect(client.query).toBeCalledWith('SELECT * FROM todos;')
    expect(client.end).toBeCalledTimes(1)

    expect(success).toBeCalledWith({
      message: '0 item(s) returned',
      data: [],
      status: true,
    })
  })

  it('should throw an error', async () => {
    const mError = new Error('Unable to retrieve rows')
    client.query.mockRejectedValueOnce(mError)

    await getTodos()

    expect(client.connect).toBeCalledTimes(1)
    expect(client.query).toBeCalledWith('SELECT * FROM todos;')
    expect(client.end).toBeCalledTimes(1)
    expect(failure).toBeCalledWith({ message: mError, status: false })
  })
})

Requests

Because Vitest runs in Node, mocking network requests is tricky; web APIs are not available, so we need something that will mimic network behavior for us. We recommend Mock Service Worker to accomplish this. It will let you mock both REST and GraphQL network requests, and is framework agnostic.

Mock Service Worker (MSW) works by intercepting the requests your tests make, allowing you to use it without changing any of your application code. In-browser, this uses the Service Worker API. In Node.js, and for Vitest, it uses node-request-interceptor. To learn more about MSW, read their introduction

Configuration

You can use it like below in your setup file

import { afterAll, afterEach, beforeAll } from 'vitest'
import { setupServer } from 'msw/node'
import { graphql, rest } from 'msw'

const posts = [
  {
    userId: 1,
    id: 1,
    title: 'first post title',
    body: 'first post body',
  },
  // ...
]

export const restHandlers = [
  rest.get('https://rest-endpoint.example/path/to/posts', (req, res, ctx) => {
    return res(ctx.status(200), ctx.json(posts))
  }),
]

const graphqlHandlers = [
  graphql.query('https://graphql-endpoint.example/api/v1/posts', (req, res, ctx) => {
    return res(ctx.data(posts))
  }),
]

const server = setupServer(...restHandlers, ...graphqlHandlers)

// Start server before all tests
beforeAll(() => server.listen({ onUnhandledRequest: 'error' }))

//  Close server after all tests
afterAll(() => server.close())

// Reset handlers after each test `important for test isolation`
afterEach(() => server.resetHandlers())

import { afterAll, afterEach, beforeAll } from 'vitest'
import { setupServer } from 'msw/node'
import { graphql, rest } from 'msw'

const posts = [
  {
    userId: 1,
    id: 1,
    title: 'first post title',
    body: 'first post body',
  },
  // ...
]

export const restHandlers = [
  rest.get('https://rest-endpoint.example/path/to/posts', (req, res, ctx) => {
    return res(ctx.status(200), ctx.json(posts))
  }),
]

const graphqlHandlers = [
  graphql.query('https://graphql-endpoint.example/api/v1/posts', (req, res, ctx) => {
    return res(ctx.data(posts))
  }),
]

const server = setupServer(...restHandlers, ...graphqlHandlers)

// Start server before all tests
beforeAll(() => server.listen({ onUnhandledRequest: 'error' }))

//  Close server after all tests
afterAll(() => server.close())

// Reset handlers after each test `important for test isolation`
afterEach(() => server.resetHandlers())

Configuring the server with onUnhandleRequest: 'error' ensures that an error is thrown whenever there is a request that does not have a corresponding request handler.

Example

We have a full working example which uses MSW: React Testing with MSW.

There is much more to MSW. You can access cookies and query parameters, define mock error responses, and much more! To see all you can do with MSW, read their documentation.

Timers

Whenever we test code that involves timeouts or intervals, instead of having our tests wait it out or timeout. We can speed up our tests by using "fake" timers by mocking calls to setTimeout and setInterval, too.

See the vi.usefaketimers api section for a more in depth detailed API description.

Example

import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'

const executeAfterTwoHours = (func) => {
  setTimeout(func, 1000 * 60 * 60 * 2) // 2 hours
}

const executeEveryMinute = (func) => {
  setInterval(func, 1000 * 60) // 1 minute
}

const mock = vi.fn(() => console.log('executed'))

describe('delayed execution', () => {
  beforeEach(() => {
    vi.useFakeTimers()
  })
  afterEach(() => {
    vi.restoreAllMocks()
  })
  it('should execute the function', () => {
    executeAfterTwoHours(mock)
    vi.runAllTimers()
    expect(mock).toHaveBeenCalledTimes(1)
  })
  it('should not execute the function', () => {
    executeAfterTwoHours(mock)
    // advancing by 2ms won't trigger the func
    vi.advanceTimersByTime(2)
    expect(mock).not.toHaveBeenCalled()
  })
  it('should execute every minute', () => {
    executeEveryMinute(mock)
    vi.advanceTimersToNextTimer()
    expect(mock).toHaveBeenCalledTimes(1)
    vi.advanceTimersToNextTimer()
    expect(mock).toHaveBeenCalledTimes(2)
  })
})

import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'

const executeAfterTwoHours = (func) => {
  setTimeout(func, 1000 * 60 * 60 * 2) // 2 hours
}

const executeEveryMinute = (func) => {
  setInterval(func, 1000 * 60) // 1 minute
}

const mock = vi.fn(() => console.log('executed'))

describe('delayed execution', () => {
  beforeEach(() => {
    vi.useFakeTimers()
  })
  afterEach(() => {
    vi.restoreAllMocks()
  })
  it('should execute the function', () => {
    executeAfterTwoHours(mock)
    vi.runAllTimers()
    expect(mock).toHaveBeenCalledTimes(1)
  })
  it('should not execute the function', () => {
    executeAfterTwoHours(mock)
    // advancing by 2ms won't trigger the func
    vi.advanceTimersByTime(2)
    expect(mock).not.toHaveBeenCalled()
  })
  it('should execute every minute', () => {
    executeEveryMinute(mock)
    vi.advanceTimersToNextTimer()
    expect(mock).toHaveBeenCalledTimes(1)
    vi.advanceTimersToNextTimer()
    expect(mock).toHaveBeenCalledTimes(2)
  })
})

Cheat Sheet

INFO

vi in the examples below is imported directly from vitest. You can also use it globally, if you set globals to true in your config.

I want to…

Spy on a method

const instance = new SomeClass()
vi.spyOn(instance, 'method')

const instance = new SomeClass()
vi.spyOn(instance, 'method')

Mock exported variables

// some-path.ts
export const getter = 'variable'

// some-path.ts
export const getter = 'variable'

// some-path.test.ts
import * as exports from 'some-path'
vi.spyOn(exports, 'getter', 'get').mockReturnValue('mocked')

// some-path.test.ts
import * as exports from 'some-path'
vi.spyOn(exports, 'getter', 'get').mockReturnValue('mocked')

Mock exported function

Example with vi.mock:

// some-path.ts
export function method() {}

// some-path.ts
export function method() {}

import { method } from 'some-path'
vi.mock('some-path', () => ({
  method: vi.fn()
}))

import { method } from 'some-path'
vi.mock('some-path', () => ({
  method: vi.fn()
}))

Example with vi.spyOn:

import * as exports from 'some-path'
vi.spyOn(exports, 'method').mockImplementation(() => {})

import * as exports from 'some-path'
vi.spyOn(exports, 'method').mockImplementation(() => {})

Mock exported class implementation

Example with vi.mock and prototype:

// some-path.ts
export class SomeClass {}

// some-path.ts
export class SomeClass {}

import { SomeClass } from 'some-path'
vi.mock('some-path', () => {
  const SomeClass = vi.fn()
  SomeClass.prototype.someMethod = vi.fn()
  return { SomeClass }
})
// SomeClass.mock.instances will have SomeClass

import { SomeClass } from 'some-path'
vi.mock('some-path', () => {
  const SomeClass = vi.fn()
  SomeClass.prototype.someMethod = vi.fn()
  return { SomeClass }
})
// SomeClass.mock.instances will have SomeClass

Example with vi.mock and return value:

import { SomeClass } from 'some-path'
vi.mock('some-path', () => {
  const SomeClass = vi.fn(() => ({
    someMethod: vi.fn()
  }))
  return { SomeClass }
})
// SomeClass.mock.returns will have returned object

import { SomeClass } from 'some-path'
vi.mock('some-path', () => {
  const SomeClass = vi.fn(() => ({
    someMethod: vi.fn()
  }))
  return { SomeClass }
})
// SomeClass.mock.returns will have returned object

Example with vi.spyOn:

import * as exports from 'some-path'
vi.spyOn(exports, 'SomeClass').mockImplementation(() => {
  // whatever suites you from first two examples
})

import * as exports from 'some-path'
vi.spyOn(exports, 'SomeClass').mockImplementation(() => {
  // whatever suites you from first two examples
})

Spy on an object returned from a function

Example using cache:

// some-path.ts
export function useObject() {
  return { method: () => true }
}

// some-path.ts
export function useObject() {
  return { method: () => true }
}

// useObject.js
import { useObject } from 'some-path'
const obj = useObject()
obj.method()

// useObject.js
import { useObject } from 'some-path'
const obj = useObject()
obj.method()

// useObject.test.js
import { useObject } from 'some-path'
vi.mock('some-path', () => {
  let _cache
  const useObject = () => {
    if (!_cache) {
      _cache = {
        method: vi.fn(),
      }
    }
    // now everytime useObject() is called it will
    // return the same object reference
    return _cache
  }
  return { useObject }
})

const obj = useObject()
// obj.method was called inside some-path
expect(obj.method).toHaveBeenCalled()

// useObject.test.js
import { useObject } from 'some-path'
vi.mock('some-path', () => {
  let _cache
  const useObject = () => {
    if (!_cache) {
      _cache = {
        method: vi.fn(),
      }
    }
    // now everytime useObject() is called it will
    // return the same object reference
    return _cache
  }
  return { useObject }
})

const obj = useObject()
// obj.method was called inside some-path
expect(obj.method).toHaveBeenCalled()

Mock part of a module

import { mocked, original } from 'some-path'
vi.mock('some-path', async () => {
  const mod = await vi.importActual<typeof import('some-path')>('some-path')
  return {
    ...mod,
    mocked: vi.fn()
  }
})
original() // has original behaviour
mocked() // is a spy function

import { mocked, original } from 'some-path'
vi.mock('some-path', async () => {
  const mod = await vi.importActual<typeof import('some-path')>('some-path')
  return {
    ...mod,
    mocked: vi.fn()
  }
})
original() // has original behaviour
mocked() // is a spy function

Mock current date

const mockDate = new Date(2022, 0, 1)
vi.setSystemTime(mockDate)
const now = new Date()
expect(now.valueOf()).toBe(mockDate.valueOf())

const mockDate = new Date(2022, 0, 1)
vi.setSystemTime(mockDate)
const now = new Date()
expect(now.valueOf()).toBe(mockDate.valueOf())

Mock global variable

vi.stubGlobal('__VERSION__', '1.0.0')
expect(__VERSION__).toBe('1.0.0')

vi.stubGlobal('__VERSION__', '1.0.0')
expect(__VERSION__).toBe('1.0.0')

Mocking

Dates

Example

Functions

Example

More

Globals

Modules

Automocking algorithm

Example

Requests

Configuration

Example

More

Timers

Example

Cheat Sheet

Mocking #

Dates #

Example #

Functions #

Example #

More #

Globals #

Modules #

Automocking algorithm #

Example #

Requests #

Configuration #

Example #

More #

Timers #

Example #

Cheat Sheet #

Mocking

Dates

Example

Functions

Example

More

Globals

Modules

Automocking algorithm

Example

Requests

Configuration

Example

More

Timers

Example

Cheat Sheet