Vitest

API Reference

The following types are used in the type signatures below

type Awaitable<T> = T | PromiseLike<T>
type TestFunction = () => Awaitable<void>

When a test function returns a promise, the runner will wait until it is resolved to collect async expectations. If the promise is rejected, the test will fail.

TIP

In Jest, TestFunction can also be of type (done: DoneCallback) => void. If this form is used, the test will not be concluded until done is called. You can achieve the same using an async function, see the Migration guide Done Callback section.

test

  • Type: (name: string, fn: TestFunction, timeout?: number) => void

  • Alias: it

    test defines a set of related expectations. It receives the test name and a function that holds the expectations to test.

    Optionally, you can provide a timeout (in milliseconds) for specifying how long to wait before terminating. The default is 5 seconds, and can be configured globally with testTimeout

    import { expect, test } from 'vitest'

test('should work as expected', () => {
  expect(Math.sqrt(4)).toBe(2)
})

test.skip

  • Type: (name: string, fn: TestFunction, timeout?: number) => void

  • Alias: it.skip

    If you want to skip running certain tests, but you don't want to delete the code due to any reason, you can use test.skip to avoid running them.

    import { assert, test } from 'vitest'

test.skip('skipped test', () => {
  // Test skipped, no error
  assert.equal(Math.sqrt(4), 3)
})

test.skipIf

  • Type: (condition: any) => Test

  • Alias: it.skipIf

    In some cases you might run tests multiple times with different environments, and some of the tests might be environment-specific. Instead of wrapping the test code with if, you can use test.skipIf to skip the test whenever the condition is truthy.

    import { assert, test } from 'vitest'

const isDev = process.env.NODE_ENV === 'development'

test.skipIf(isDev)('prod only test', () => {
  // this test only runs in production
})

test.runIf

  • Type: (condition: any) => Test

  • Alias: it.runIf

    Oppsit of test.skipIf.

    import { assert, test } from 'vitest'

const isDev = process.env.NODE_ENV === 'development'

test.runIf(isDev)('dev only test', () => {
  // this test only runs in development
})

test.only

  • Type: (name: string, fn: TestFunction, timeout?: number) => void

  • Alias: it.only

    Use test.only to only run certain tests in a given suite. This is useful when debugging.

    Optionally, you can provide a timeout (in milliseconds) for specifying how long to wait before terminating. The default is 5 seconds, and can be configured globally with testTimeout.

    import { assert, test } from 'vitest'

test.only('test', () => {
  // Only this test (and others marked with only) are run
  assert.equal(Math.sqrt(4), 2)
})

test.concurrent

  • Type: (name: string, fn: TestFunction, timeout?: number) => void

  • Alias: it.concurrent

    test.concurrent marks consecutive tests to be run them in parallel. It receives the test name, an async function with the tests to collect, and an optional timeout (in milliseconds).

    import { describe, test } from 'vitest'

// The two tests marked with concurrent will be run in parallel
describe('suite', () => {
  test('serial test', async () => { /* ... */ })
  test.concurrent('concurrent test 1', async () => { /* ... */ })
  test.concurrent('concurrent test 2', async () => { /* ... */ })
})

    test.skip, test.only, and test.todo works with concurrent tests. All the following combinations are valid:

    test.concurrent(/* ... */)
test.skip.concurrent(/* ... */) // or test.concurrent.skip(/* ... */)
test.only.concurrent(/* ... */) // or test.concurrent.only(/* ... */)
test.todo.concurrent(/* ... */) // or test.concurrent.todo(/* ... */)

    When using Snapshots with async concurrent tests, due to the limitation of JavaScript, you need to use the expect from the Test Context to ensure the right test is being detected.

    test.concurrent('test 1', async ({ expect }) => {
  expect(foo).toMatchSnapshot()
})
test.concurrent('test 2', async ({ expect }) => {
  expect(foo).toMatchSnapshot()
})

test.todo

  • Type: (name: string) => void

  • Alias: it.todo

    Use test.todo to stub tests to be implemented later. An entry will be shown in the report for the tests so you know how many tests you still need to implement.

    // An entry will be shown in the report for this test
test.todo('unimplemented test')

test.fails

  • Type: (name: string, fn: TestFunction, timeout?: number) => void

  • Alias: it.fails

    Use test.fails to indicate that an assertion will fail explicitly.

    import { expect, test } from 'vitest'
const myAsyncFunc = () => new Promise(resolve => resolve(1))
test.fails('fail test', () => {
  expect(myAsyncFunc()).rejects.toBe(1)
})

test.each

  • Type: (cases: ReadonlyArray<T>) => void

  • Alias: it.each

    Use test.each when you need to run the same test with different variables. You can inject parameters with printf formmatting in the test name in the order of the test function parameters.

    • %s: string
    • %d: number
    • %i: integer
    • %f: floating point value
    • %j: json
    • %o: object
    • %#: index of the test case
    • %%: single percent sign ('%')
    test.each([
  [1, 1, 2],
  [1, 2, 3],
  [2, 1, 3],
])('add(%i, %i) -> %i', (a, b, expected) => {
  expect(a + b).toBe(expected)
})

// this will return
// √ add(1, 1) -> 2
// √ add(1, 2) -> 3
// √ add(2, 1) -> 3

describe

When you use test in the top level of file, they are collected as part of the implicit suite for it. Using describe you can define a new suite in the current context, as a set of related tests and other nested suites. A suite lets you organize your tests so reports are more clear.

import { describe, expect, test } from 'vitest'

const person = {
  isActive: true,
  age: 32,
}

describe('person', () => {
  test('person is defined', () => {
    expect(person).toBeDefined()
  })

  test('is active', () => {
    expect(person.isActive).toBeTruthy()
  })

  test('age limit', () => {
    expect(person.age).toBeLessThanOrEqual(32)
  })
})

You can also nest describe blocks if you have a hierarchy of tests:

import { describe, expect, test } from 'vitest'

const numberToCurrency = (value) => {
  if (typeof value !== 'number')
    throw new Error('Value must be a number')

  return value.toFixed(2).toString().replace(/\B(?=(\d{3})+(?!\d))/g, ',')
}

describe('numberToCurrency', () => {
  describe('given an invalid number', () => {
    test('composed of non-numbers to throw error', () => {
      expect(() => numberToCurrency('abc')).toThrow()
    })
  })

  describe('given a valid number', () => {
    test('returns the correct currency format', () => {
      expect(numberToCurrency(10000)).toBe('10,000.00')
    })
  })
})

describe.skip

  • Type: (name: string, fn: TestFunction) => void

    Use describe.skip in a suite to avoid running a particular describe block.

    import { assert, describe, test } from 'vitest'

describe.skip('skipped suite', () => {
  test('sqrt', () => {
    // Suite skipped, no error
    assert.equal(Math.sqrt(4), 3)
  })
})

describe.only

  • Type: (name: string, fn: TestFunction) => void

    Use describe.only to only run certain suites

    // Only this suite (and others marked with only) are run
describe.only('suite', () => {
  test('sqrt', () => {
    assert.equal(Math.sqrt(4), 3)
  })
})

describe('other suite', () => {
  // ... will be skipped
})

describe.concurrent

  • Type: (name: string, fn: TestFunction, timeout?: number) => void

    describe.concurrent in a suite marks every tests as concurrent

    // All tests within this suite will be run in parallel
describe.concurrent('suite', () => {
  test('concurrent test 1', async () => { /* ... */ })
  test('concurrent test 2', async () => { /* ... */ })
  test.concurrent('concurrent test 3', async () => { /* ... */ })
})

    .skip, .only, and .todo works with concurrent suites. All the following combinations are valid:

    describe.concurrent(/* ... */)
describe.skip.concurrent(/* ... */) // or describe.concurrent.skip(/* ... */)
describe.only.concurrent(/* ... */) // or describe.concurrent.only(/* ... */)
describe.todo.concurrent(/* ... */) // or describe.concurrent.todo(/* ... */)

describe.todo

  • Type: (name: string) => void

    Use describe.todo to stub suites to be implemented later. An entry will be shown in the report for the tests so you know how many tests you still need to implement.

    // An entry will be shown in the report for this suite
describe.todo('unimplemented suite')

describe.each

  • Type: (cases: ReadonlyArray<T>): (name: string, fn: (...args: T[]) => void) => void

    Use describe.each if you have more than one test that depends on the same data.

    describe.each([
  { a: 1, b: 1, expected: 2 },
  { a: 1, b: 2, expected: 3 },
  { a: 2, b: 1, expected: 3 },
])('describe object add(%i, %i)', ({ a, b, expected }) => {
  test(`returns ${expected}`, () => {
    expect(a + b).toBe(expected)
  })

  test(`returned value not be greater than ${expected}`, () => {
    expect(a + b).not.toBeGreaterThan(expected)
  })

  test(`returned value not be less than ${expected}`, () => {
    expect(a + b).not.toBeLessThan(expected)
  })
})

expect

  • Type: ExpectStatic & (actual: any) => Assertions

    expect is used to create assertions. In this context assertions are functions that can be called to assert a statement. Vitest provides chai assertions by default and also Jest compatible assertions build on top of chai.

    For example, this code asserts that an input value is equal to 2. If it's not, assertion will throw an error, and the test will fail.

    import { expect } from 'vitest'

const input = Math.sqrt(4)

expect(input).to.equal(2) // chai API
expect(input).toBe(2) // jest API

    Technically this example doesn't use test function, so in the console you will see Nodejs error instead of Vitest output. To learn more about test, please read next chapter.

    Also, expect can be used statically to access matchers functions, described later, and more.

not

Using not will negate the assertion. For example, this code asserts that an input value is not equal to 2. If it's equal, assertion will throw an error, and the test will fail.

import { expect, test } from 'vitest'

const input = Math.sqrt(16)

expect(input).not.to.equal(2) // chai API
expect(input).not.toBe(2) // jest API

toBe

  • Type: (value: any) => Awaitable<void>

    toBe can be used to assert if primitives are equal or that objects share the same reference. It is equivalent of calling expect(Object.is(3, 3)).toBe(true). If the objects are not the same, but you want check if their structures are identical, you can use toEqual.

    For example, the code below checks if the trader has 13 apples.

    import { expect, test } from 'vitest'

const stock = {
  type: 'apples',
  count: 13,
}

test('stock has 13 apples', () => {
  expect(stock.type).toBe('apples')
  expect(stock.count).toBe(13)
})

test('stocks are the same', () => {
  const refStock = stock // same reference

  expect(stock).toBe(refStock)
})

    Try not to use toBe with floating-point numbers. Since JavaScript rounds them, 0.1 + 0.2 is not strictly 0.3. To reliably assert floating-point numbers, use toBeCloseTo assertion.

toBeCloseTo

  • Type: (value: number, numDigits?: number) => Awaitable<void>

    Use toBeCloseTo to compare floating-point numbers. The optional numDigits argument limits the number of digits to check after the decimal point. For example:

    import { expect, test } from 'vitest'

test.fails('decimals are not equal in javascript', () => {
  expect(0.2 + 0.1).toBe(0.3) // 0.2 + 0.1 is 0.30000000000000004
})

test('decimals are rounded to 5 after the point', () => {
  // 0.2 + 0.1 is 0.30000 | "000000000004" removed
  expect(0.2 + 0.1).toBeCloseTo(0.3, 5)
  // nothing from 0.30000000000000004 is removed
  expect(0.2 + 0.1).not.toBeCloseTo(0.3, 50)
})

toBeDefined

  • Type: () => Awaitable<void>

    toBeDefined asserts that the value is not equal to undefined. Useful use case would be to check if function returned anything.

    import { expect, test } from 'vitest'

const getApples = () => 3

test('function returned something', () => {
  expect(getApples()).toBeDefined()
})

toBeUndefined

  • Type: () => Awaitable<void>

    Opposite of toBeDefined, toBeUndefined asserts that the value is equal to undefined. Useful use case would be to check if function hasn't returned anything.

    import { expect, test } from 'vitest'

function getApplesFromStock(stock) {
  if (stock === 'Bill')
    return 13
}

test('mary doesn\'t have a stock', () => {
  expect(getApplesFromStock('Mary')).toBeUndefined()
})

toBeTruthy

  • Type: () => Awaitable<void>

    toBeTruthy asserts that the value is true, when converted to boolean. Useful if you don't care for the value, but just want to know it can be converted to true.

    For example having this code you don't care for the return value of stocks.getInfo - it maybe complex object, a string or anything else. The code will still work.

    import { Stocks } from './stocks'
const stocks = new Stocks()
stocks.sync('Bill')
if (stocks.getInfo('Bill'))
  stocks.sell('apples', 'Bill')

    So if you want to test that stocks.getInfo will be truthy, you could write:

    import { expect, test } from 'vitest'
import { Stocks } from './stocks'
const stocks = new Stocks()

test('if we know Bill stock, sell apples to him', () => {
  stocks.sync('Bill')
  expect(stocks.getInfo('Bill')).toBeTruthy()
})

    Everything in JavaScript is truthy, except false, 0, '', null, undefined, and NaN.

toBeFalsy

  • Type: () => Awaitable<void>

    toBeFalsy asserts that the value is false, when converted to boolean. Useful if you don't care for the value, but just want to know it can be converted to false.

    For example having this code you don't care for the return value of stocks.stockFailed - it may return any falsy value, but the code will still work.

    import { Stocks } from './stocks'
const stocks = new Stocks()
stocks.sync('Bill')
if (!stocks.stockFailed('Bill'))
  stocks.sell('apples', 'Bill')

    So if you want to test that stocks.stockFailed will be falsy, you could write:

    import { expect, test } from 'vitest'
import { Stocks } from './stocks'
const stocks = new Stocks()

test('if Bill stock hasnt failed, sell apples to him', () => {
  stocks.syncStocks('Bill')
  expect(stocks.stockFailed('Bill')).toBeFalsy()
})

    Everything in JavaScript is truthy, except false, 0, '', null, undefined, and NaN.

toBeNull

  • Type: () => Awaitable<void>

    toBeNull simply asserts if something is null. Alias for .toBe(null).

    import { expect, test } from 'vitest'

function apples() {
  return null
}

test('we dont have apples', () => {
  expect(apples()).toBeNull()
})

toBeNaN

  • Type: () => Awaitable<void>

    toBeNaN simply asserts if something is NaN. Alias for .toBe(NaN).

    import { expect, test } from 'vitest'

let i = 0

function getApplesCount() {
  i++
  return i > 1 ? NaN : i
}

test('getApplesCount has some unusual side effects...', () => {
  expect(getApplesCount()).not.toBeNaN()
  expect(getApplesCount()).toBeNaN()
})

toBeTypeOf

  • Type: (c: 'bigint' | 'boolean' | 'function' | 'number' | 'object' | 'string' | 'symbol' | 'undefined') => Awaitable<void>

    toBeTypeOf asserts if an actual value is of type of received type.

    import { expect, test } from 'vitest'
const actual = 'stock'

test('stock is type of string', () => {
  expect(actual).toBeTypeOf('string')
})

toBeInstanceOf

  • Type: (c: any) => Awaitable<void>

    toBeInstanceOf asserts if an actual value is instance of received class.

    import { expect, test } from 'vitest'
import { Stocks } from './stocks'
const stocks = new Stocks()

test('stocks are instance of Stocks', () => {
  expect(stocks).toBeInstanceOf(Stocks)
})

toBeGreaterThan

  • Type: (n: number | bigint) => Awaitable<void>

    toBeGreaterThan asserts if actual value is greater than received one. Equal values will fail the test.

    import { expect, test } from 'vitest'
import { getApples } from './stock'

test('have more then 10 apples', () => {
  expect(getApples()).toBeGreaterThan(10)
})

toBeGreaterThanOrEqual

  • Type: (n: number | bigint) => Awaitable<void>

    toBeGreaterThanOrEqual asserts if actual value is greater than received one or equal to it.

    import { expect, test } from 'vitest'
import { getApples } from './stock'

test('have 11 apples or more', () => {
  expect(getApples()).toBeGreaterThanOrEqual(11)
})

toBeLessThan

  • Type: (n: number | bigint) => Awaitable<void>

    toBeLessThan asserts if actual value is less than received one. Equal values will fail the test.

    import { expect, test } from 'vitest'
import { getApples } from './stock'

test('have less then 20 apples', () => {
  expect(getApples()).toBeLessThan(20)
})

toBeLessThanOrEqual

  • Type: (n: number | bigint) => Awaitable<void>

    toBeLessThanOrEqual asserts if actual value is less than received one or equal to it.

    import { expect, test } from 'vitest'
import { getApples } from './stock'

test('have 11 apples or less', () => {
  expect(getApples()).toBeLessThanOrEqual(11)
})

toEqual

  • Type: (received: any) => Awaitable<void>

    toEqual asserts if actual value is equal to received one or has the same structure, if it is an object (compares them recursively). You can see the difference between toEqual and toBe in this example:

    import { expect, test } from 'vitest'

const stockBill = {
  type: 'apples',
  count: 13,
}

const stockMary = {
  type: 'apples',
  count: 13,
}

test('stocks have the same properties', () => {
  expect(stockBill).toEqual(stockMary)
})

test('stocks are not the same', () => {
  expect(stockBill).not.toBe(stockMary)
})

    WARNING

    A deep equality will not be performed for Error objects. To test if something was thrown, use toThrow assertion.

toStrictEqual

  • Type: (received: any) => Awaitable<void>

    toStrictEqual asserts if actual value is equal to received one or has the same structure, if it is an object (compares them recursively), and of the same type.

    Differences from .toEqual:

    • Keys with undefined properties are checked. e.g. {a: undefined, b: 2} does not match {b: 2} when using .toStrictEqual.
    • Array sparseness is checked. e.g. [, 1] does not match [undefined, 1] when using .toStrictEqual.
    • Object types are checked to be equal. e.g. A class instance with fields a and b will not equal a literal object with fields a and b.
    import { expect, test } from 'vitest'

class Stock {
  constructor(type) {
    this.type = type
  }
}

test('structurally the same, but semantically different', () => {
  expect(new Stock('apples')).toEqual({ type: 'apples' })
  expect(new Stock('apples')).not.toStrictEqual({ type: 'apples' })
})

toContain

  • Type: (received: string) => Awaitable<void>

    toContain asserts if actual value is in an array. toContain can also check whether a string is a substring of another string.

    import { expect, test } from 'vitest'
import { getAllFruits } from './stock'

test('the fruit list contains orange', () => {
  expect(getAllFruits()).toContain('orange')
})

toContainEqual

  • Type: (received: any) => Awaitable<void>

    toContainEqual asserts if an item with a specific structure and values is contained in an array. It works like toEqual inside for each element.

    import { expect, test } from 'vitest'
import { getFruitStock } from './stock'

test('apple available', () => {
  expect(getFruitStock()).toContainEqual({ fruit: 'apple', count: 5 })
})

toHaveLength

  • Type: (received: number) => Awaitable<void>

    toHaveLength asserts if an object has a .length property and it is set to a certain numeric value.

    import { expect, test } from 'vitest'

test('toHaveLength', () => {
  expect('abc').toHaveLength(3)
  expect([1, 2, 3]).toHaveLength(3)

  expect('').not.toHaveLength(3) // doesn't have .length of 3
  expect({ length: 3 }).toHaveLength(3)
})

toHaveProperty

  • Type: (key: any, received?: any) => Awaitable<void>

    toHaveProperty asserts if a property at provided reference key exists for an object.

    You can provide an optional value argument also known as deep equality, like the toEqual matcher to compare the received property value.

    import { expect, test } from 'vitest'

const invoice = {
  'isActive': true,
  'P.O': '12345',
  'customer': {
    first_name: 'John',
    last_name: 'Doe',
    location: 'China',
  },
  'total_amount': 5000,
  'items': [
    {
      type: 'apples',
      quantity: 10,
    },
    {
      type: 'oranges',
      quantity: 5,
    },
  ],
}

test('John Doe Invoice', () => {
  expect(invoice).toHaveProperty('isActive') // assert that the key exists
  expect(invoice).toHaveProperty('total_amount', 5000) // assert that the key exists and the value is equal

  expect(invoice).not.toHaveProperty('account') // assert that this key does not exist

  // Deep referencing using dot notation
  expect(invoice).toHaveProperty('customer.first_name')
  expect(invoice).toHaveProperty('customer.last_name', 'Doe')
  expect(invoice).not.toHaveProperty('customer.location', 'India')

  // Deep referencing using an array containing the key
  expect(invoice).toHaveProperty('items[0].type', 'apples')
  expect(invoice).toHaveProperty('items.0.type', 'apples') // dot notation also works

  // Wrap your key in an array to avoid the key from being parsed as a deep reference
  expect(invoice).toHaveProperty(['P.O'], '12345')
})

toMatch

  • Type: (received: string | regexp) => Awaitable<void>

    toMatch asserts if a string matches a regular expression or a string.

    import { expect, test } from 'vitest'

test('top fruits', () => {
  expect('top fruits include apple, orange and grape').toMatch(/apple/)
  expect('applefruits').toMatch('fruit') // toMatch also accepts a string
})

toMatchObject

  • Type: (received: object | array) => Awaitable<void>

    toMatchObject asserts if an object matches a subset of the properties of an object.

    You can also pass an array of objects. This is useful if you want to check that two arrays match in their number of elements, as opposed to arrayContaining, which allows for extra elements in the received array.

    import { expect, test } from 'vitest'

const johnInvoice = {
  isActive: true,
  customer: {
    first_name: 'John',
    last_name: 'Doe',
    location: 'China',
  },
  total_amount: 5000,
  items: [
    {
      type: 'apples',
      quantity: 10,
    },
    {
      type: 'oranges',
      quantity: 5,
    },
  ],
}

const johnDetails = {
  customer: {
    first_name: 'John',
    last_name: 'Doe',
    location: 'China',
  },
}

test('invoice has john personal details', () => {
  expect(johnInvoice).toMatchObject(johnDetails)
})

test('the number of elements must match exactly', () => {
  // Assert that an array of object matches
  expect([{ foo: 'bar' }, { baz: 1 }]).toMatchObject([
    { foo: 'bar' },
    { baz: 1 },
  ])
})

toThrowError

  • Type: (received: any) => Awaitable<void>

    toThrowError asserts if a function throws an error when it is called.

    For example, if we want to test that getFruitStock('pineapples') throws, we could write:

    You can provide an optional argument to test that a specific error is thrown:

    • regular expression: error message matches the pattern
    • string: error message includes the substring

    TIP

    You must wrap the code in a function, otherwise the error will not be caught, and the assertion will fail.

    import { expect, test } from 'vitest'

function getFruitStock(type) {
  if (type === 'pineapples')
    throw new DiabetesError('Pineapples is not good for people with diabetes')

  // Do some other stuff
}

test('throws on pineapples', () => {
  // Test that the error message says "diabetes" somewhere: these are equivalent
  expect(() => getFruitStock('pineapples')).toThrowError(/diabetes/)
  expect(() => getFruitStock('pineapples')).toThrowError('diabetes')

  // Test the exact error message
  expect(() => getFruitStock('pineapples')).toThrowError(
    /^Pineapples is not good for people with diabetes$/,
  )
})

toMatchSnapshot

  • Type: (hint?: string) => void

    This ensures that a value matches the most recent snapshot.

    You can provide an optional hint string argument that is appended to the test name. Although Vitest always appends a number at the end of a snapshot name, short descriptive hints might be more useful than numbers to differentiate multiple snapshots in a single it or test block. Vitest sorts snapshots by name in the corresponding .snap file.

    TIP

    When snapshot mismatch and causing the test failing, if the mismatch is expected, you can press u key to update the snapshot for once. Or you can pass -u or --update CLI options to make Vitest always update the tests.

    import { expect, test } from 'vitest'

test('matches snapshot', () => {
  const data = { foo: new Set(['bar', 'snapshot']) }
  expect(data).toMatchSnapshot()
})

toMatchInlineSnapshot

  • Type: (snapshot?: string) => void

    This ensures that a value matches the most recent snapshot.

    Vitest adds and updates the inlineSnapshot string argument to the matcher in the test file (instead of an external .snap file).

    import { expect, test } from 'vitest'

test('matches inline snapshot', () => {
  const data = { foo: new Set(['bar', 'snapshot']) }
  // Vitest will update following content when updating the snapshot
  expect(data).toMatchInlineSnapshot(`
    {
      "foo": Set {
        "bar",
        "snapshot",
      },
    }
  `)
})

toThrowErrorMatchingSnapshot

  • Type: (snapshot?: string) => void

    The same as [toMatchSnapshot](#toMatchSnapshot), but expects the same value as [toThrowError](#toThrowError).

    If the function throws an Error, the snapshot will be the error message. Otherwise, snapshot will be the value thrown by the function.

toThrowErrorMatchingInlineSnapshot

  • Type: (snapshot?: string) => void

    The same as [toMatchInlineSnapshot](#toMatchInlineSnapshot), but expects the same value as [toThrowError](#toThrowError).

    If the function throws an Error, the snapshot will be the error message. Otherwise, snapshot will be the value thrown by the function.

toHaveBeenCalled

  • Type: () => Awaitable<void>

    This assertion is useful for testing that a function has been called. Requires a spy function to be passed to expect.

    import { expect, test, vi } from 'vitest'

const market = {
  buy(subject: string, amount: number) {
    // ...
  },
}

test('spy function', () => {
  const buySpy = vi.spyOn(market, 'buy')

  expect(buySpy).not.toHaveBeenCalled()

  market.buy('apples', 10)

  expect(buySpy).toHaveBeenCalled()
})

toHaveBeenCalledTimes

  • Type: (amount: number) => Awaitable<void>

This assertion checks if a function was called a certain amount of times. Requires a spy function to be passed to expect.

import { expect, test, vi } from 'vitest'

const market = {
  buy(subject: string, amount: number) {
    // ...
  },
}

test('spy function called two times', () => {
  const buySpy = vi.spyOn(market, 'buy')

  market.buy('apples', 10)
  market.buy('apples', 20)

  expect(buySpy).toHaveBeenCalledTimes(2)
})

toHaveBeenCalledWith

  • Type: (...args: any[]) => Awaitable<void>

This assertion checks if a function was called at least once with certain parameters. Requires a spy function to be passed to expect.

import { expect, test, vi } from 'vitest'

const market = {
  buy(subject: string, amount: number) {
    // ...
  },
}

test('spy function', () => {
  const buySpy = vi.spyOn(market, 'buy')

  market.buy('apples', 10)
  market.buy('apples', 20)

  expect(buySpy).toHaveBeenCalledWith('apples', 10)
  expect(buySpy).toHaveBeenCalledWith('apples', 20)
})

toHaveBeenLastCalledWith

  • Type: (...args: any[]) => Awaitable<void>

This assertion checks if a function was called with certain parameters at it's last invocation. Requires a spy function to be passed to expect.

import { expect, test, vi } from 'vitest'

const market = {
  buy(subject: string, amount: number) {
    // ...
  },
}

test('spy function', () => {
  const buySpy = vi.spyOn(market, 'buy')

  market.buy('apples', 10)
  market.buy('apples', 20)

  expect(buySpy).not.toHaveBeenLastCalledWith('apples', 10)
  expect(buySpy).toHaveBeenLastCalledWith('apples', 20)
})

toHaveBeenNthCalledWith

  • Type: (time: number, ...args: any[]) => Awaitable<void>

This assertion checks if a function was called with certain parameters at the certain time. The count starts at 1. So, to check the second entry, you would write .toHaveBeenNthCalledWith(2, ...).

Requires a spy function to be passed to expect.

import { expect, test, vi } from 'vitest'

const market = {
  buy(subject: string, amount: number) {
    // ...
  },
}

test('first call of spy function called with right params', () => {
  const buySpy = vi.spyOn(market, 'buy')

  market.buy('apples', 10)
  market.buy('apples', 20)

  expect(buySpy).toHaveBeenNthCalledWith(1, 'apples', 10)
})

toHaveReturned

  • Type: () => Awaitable<void>

This assertion checks if a function has successfully returned a value at least once (i.e., did not throw an error). Requires a spy function to be passed to expect.

import { expect, test, vi } from 'vitest'

const getApplesPrice = (amount: number) => {
  const PRICE = 10
  return amount * PRICE
}

test('spy function returned a value', () => {
  const getPriceSpy = vi.fn(getApplesPrice)

  const price = getPriceSpy(10)

  expect(price).toBe(100)
  expect(getPriceSpy).toHaveReturned()
})

toHaveReturnedTimes

  • Type: (amount: number) => Awaitable<void>

This assertion checks if a function has successfully returned a value exact amount of times (i.e., did not throw an error). Requires a spy function to be passed to expect.

import { expect, test, vi } from 'vitest'

test('spy function returns a value two times', () => {
  const sell = vi.fn((product: string) => ({ product }))

  sell('apples')
  sell('bananas')

  expect(sell).toHaveReturnedTimes(2)
})

toHaveReturnedWith

  • Type: (returnValue: any) => Awaitable<void>

You can call this assertion to check if a function has successfully returned a value with certain parameters at least once. Requires a spy function to be passed to expect.

import { expect, test, vi } from 'vitest'

test('spy function returns a product', () => {
  const sell = vi.fn((product: string) => ({ product }))

  sell('apples')

  expect(sell).toHaveReturnedWith({ procuct: 'apples' })
})

toHaveLastReturnedWith

  • Type: (returnValue: any) => Awaitable<void>

You can call this assertion to check if a function has successfully returned a value with certain parameters on it's last invoking. Requires a spy function to be passed to expect.

import { expect, test, vi } from 'vitest'

test('spy function returns bananas on a last call', () => {
  const sell = vi.fn((product: string) => ({ product }))

  sell('apples')
  sell('bananas')

  expect(sell).toHaveLastReturnedWith({ procuct: 'bananas' })
})

toHaveNthReturnedWith

  • Type: (time: number, returnValue: any) => Awaitable<void>

You can call this assertion to check if a function has successfully returned a value with certain parameters on a certain call. Requires a spy function to be passed to expect.

import { expect, test, vi } from 'vitest'

test('spy function returns bananas on second call', () => {
  const sell = vi.fn((product: string) => ({ product }))

  sell('apples')
  sell('bananas')

  expect(sell).toHaveNthReturnedWith(2, { product: 'bananas' })
})

toSatisfy

  • Type: (predicate: (value: any) => boolean) => Awaitable<void>

This assertion checks if a value satisfies a certain predicate.

describe('toSatisfy()', () => {
  const isOdd = (value: number) => value % 2 !== 0

  it('pass with 0', () => {
    expect(1).toSatisfy(isOdd)
  })

  it('pass with negotiation', () => {
    expect(2).not.toSatisfy(isOdd)
  })
})

resolves

  • Type: Promisify<Assertions>

    resolves is intended to remove boilerplate when asserting asynchronous code. Use it to unwrap value from the pending promise and assert its value with usual assertions. If the promise rejects, the assertion will fail.

    It returns the same Assertions object, but all matchers now return Promise, so you would need to await it. Also works with chai assertions.

    For example, if you have a function, that makes an API call and returns some data, you may use this code to assert its return value:

    import { expect, test } from 'vitest'

function buyApples() {
  return fetch('/buy/apples').then(r => r.json())
}

test('buyApples returns new stock id', async () => {
  // toEqual returns a promise now, so you HAVE to await it
  await expect(buyApples()).resolves.toEqual({ id: 1 }) // jest API
  await expect(buyApples()).resolves.to.equal({ id: 1 }) // chai API
})

    WARNING

    If the assertion is not awaited, then you will have a false-positive test that will pass every time. To make sure that assertions are actually called, you may use expect.assertions(number).

rejects

  • Type: Promisify<Assertions>

    rejects is intended to remove boilerplate when asserting asynchronous code. Use it to unwrap reason why promise was rejected, and assert its value with usual assertions. If promise successfully resolves, the assertion will fail.

    It returns the same Assertions object, but all matchers are now return Promise, so you would need to await it. Also works with chai assertions.

    For example, if you have a function that fails when you call it, you may use this code to assert the reason:

    import { expect, test } from 'vitest'

function buyApples(id) {
  if (!id)
    throw new Error('no id')
}

test('buyApples throws an error when no id provided', async () => {
  // toThrow returns a promise now, so you HAVE to await it
  await expect(buyApples()).rejects.toThrow('no id')
})

    WARNING

    If the assertion is not awaited, then you will have a false-positive test that will pass every time. To make sure that assertions are actually happened, you may use expect.assertions(number).

expect.assertions

  • Type: (count: number) => void

    After the test has passed or failed verifies that curtain number of assertions was called during a test. Useful case would be to check if an asynchronous code was called.

    For examples, if we have a function than asynchronously calls two matchers, we can assert that they were actually called.

    import { expect, test } from 'vitest'

async function doAsync(...cbs) {
  await Promise.all(
    cbs.map((cb, index) => cb({ index })),
  )
}

test('all assertions are called', async () => {
  expect.assertions(2)
  function callback1(data) {
    expect(data).toBeTruthy()
  }
  function callback2(data) {
    expect(data).toBeTruthy()
  }

  await doAsync(callback1, callback2)
})

expect.hasAssertions

  • Type: (count: number) => void

    After the test has passed or failed verifies that at least one assertion was called during a test. Useful case would be to check if an asynchronous code was called.

    For example, if you have a code that calls a callback, we can make an assertion inside a callback, but the test will always pass, if we don't check if an assertion was called.

    import { expect, test } from 'vitest'
import { db } from './db'

const cbs = []

function onSelect(cb) {
  cbs.push(cb)
}

// after selecting from db, we call all callbacks
function select(id) {
  return db.select({ id }).then((data) => {
    return Promise.all(
      cbs.map(cb => cb(data)),
    )
  })
}

test('callback was called', async () => {
  expect.hasAssertions()
  onSelect((data) => {
    // should be called on select
    expect(data).toBeTruthy()
  })
  // if not awaited, test will fail
  // if you dont have expect.hasAssertions(), test will pass
  await select(3)
})

expect.addSnapshotSerializer

  • Type: (plugin: PrettyFormatPlugin) => void

    This method adds custom serializers that are called when creating a snapshot. This is advanced feature - if you want to know more, please read a guide on custom serializers.

    If you are adding custom serializers, you should call this method inside setupFiles. This will affect every snapshot.

    TIP

    If you previously used Vue CLI with Jest, you might want to install jest-serializer-vue. Otherwise, your snapshots will be wrapped in a string, which cases " to be escaped.

expect.extend

  • Type: (matchers: MatchersObject) => void

    You can extend default matchers with your own. This function is used to extend the matchers object with custom matchers.

    When you define matchers that way, you also create asymmetric matchers that can be used like expect.stringContaining.

    import { expect, test } from 'vitest'

test('custom matchers', () => {
  expect.extend({
    toBeFoo: (received, expected) => {
      if (received !== 'foo') {
        return {
          message: () => `expected ${received} to be foo`,
          pass: false,
        }
      }
    },
  })

  expect('foo').toBeFoo()
  expect({ foo: 'foo' }).toEqual({ foo: expect.toBeFoo() })
})

    If you want your matchers to appear in every test, you should call this method inside setupFiles.

    This function is compatible with Jest's expect.extend, so any library that uses it to create custom matchers will work with Vitest.

    If you are using TypeScript, you can extend default Matchers interface with the code bellow:

    interface CustomMatchers<R = unknown> {
  toBeFoo(): R
}

declare global {
  namespace Vi {
    interface Assertion extends CustomMatchers {}
    interface AsymmetricMatchersContaining extends CustomMatchers {}
  }
}

    Note: augmenting jest.Matchers interface will also work.

    TIP

    If you want to know more, checkout guide on extending matchers.

Setup and Teardown

These functions allow you to hook into the life cycle of tests to avoid repeating setup and teardown code. They apply to the current context: the file if they are used at the top-level or the current suite if they are inside a describe block.

beforeEach

  • Type: beforeEach(fn: () => Awaitable<void>, timeout?: number)

    Register a callback to be called before each of the tests in the current context runs. If the function returns a promise, Vitest waits until the promise resolve before running the test.

    Optionally, you can pass a timeout (in milliseconds) defining how long to wait before terminating. The default is 5 seconds.

    import { beforeEach } from 'vitest'

beforeEach(async () => {
  // Clear mocks and add some testing data after before each test run
  await stopMocking()
  await addUser({ name: 'John' })
})

    Here, the beforeEach ensures that user is added for each test.

    Since Vitest v0.10.0, beforeEach also accepts an optional cleanup function (equivalent to afterEach).

    import { beforeEach } from 'vitest'

beforeEach(async () => {
  // called once before all tests run
  await prepareSomething()

  // clean up function, called once after all tests run
  return async () => {
    await resetSomething()
  }
})

afterEach

  • Type: afterEach(fn: () => Awaitable<void>, timeout?: number)

    Register a callback to be called after each one of the tests in the current context completes. If the function returns a promise, Vitest waits until the promise resolve before continuing.

    Optionally, you can a timeout (in milliseconds) for specifying how long to wait before terminating. The default is 5 seconds.

    import { afterEach } from 'vitest'

afterEach(async () => {
  await clearTestingData() // clear testing data after each test run
})

    Here, the afterEach ensures that testing data is cleared after each test runs.

beforeAll

  • Type: beforeAll(fn: () => Awaitable<void>, timeout?: number)

    Register a callback to be called once before starting to run all tests in the current context. If the function returns a promise, Vitest waits until the promise resolve before running tests.

    Optionally, you can provide a timeout (in milliseconds) for specifying how long to wait before terminating. The default is 5 seconds.

    import { beforeAll } from 'vitest'

beforeAll(async () => {
  await startMocking() // called once before all tests run
})

    Here the beforeAll ensures that the mock data is set up before tests run.

    Since Vitest v0.10.0, beforeAll also accepts an optional cleanup function (equivalent to afterAll).

    import { beforeAll } from 'vitest'

beforeAll(async () => {
  // called once before all tests run
  await startMocking()

  // clean up function, called once after all tests run
  return async () => {
    await stopMocking()
  }
})

afterAll

  • Type: afterAll(fn: () => Awaitable<void>, timeout?: number)

    Register a callback to be called once after all tests have run in the current context. If the function returns a promise, Vitest waits until the promise resolve before continuing.

    Optionally, you can provide a timeout (in milliseconds) for specifying how long to wait before terminating. The default is 5 seconds.

    import { afterAll } from 'vitest'

afterAll(async () => {
  await stopMocking() // this method is called after all tests run
})

    Here the afterAll ensures that stopMocking method is called after all tests run.

Vi

Vitest provides utility functions to help you out through it's vi helper. You can import { vi } from 'vitest' or access it globally (when globals configuration is enabled).

vi.advanceTimersByTime

  • Type: (ms: number) => Vitest

    Works just like runAllTimers, but will end after passed milliseconds. For example this will log 1, 2, 3 and will not throw:

    let i = 0
setInterval(() => console.log(++i), 50)

vi.advanceTimersByTime(150)

vi.advanceTimersToNextTimer

  • Type: () => Vitest

    Will call next available timer. Useful to make assertions between each timer call. You can chain call it to manage timers by yourself.

    let i = 0
setInterval(() => console.log(++i), 50)

vi.advanceTimersToNextTimer() // log 1
  .advanceTimersToNextTimer() // log 2
  .advanceTimersToNextTimer() // log 3

vi.clearAllTimers

Removes all timers that are scheduled to run. These timers will never run in the future.

vi.fn

  • Type: (fn: Function) => CallableMockInstance

    Creates a spy on a function, though can be initiated without one. Every time a function is invoked, it stores its call arguments, returns and instances. Also, you can manipulate its behavior with methods. If no function is given, mock will return undefined, when invoked.

    const getApples = vi.fn(() => 0)

getApples()

expect(getApples).toHaveBeenCalled()
expect(getApples).toHaveReturnedWith(0)

getApples.mockReturnValueOnce(5)

const res = getApples()
expect(res).toBe(5)
expect(getApples).toHaveNthReturnedWith(2, 5)

vi.getMockedSystemTime

  • Type: () => Date | null

    Returns mocked current date that was set using setSystemTime. If date is not mocked, will return null.

vi.getRealSystemTime

  • Type: () => number

    When using vi.useFakeTimers, Date.now calls are mocked. If you need to get real time in milliseconds, you can call this function.

vi.mock

  • Type: (path: string, factory?: () => unknown) => void

    Makes all imports to passed module to be mocked. Inside a path you can use configured Vite aliases.

    • If factory is defined, will return its result. Factory function can be asynchronous. You may call vi.importActual inside to get the original module. The call to vi.mock is hoisted to the top of the file, so you don't have access to variables declared in the global file scope!
    • If mocking a module with a default export, you'll need to provide a default key within the returned factory function object. This is an ES modules specific caveat, therefore jest documentation may differ as jest uses commonJS modules. Example:
    vi.mock('path', () => {
  return {
    default: { myDefaultKey: vi.fn() },
    namedExport: vi.fn(),
    // etc...
  }
})
    • If __mocks__ folder with file of the same name exist, all imports will return its exports. For example, vi.mock('axios') with <root>/__mocks__/axios.ts folder will return everything exported from axios.ts.
    • If there is no __mocks__ folder or a file with the same name inside, will call original module and mock it. (For the rules applied, see algorithm.)

vi.setSystemTime

  • Type: (date: string | number | Date) => void

    Sets current date to the one that was passed. All Date calls will return this date.

    Useful if you need to test anything that depends on the current date - for example luxon calls inside your code.

    const date = new Date(1998, 11, 19)

vi.useFakeTimers()
vi.setSystemTime(date)

expect(Date.now()).toBe(date.valueOf())

vi.useRealTimers()

vi.mocked

  • Type: <T>(obj: T, deep?: boolean) => MaybeMockedDeep<T>

    Type helper for TypeScript. In reality just returns the object that was passed.

    import example from './example'
vi.mock('./example')

test('1+1 equals 2', async () => {
  vi.mocked(example.calc).mockRestore()

  const res = example.calc(1, '+', 1)

  expect(res).toBe(2)
})

vi.importActual

  • Type: <T>(path: string) => Promise<T>

    Imports module, bypassing all checks if it should be mocked. Can be useful if you want to mock module partially.

    vi.mock('./example', async () => {
  const axios = await vi.importActual('./example')

  return { ...axios, get: vi.fn() }
})

vi.importMock

  • Type: <T>(path: string) => Promise<MaybeMockedDeep<T>>

    Imports a module with all of its properties (including nested properties) mocked. Follows the same rules that vi.mock follows. For the rules applied, see algorithm.

vi.resetModules

  • Type: () => Vitest

    Resets modules registry by clearing cache of all modules. Might be useful to isolate modules where local state conflicts between tests.

    import { vi } from 'vitest'

beforeAll(() => {
  vi.resetModules()
})

test('change state', async () => {
  const mod = await import('./some/path')
  mod.changeLocalState('new value')
  expect(mod.getlocalState()).toBe('new value')
})

test('module has old state', async () => {
  const mod = await import('./some/path')
  expect(mod.getlocalState()).toBe('old value')
})

vi.restoreCurrentDate

  • Type: () => void

    Restores Date back to its native implementation.

vi.runAllTicks

  • Type: () => Vitest

    Calls every microtask. These are usually queued by proccess.nextTick. This will also run all microtasks scheduled by themselves.

vi.runAllTimers

  • Type: () => Vitest

    This method will invoke every initiated timer until the timers queue is empty. It means that every timer called during runAllTimers will be fired. If you have an infinite interval, it will throw after 10 000 tries. For example this will log 1, 2, 3:

    let i = 0
setTimeout(() => console.log(++i))
const interval = setInterval(() => {
  console.log(++i)
  if (i === 2)
    clearInterval(interval)

}, 50)

vi.runAllTimers()

vi.runOnlyPendingTimers

  • Type: () => Vitest

    This method will call every timer that was initiated after vi.useFakeTimers() call. It will not fire any timer that was initiated during its call. For example this will only log 1:

    let i = 0
setInterval(() => console.log(++i), 50)

vi.runOnlyPendingTimers()

vi.spyOn

  • Type: <T, K extends keyof T>(object: T, method: K, accessType?: 'get' | 'set') => MockInstance

    Creates a spy on a method or getter/setter of an object.

    let apples = 0
const obj = {
  getApples: () => 13,
}

const spy = vi.spyOn(obj, 'getApples').mockImplementation(() => apples)
apples = 1

expect(obj.getApples()).toBe(1)

expect(spy).toHaveBeenCalled()
expect(spy).toHaveReturnedWith(1)

vi.stubGlobal

  • Type: (key: keyof globalThis & Window, value: any) => Vitest

    Puts a value on global variable. If you are using jsdom or happy-dom, also puts the value on window object.

    Read more in "Mocking Globals" section.

vi.unmock

  • Type: (path: string) => void

    Removes module from mocked registry. All subsequent calls to import will return original module even if it was mocked.

vi.useFakeTimers

  • Type: () => Vitest

    To enable mocking timers, you need to call this method. It will wrap all further calls to timers (such as setTimeout, setInterval, clearTimeout, clearInterval, nextTick, setImmediate, clearImmediate, and Date), until vi.useRealTimers() is called.

    The implementation is based internally on @sinonjs/fake-timers.

vi.useRealTimers

  • Type: () => Vitest

    When timers are run out, you may call this method to return mocked timers to its original implementations. All timers that were run before will not be restored.

MockInstance Methods

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:

    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.

    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:

    const myMockFn = vi
  .fn(() => 'default')
  .mockImplementationOnce(() => 'first call')
  .mockImplementationOnce(() => 'second call')

// 'first call', 'second call', 'default', 'default'
console.log(myMockFn(), myMockFn(), myMockFn(), myMockFn())

mockRejectedValue

  • Type: (value: any) => MockInstance

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

    test('async test', async () => {
  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.

    test('async test', async () => {
  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 as 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 mockRestore 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.

    test('async test', async () => {
  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.

    test('async test', async () => {
  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.

    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 passed value. When there are no more mockReturnValueOnce values to use, calls a function specified by mockImplementation or other mockReturn* methods.

    const myMockFn = vi
  .fn()
  .mockReturnValue('default')
  .mockReturnValueOnce('first call')
  .mockReturnValueOnce('second call')

// 'first call', 'second call', 'default', 'default'
console.log(myMockFn(), myMockFn(), myMockFn(), myMockFn())

MockInstance Properties

mock.calls

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

If a function was invoked twice with the following arguments fn(arg1, arg2), fn(arg3, arg4) in that order, then mock.calls will be:

[
  ['arg1', 'arg2'],
  ['arg3', 'arg4'],
]

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 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 returned value or thrown error.

If function returned result, then threw an error, then mock.results will be:

[
  {
    type: 'return',
    value: '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.

For example, if mock was instantiated with new MyClass(), then mock.instances will be an array of one value:

import { expect, vi } from 'vitest'

const MyClass = vi.fn()

const a = new MyClass()

expect(MyClass.mock.instances[0]).toBe(a)

If you return a value from constructor, it will not be in instances array, but instead on results:

import { expect, vi } from 'vitest'

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

const a = new Spy()

expect(Spy.mock.instances[0]).not.toBe(a)
expect(Spy.mock.results[0]).toBe(a)