expect

The following types are used in the type signatures below

type Awaitable<T> = T | PromiseLike<T>

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 built on top of chai.

For example, this code asserts that an input value is equal to 2. If it's not, the 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 Node.js error instead of Vitest output. To learn more about test, please read Test API Reference.

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

WARNING

expect has no effect on testing types, if the expression doesn't have a type error. If you want to use Vitest as type checker, use expectTypeOf or assertType.

soft

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

expect.soft functions similarly to expect, but instead of terminating the test execution upon a failed assertion, it continues running and marks the failure as a test failure. All errors encountered during the test will be displayed until the test is completed.

import { expect, test } from 'vitest'

test('expect.soft test', () => {
  expect.soft(1 + 1).toBe(3) // mark the test as fail and continue
  expect.soft(1 + 2).toBe(4) // mark the test as fail and continue
})
// reporter will report both errors at the end of the run

It can also be used with expect. if expect assertion fails, the test will be terminated and all errors will be displayed.

import { expect, test } from 'vitest'

test('expect.soft test', () => {
  expect.soft(1 + 1).toBe(3) // mark the test as fail and continue
  expect(1 + 2).toBe(4) // failed and terminate the test, all previous errors will be output
  expect.soft(1 + 3).toBe(5) // do not run
})

WARNING

expect.soft can only be used inside the test function.

poll

interface ExpectPoll extends ExpectStatic {
  (actual: () => T, options: { interval; timeout; message }): Promise<Assertions<T>>
}

expect.poll reruns the assertion until it is succeeded. You can configure how many times Vitest should rerun the expect.poll callback by setting interval and timeout options.

If an error is thrown inside the expect.poll callback, Vitest will retry again until the timeout runs out.

import { expect, test } from 'vitest'

test('element exists', async () => {
  asyncInjectElement()

  await expect.poll(() => document.querySelector('.element')).toBeTruthy()
})

WARNING

expect.poll makes every assertion asynchronous, so do not forget to await it otherwise you might get unhandled promise rejections.

expect.poll doesn't work with several matchers:

• Snapshot matchers are not supported because they will always succeed. If your condition is flaky, consider using vi.waitFor instead to resolve it first:

import { expect, vi } from 'vitest'

const flakyValue = await vi.waitFor(() => getFlakyValue())
expect(flakyValue).toMatchSnapshot()

• .resolves and .rejects are not supported. expect.poll already awaits the condition if it's asynchronous.
• toThrow and its aliases are not supported because the expect.poll condition is always resolved before the matcher gets the value

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, the 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 to 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'

function getApples() {
  return 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: string) {
  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 a complex object, a string, or anything else. The code will still work.

import { Stocks } from './stocks.js'

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.js'

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, null, undefined, NaN, 0, -0, 0n, "" and document.all.

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 if 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.js'

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.js'

const stocks = new Stocks()

test('if Bill stock hasn\'t failed, sell apples to him', () => {
  stocks.syncStocks('Bill')
  expect(stocks.stockFailed('Bill')).toBeFalsy()
})

Everything in JavaScript is truthy, except false, null, undefined, NaN, 0, -0, 0n, "" and document.all.

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 don\'t 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 ? Number.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.js'

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 './stocks.js'

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 './stocks.js'

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 './stocks.js'

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 './stocks.js'

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. Only the message property of an Error is considered for equality. To customize equality to check properties other than message, use expect.addEqualityTesters. To test if something was thrown, use toThrowError assertion.

toStrictEqual

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

toStrictEqual asserts if the actual value is equal to the 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 the actual value is in an array. toContain can also check whether a string is a substring of another string. Since Vitest 1.0, if you are running tests in a browser-like environment, this assertion can also check if class is contained in a classList, or an element is inside another one.

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

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

  const element = document.querySelector('#el')
  // element has a class
  expect(element.classList).toContain('flex')
  // element is inside another one
  expect(document.querySelector('#wrapper')).toContain(element)
})

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 './stocks.js'

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

  // Deep referencing using an array containing the keyPath
  expect(invoice).toHaveProperty(['items', 0, 'type'], 'apples')
  expect(invoice).toHaveProperty(['items', '0', 'type'], 'apples') // string 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>

• Alias: toThrow

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

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 test will fail.

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

import { expect, test } from 'vitest'

function getFruitStock(type: string) {
  if (type === 'pineapples') {
    throw new Error('Pineapples are not in stock')
  }

  // Do some other stuff
}

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

  // Test the exact error message
  expect(() => getFruitStock('pineapples')).toThrowError(
    /^Pineapples are not in stock$/,
  )
})

TIP

To test async functions, use in combination with rejects.

function getAsyncFruitStock() {
  return Promise.reject(new Error('empty'))
}

test('throws on pineapples', async () => {
  await expect(() => getAsyncFruitStock()).rejects.toThrowError('empty')
})

toMatchSnapshot

• Type: <T>(shape?: Partial<T> | string, message?: 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 a snapshot mismatches and causes the test to fail, if the mismatch is expected, you can press u key to update the snapshot 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()
})

You can also provide a shape of an object, if you are testing just a shape of an object, and don't need it to be 100% compatible:

import { expect, test } from 'vitest'

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

toMatchInlineSnapshot

• Type: <T>(shape?: Partial<T> | string, snapshot?: string, message?: 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",
      },
    }
  `)
})

You can also provide a shape of an object, if you are testing just a shape of an object, and don't need it to be 100% compatible:

import { expect, test } from 'vitest'

test('matches snapshot', () => {
  const data = { foo: new Set(['bar', 'snapshot']) }
  expect(data).toMatchInlineSnapshot(
    { foo: expect.any(Set) },
    `
    {
      "foo": Any<Set>,
    }
  `
  )
})

toMatchFileSnapshot

• Type: <T>(filepath: string, message?: string) => Promise<void>

Compare or update the snapshot with the content of a file explicitly specified (instead of the .snap file).

import { expect, it } from 'vitest'

it('render basic', async () => {
  const result = renderHTML(h('div', { class: 'foo' }))
  await expect(result).toMatchFileSnapshot('./test/basic.output.html')
})

Note that since file system operation is async, you need to use await with toMatchFileSnapshot(). If await is not used, Vitest treats it like expect.soft, meaning the code after the statement will continue to run even if the snapshot mismatches. After the test finishes, Vitest will check the snapshot and fail if there is a mismatch.

toThrowErrorMatchingSnapshot

• Type: (message?: string) => void

The same as toMatchSnapshot, but expects the same value as toThrowError.

toThrowErrorMatchingInlineSnapshot

• Type: (snapshot?: string, message?: string) => void

The same as toMatchInlineSnapshot, but expects the same value as toThrowError.

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)
})

toHaveBeenCalledExactlyOnceWith 2.2.0+

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

This assertion checks if a function was called exactly once and 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)

  expect(buySpy).toHaveBeenCalledExactlyOnceWith('apples', 10)
})

toHaveBeenLastCalledWith

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

This assertion checks if a function was called with certain parameters at its 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'

function 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 an 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({ product: 'apples' })
})

toHaveLastReturnedWith

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

You can call this assertion to check if a function has successfully returned a certain value when it was last invoked. 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({ product: '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' })
})

toHaveResolved

• Type: () => Awaitable<void>

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

If the function returned a promise, but it was not resolved yet, this will fail.

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

async function getApplesPrice(amount: number) {
  return amount * await db.get('price')
}

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

  const price = await getPriceSpy(10)

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

toHaveResolvedTimes

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

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

This will only count resolved promises. If the function returned a promise, but it was not resolved yet, it will not be counted.

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

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

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

  expect(sell).toHaveResolvedTimes(2)
})

toHaveResolvedWith

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

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

If the function returned a promise, but it was not resolved yet, this will fail.

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

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

  await sell('apples')

  expect(sell).toHaveResolvedWith({ product: 'apples' })
})

toHaveLastResolvedWith

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

You can call this assertion to check if a function has successfully resolved a certain value when it was last invoked. Requires a spy function to be passed to expect.

If the function returned a promise, but it was not resolved yet, this will fail.

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

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

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

  expect(sell).toHaveLastResolvedWith({ product: 'bananas' })
})

toHaveNthResolvedWith

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

You can call this assertion to check if a function has successfully resolved a certain value on a specific invokation. Requires a spy function to be passed to expect.

If the function returned a promise, but it was not resolved yet, this will fail.

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

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

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

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

toSatisfy

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

This assertion checks if a value satisfies a certain predicate.

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

const isOdd = (value: number) => value % 2 !== 0

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

  it('pass with negation', () => {
    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'

async 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 the promise was rejected, and assert its value with usual assertions. If the promise successfully resolves, 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 fails when you call it, you may use this code to assert the reason:

import { expect, test } from 'vitest'

async 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 were actually called, you can use expect.assertions(number).

expect.assertions

• Type: (count: number) => void

After the test has passed or failed verify that a certain number of assertions was called during a test. A useful case would be to check if an asynchronous code was called.

For example, if we have a function that 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)
})

WARNING

When using assertions with async concurrent tests, expect from the local Test Context must be used to ensure the right test is detected.

expect.hasAssertions

• Type: () => void

After the test has passed or failed verify that at least one assertion was called during a test. A 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.js'

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 don't have expect.hasAssertions(), test will pass
  await select(3)
})

expect.unreachable

• Type: (message?: string) => never

This method is used to assert that a line should never be reached.

For example, if we want to test that build() throws due to receiving directories having no src folder, and also handle each error separately, we could do this:

import { expect, test } from 'vitest'

async function build(dir) {
  if (dir.includes('no-src')) {
    throw new Error(`${dir}/src does not exist`)
  }
}

const errorDirs = [
  'no-src-folder',
  // ...
]

test.each(errorDirs)('build fails with "%s"', async (dir) => {
  try {
    await build(dir)
    expect.unreachable('Should not pass build')
  }
  catch (err: any) {
    expect(err).toBeInstanceOf(Error)
    expect(err.stack).toContain('build')

    switch (dir) {
      case 'no-src-folder':
        expect(err.message).toBe(`${dir}/src does not exist`)
        break
      default:
        // to exhaust all error tests
        expect.unreachable('All error test must be handled')
        break
    }
  }
})

expect.anything

• Type: () => any

This asymmetric matcher, when used with equality check, will always return true. Useful, if you just want to be sure that the property exist.

import { expect, test } from 'vitest'

test('object has "apples" key', () => {
  expect({ apples: 22 }).toEqual({ apples: expect.anything() })
})

expect.any

• Type: (constructor: unknown) => any

This asymmetric matcher, when used with an equality check, will return true only if the value is an instance of a specified constructor. Useful, if you have a value that is generated each time, and you only want to know that it exists with a proper type.

import { expect, test } from 'vitest'
import { generateId } from './generators.js'

test('"id" is a number', () => {
  expect({ id: generateId() }).toEqual({ id: expect.any(Number) })
})

expect.closeTo

• Type: (expected: any, precision?: number) => any

expect.closeTo is useful when comparing floating point numbers in object properties or array item. If you need to compare a number, please use .toBeCloseTo instead.

The optional precision argument limits the number of digits to check after the decimal point. For the default value 2, the test criterion is Math.abs(expected - received) < 0.005 (that is, 10 ** -2 / 2).

For example, this test passes with a precision of 5 digits:

test('compare float in object properties', () => {
  expect({
    title: '0.1 + 0.2',
    sum: 0.1 + 0.2,
  }).toEqual({
    title: '0.1 + 0.2',
    sum: expect.closeTo(0.3, 5),
  })
})

expect.arrayContaining

• Type: <T>(expected: T[]) => any

When used with an equality check, this asymmetric matcher will return true if the value is an array and contains specified items.

import { expect, test } from 'vitest'

test('basket includes fuji', () => {
  const basket = {
    varieties: [
      'Empire',
      'Fuji',
      'Gala',
    ],
    count: 3
  }
  expect(basket).toEqual({
    count: 3,
    varieties: expect.arrayContaining(['Fuji'])
  })
})

TIP

You can use expect.not with this matcher to negate the expected value.

expect.objectContaining

• Type: (expected: any) => any

When used with an equality check, this asymmetric matcher will return true if the value has a similar shape.

import { expect, test } from 'vitest'

test('basket has empire apples', () => {
  const basket = {
    varieties: [
      {
        name: 'Empire',
        count: 1,
      }
    ],
  }
  expect(basket).toEqual({
    varieties: [
      expect.objectContaining({ name: 'Empire' }),
    ]
  })
})

TIP

You can use expect.not with this matcher to negate the expected value.

expect.stringContaining

• Type: (expected: any) => any

When used with an equality check, this asymmetric matcher will return true if the value is a string and contains a specified substring.

import { expect, test } from 'vitest'

test('variety has "Emp" in its name', () => {
  const variety = {
    name: 'Empire',
    count: 1,
  }
  expect(variety).toEqual({
    name: expect.stringContaining('Emp'),
    count: 1,
  })
})

TIP

You can use expect.not with this matcher to negate the expected value.

expect.stringMatching

• Type: (expected: any) => any

When used with an equality check, this asymmetric matcher will return true if the value is a string and contains a specified substring or if the string matches a regular expression.

import { expect, test } from 'vitest'

test('variety ends with "re"', () => {
  const variety = {
    name: 'Empire',
    count: 1,
  }
  expect(variety).toEqual({
    name: expect.stringMatching(/re$/),
    count: 1,
  })
})

TIP

You can use expect.not with this matcher to negate the expected value.

expect.addSnapshotSerializer

• Type: (plugin: PrettyFormatPlugin) => void

This method adds custom serializers that are called when creating a snapshot. This is an 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() })
})

TIP

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, since Vitest 0.31.0 you can extend default Assertion interface in an ambient declaration file (e.g: vitest.d.ts) with the code below:

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

declare module 'vitest' {
  interface Assertion<T = any> extends CustomMatchers<T> {}
  interface AsymmetricMatchersContaining extends CustomMatchers {}
}

WARNING

Don't forget to include the ambient declaration file in your tsconfig.json.

TIP

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

expect.addEqualityTesters

• Type: (tester: Array<Tester>) => void

You can use this method to define custom testers, which are methods used by matchers, to test if two objects are equal. It is compatible with Jest's expect.addEqualityTesters.

import { expect, test } from 'vitest'

class AnagramComparator {
  public word: string

  constructor(word: string) {
    this.word = word
  }

  equals(other: AnagramComparator): boolean {
    const cleanStr1 = this.word.replace(/ /g, '').toLowerCase()
    const cleanStr2 = other.word.replace(/ /g, '').toLowerCase()

    const sortedStr1 = cleanStr1.split('').sort().join('')
    const sortedStr2 = cleanStr2.split('').sort().join('')

    return sortedStr1 === sortedStr2
  }
}

function isAnagramComparator(a: unknown): a is AnagramComparator {
  return a instanceof AnagramComparator
}

function areAnagramsEqual(a: unknown, b: unknown): boolean | undefined {
  const isAAnagramComparator = isAnagramComparator(a)
  const isBAnagramComparator = isAnagramComparator(b)

  if (isAAnagramComparator && isBAnagramComparator) {
    return a.equals(b)
  }
  else if (isAAnagramComparator === isBAnagramComparator) {
    return undefined
  }
  else {
    return false
  }
}

expect.addEqualityTesters([areAnagramsEqual])

test('custom equality tester', () => {
  expect(new AnagramComparator('listen')).toEqual(new AnagramComparator('silent'))
})