mirror of https://github.com/vitest-dev/vitest.git synced 2025-12-08 18:26:03 +00:00

docs: update API mocking documentation (#2553 )

* docs: update API mocking documentation

* Apply suggestions from code review

Co-authored-by: Anjorin Damilare <damilareanjorin1@gmail.com>

* chore: more docs

* chore: cleanup

Co-authored-by: Anjorin Damilare <damilareanjorin1@gmail.com>

2022-12-22 16:35:15 +03:00

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API Reference

The following types are used in the type signatures below

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

interface TestOptions {
  timeout?: number
  retry?: number
}

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 | TestOptions) => 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 | TestOptions) => 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
})
```

::: warning You cannot use this syntax, when using Vitest as type checker. :::

test.runIf

Type: (condition: any) => Test

Alias: it.runIf

Opposite 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
})

::: warning You cannot use this syntax, when using Vitest as type checker. :::

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)
})
```
Sometimes it is very useful to run only tests in a certain file, ignoring all other tests from the whole test suite, which pollute the output.

In order to do that run vitest with specific file containing the tests in question.
```
# vitest interesting.test.ts
```

test.concurrent

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

Alias: it.concurrent

test.concurrent marks consecutive tests to be run 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()
})

::: warning You cannot use this syntax, when using Vitest as type checker. :::

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', async () => {
  await expect(myAsyncFunc()).rejects.toBe(1)
})

::: warning You cannot use this syntax, when using Vitest as type checker. :::

test.each

Type: (cases: ReadonlyArray<T>, ...args: any[]) => void

Alias: it.each

Use test.each when you need to run the same test with different variables. You can inject parameters with printf formatting 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

You can also access object properties with $ prefix, if you are using objects as arguments:

test.each([
  { a: 1, b: 1, expected: 2 },
  { a: 1, b: 2, expected: 3 },
  { a: 2, b: 1, expected: 3 },
])('add($a, $b) -> $expected', ({ a, b, expected }) => {
  expect(a + b).toBe(expected)
})

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

You can also access Object attributes with ., if you are using objects as arguments:

test.each`
a               | b      | expected
${{ val: 1 }}   | ${'b'} | ${'1b'}
${{ val: 2 }}   | ${'b'} | ${'2b'}
${{ val: 3 }}   | ${'b'} | ${'3b'}
`('add($a.val, $b) -> $expected', ({ a, b, expected }) => {
  expect(a.val + b).toBe(expected)
})

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

Starting from Vitest 0.25.3, you can also use template string table.

First row should be column names, separated by |;
One or more subsequent rows of data supplied as template literal expressions using ${value} syntax.

test.each`
  a               | b      | expected
  ${1}            | ${1}   | ${2}
  ${'a'}          | ${'b'} | ${'ab'}
  ${[]}           | ${'b'} | ${'b'}
  ${{}}           | ${'b'} | ${'[object Object]b'}
  ${{ asd: 1 }}   | ${'b'} | ${'[object Object]b'}
`('returns $expected when $a is added $b', ({ a, b, expected }) => {
  expect(a + b).toBe(expected)
})

If you want to have access to TestContext, use describe.each with a single test.

::: warning You cannot use this syntax, when using Vitest as type checker. :::

bench

Type: (name: string, fn: BenchFunction, options?: BenchOptions) => void

bench defines a benchmark. In Vitest terms benchmark is a function that defines a series of operations. Vitest runs this function multiple times to display different performance results.

Vitest uses tinybench library under the hood, inheriting all its options that can be used as a third argument.

import { bench } from 'vitest'

bench('normal sorting', () => {
  const x = [1, 5, 4, 2, 3]
  x.sort((a, b) => {
    return a - b
  })
}, { time: 1000 })

export interface Options {
  /**
   * time needed for running a benchmark task (milliseconds)
   * @default 500
   */
  time?: number

  /**
   * number of times that a task should run if even the time option is finished
   * @default 10
   */
  iterations?: number

  /**
   * function to get the current timestamp in milliseconds
   */
  now?: () => number

  /**
   * An AbortSignal for aborting the benchmark
   */
  signal?: AbortSignal

  /**
   * warmup time (milliseconds)
   * @default 100ms
   */
  warmupTime?: number

  /**
   * warmup iterations
   * @default 5
   */
  warmupIterations?: number

  /**
   * setup function to run before each benchmark task (cycle)
   */
  setup?: Hook

  /**
   * teardown function to run after each benchmark task (cycle)
   */
  teardown?: Hook
}

bench.skip

Type: (name: string, fn: BenchFunction, options?: BenchOptions) => void

You can use bench.skip syntax to skip running certain benchmarks.

import { bench } from 'vitest'

bench.skip('normal sorting', () => {
  const x = [1, 5, 4, 2, 3]
  x.sort((a, b) => {
    return a - b
  })
})

bench.only

Type: (name: string, fn: BenchFunction, options?: BenchOptions) => void

Use bench.only to only run certain benchmarks in a given suite. This is useful when debugging.

import { bench } from 'vitest'

bench.only('normal sorting', () => {
  const x = [1, 5, 4, 2, 3]
  x.sort((a, b) => {
    return a - b
  })
})

bench.todo

Type: (name: string) => void

Use bench.todo to stub benchmarks to be implemented later.

import { bench } from 'vitest'

bench.todo('unimplemented test')

describe

When you use test or bench 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 or benchmarks and other nested suites. A suite lets you organize your tests and benchmarks so reports are more clear.

// basic.spec.ts
// organizing tests

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)
  })
})

// basic.bench.ts
// organizing benchmarks

import { bench, describe } from 'vitest'

describe('sort', () => {
  bench('normal', () => {
    const x = [1, 5, 4, 2, 3]
    x.sort((a, b) => {
      return a - b
    })
  })

  bench('reverse', () => {
    const x = [1, 5, 4, 2, 3]
    x.reverse().sort((a, b) => {
      return a - b
    })
  })
})

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

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')).toThrowError()
    })
  })

  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, options?: number | TestOptions) => 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, options?: number | TestOptions) => 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
})
```
Sometimes it is very useful to run only tests in a certain file, ignoring all other tests from the whole test suite, which pollute the output.

In order to do that run vitest with specific file containing the tests in question.
```
# vitest interesting.test.ts
```

describe.concurrent

Type: (name: string, fn: TestFunction, options?: number | TestOptions) => 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(/* ... */)

::: warning You cannot use this syntax, when using Vitest as type checker. :::

describe.shuffle

Type: (name: string, fn: TestFunction, options?: number | TestOptions) => void

Vitest provides a way to run all tests in random order via CLI flag --sequence.shuffle or config option sequence.shuffle, but if you want to have only part of your test suite to run tests in random order, you can mark it with this flag.
```
describe.shuffle('suite', () => {
  test('random test 1', async () => { /* ... */ })
  test('random test 2', async () => { /* ... */ })
  test('random test 3', async () => { /* ... */ })
})
// order depends on sequence.seed option in config (Date.now() by default)
```

.skip, .only, and .todo works with random suites.

::: warning You cannot use this syntax, when using Vitest as type checker. :::

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>, ...args: any[]): (name: string, fn: (...args: T[]) => void, options?: number | TestOptions) => 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($a, $b)', ({ 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)
  })
})

Starting from Vitest 0.25.3, you can also use template string table.

First row should be column names, separated by |;
One or more subsequent rows of data supplied as template literal expressions using ${value} syntax.

describe.each`
  a               | b      | expected
  ${1}            | ${1}   | ${2}
  ${'a'}          | ${'b'} | ${'ab'}
  ${[]}           | ${'b'} | ${'b'}
  ${{}}           | ${'b'} | ${'[object Object]b'}
  ${{ asd: 1 }}   | ${'b'} | ${'[object Object]b'}
`('describe template string add($a, $b)', ({ a, b, expected }) => {
  test(`returns ${expected}`, () => {
    expect(a + b).toBe(expected)
  })
})

::: warning You cannot use this syntax, when using Vitest as type checker. :::

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.

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

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 hasn\'t 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 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 ? 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 toThrowError 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: <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 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()
})
```
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>,
    }
  `
  )
})

toThrowErrorMatchingSnapshot

Type: (message?: string) => void

The same as toMatchSnapshot, but expects the same value as 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, message?: string) => void

The same as toMatchInlineSnapshot, but expects the same value as 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({ product: '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({ 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' })
})

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'

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 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'

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 are actually happened, you may use expect.assertions(number). :::

expect.assertions

Type: (count: number) => void

After the test has passed or failed verifies that certain number of assertions was called during a test. 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)
})

expect.hasAssertions

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

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 equality check, will return true only if value is an instance of specified constructor. Useful, if you have a value that is generated each time, and you only want to know that it exist with a proper type.
```
import { expect, test } from 'vitest'
import { generateId } from './generators'

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

expect.arrayContaining

Type: <T>(expected: T[]) => any

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

import { expect, test } from 'vitest'

test('variety has "Emp" in its name', () => {
  const variety = {
    name: 'Empire',
    count: 1,
  }
  expect(basket).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 equality check, this asymmetric matcher will return true if value is a string and contains specified substring or the string matches regular expression.

import { expect, test } from 'vitest'

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

expectTypeOf

Type: <T>(a: unknown) => ExpectTypeOf

not

Type: ExpectTypeOf

You can negate all assertions, using .not property.

toEqualTypeOf

Type: <T>(expected: T) => void

This matcher will check, if types are fully equal to each other. This matcher will not fail, if two objects have different values, but the same type, but will fail, if object is missing a property.

import { expectTypeOf } from 'vitest'

expectTypeOf({ a: 1 }).toEqualTypeOf<{ a: number }>()
expectTypeOf({ a: 1 }).toEqualTypeOf({ a: 1 })
expectTypeOf({ a: 1 }).toEqualTypeOf({ a: 2 })
expectTypeOf({ a: 1, b: 1 }).not.toEqualTypeOf<{ a: number }>()

toMatchTypeOf

Type: <T>(expected: T) => void

This matcher checks if expect type extends provided type. It is different from toEqual and is more similar to expect's toMatch. With this matcher you can check, if an object "matches" a type.

import { expectTypeOf } from 'vitest'

expectTypeOf({ a: 1, b: 1 }).toMatchTypeOf({ a: 1 })
expectTypeOf<number>().toMatchTypeOf<string | number>()
expectTypeOf<string | number>().not.toMatchTypeOf<number>()

extract

Type: ExpectTypeOf<ExtractedUnion>

You can use .extract to narrow down types for further testing.

import { expectTypeOf } from 'vitest'

type ResponsiveProp<T> = T | T[] | { xs?: T; sm?: T; md?: T }
const getResponsiveProp = <T>(_props: T): ResponsiveProp<T> => ({})
interface CSSProperties { margin?: string; padding?: string }

const cssProperties: CSSProperties = { margin: '1px', padding: '2px' }

expectTypeOf(getResponsiveProp(cssProperties))
  .extract<{ xs?: any }>() // extracts the last type from a union
  .toEqualTypeOf<{ xs?: CSSProperties; sm?: CSSProperties; md?: CSSProperties }>()

expectTypeOf(getResponsiveProp(cssProperties))
  .extract<unknown[]>() // extracts an array from a union
  .toEqualTypeOf<CSSProperties[]>()

::: warning If no type is found in the union, .extract will return never. :::

exclude

Type: ExpectTypeOf<NonExcludedUnion>

You can use .exclude to remove types from a union for further testing.

import { expectTypeOf } from 'vitest'

type ResponsiveProp<T> = T | T[] | { xs?: T; sm?: T; md?: T }
const getResponsiveProp = <T>(_props: T): ResponsiveProp<T> => ({})
interface CSSProperties { margin?: string; padding?: string }

const cssProperties: CSSProperties = { margin: '1px', padding: '2px' }

expectTypeOf(getResponsiveProp(cssProperties))
  .exclude<unknown[]>()
  .exclude<{ xs?: unknown }>() // or just .exclude<unknown[] | { xs?: unknown }>()
  .toEqualTypeOf<CSSProperties>()

::: warning If no type is found in the union, .exclude will return never. :::

returns

Type: ExpectTypeOf<ReturnValue>

You can use .returns to extract return value of a function type.

import { expectTypeOf } from 'vitest'

expectTypeOf(() => {}).returns.toBeVoid()
expectTypeOf((a: number) => [a, a]).returns.toEqualTypeOf([1, 2])

::: warning If used on a non-function type, it will return never, so you won't be able to chain it with other matchers. :::

parameters

Type: ExpectTypeOf<Parameters>

You can extract function arguments with .parameters to perform assertions on its value. Parameters are returned as an array.

import { expectTypeOf } from 'vitest'

type NoParam = () => void
type HasParam = (s: string) => void

expectTypeOf<NoParam>().parameters.toEqualTypeOf<[]>()
expectTypeOf<HasParam>().parameters.toEqualTypeOf<[string]>()

::: warning If used on a non-function type, it will return never, so you won't be able to chain it with other matchers. :::

::: tip You can also use .toBeCallableWith matcher as a more expressive assertion. :::

parameter

Type: (nth: number) => ExpectTypeOf

You can extract a certain function argument with .parameter(number) call to perform other assertions on it.

import { expectTypeOf } from 'vitest'

const foo = (a: number, b: string) => [a, b]

expectTypeOf(foo).parameter(0).toBeNumber()
expectTypeOf(foo).parameter(1).toBeString()

::: warning If used on a non-function type, it will return never, so you won't be able to chain it with other matchers. :::

constructorParameters

Type: ExpectTypeOf<ConstructorParameters>

You can extract constructor parameters as an array of values and perform assertions on them with this method.

import { expectTypeOf } from 'vitest'

expectTypeOf(Date).constructorParameters.toEqualTypeOf<[] | [string | number | Date]>()

::: warning If used on a non-function type, it will return never, so you won't be able to chain it with other matchers. :::

::: tip You can also use .toBeConstructibleWith matcher as a more expressive assertion. :::

instance

Type: ExpectTypeOf<ConstructableInstance>

This property gives access to matchers that can be performed on an instance of the provided class.

import { expectTypeOf } from 'vitest'

expectTypeOf(Date).instance.toHaveProperty('toISOString')

::: warning If used on a non-function type, it will return never, so you won't be able to chain it with other matchers. :::

items

Type: ExpectTypeOf<T>

You can get array item type with .items to perform further assertions.

import { expectTypeOf } from 'vitest'

expectTypeOf([1, 2, 3]).items.toEqualTypeOf<number>()
expectTypeOf([1, 2, 3]).items.not.toEqualTypeOf<string>()

resolves

Type: ExpectTypeOf<ResolvedPromise>

This matcher extracts resolved value of a Promise, so you can perform other assertions on it.

import { expectTypeOf } from 'vitest'

const asyncFunc = async () => 123

expectTypeOf(asyncFunc).returns.resolves.toBeNumber()
expectTypeOf(Promise.resolve('string')).resolves.toBeString()

::: warning If used on a non-promise type, it will return never, so you won't be able to chain it with other matchers. :::

guards

Type: ExpectTypeOf<Guard>

This matcher extracts guard value (e.g., v is number), so you can perform assertions on it.

import { expectTypeOf } from 'vitest'

const isString = (v: any): v is string => typeof v === 'string'
expectTypeOf(isString).guards.toBeString()

::: warning Returns never, if the value is not a guard function, so you won't be able to chain it with other matchers. :::

asserts

Type: ExpectTypeOf<Assert>

This matcher extracts assert value (e.g., assert v is number), so you can perform assertions on it.

import { expectTypeOf } from 'vitest'

const assertNumber = (v: any): asserts v is number => {
  if (typeof v !== 'number')
    throw new TypeError('Nope !')
}

expectTypeOf(assertNumber).asserts.toBeNumber()

::: warning Returns never, if the value is not an assert function, so you won't be able to chain it with other matchers. :::

toBeAny

Type: () => void

With this matcher you can check, if provided type is any type. If the type is too specific, the test will fail.

import { expectTypeOf } from 'vitest'

expectTypeOf<any>().toBeAny()
expectTypeOf({} as any).toBeAny()
expectTypeOf('string').not.toBeAny()

toBeUnknown

Type: () => void

This matcher checks, if provided type is unknown type.

import { expectTypeOf } from 'vitest'

expectTypeOf().toBeUnknown()
expectTypeOf({} as unknown).toBeUnknown()
expectTypeOf('string').not.toBeUnknown()

toBeNever

Type: () => void

This matcher checks, if provided type is a never type.

import { expectTypeOf } from 'vitest'

expectTypeOf<never>().toBeNever()
expectTypeOf((): never => {}).returns.toBeNever()

toBeFunction

Type: () => void

This matcher checks, if provided type is a functon.

import { expectTypeOf } from 'vitest'

expectTypeOf(42).not.toBeFunction()
expectTypeOf((): never => {}).toBeFunction()

toBeObject

Type: () => void

This matcher checks, if provided type is an object.

import { expectTypeOf } from 'vitest'

expectTypeOf(42).not.toBeObject()
expectTypeOf({}).toBeObject()

toBeArray

Type: () => void

This matcher checks, if provided type is Array<T>.

import { expectTypeOf } from 'vitest'

expectTypeOf(42).not.toBeArray()
expectTypeOf([]).toBeArray()
expectTypeOf([1, 2]).toBeArray()
expectTypeOf([{}, 42]).toBeArray()

toBeString

Type: () => void

This matcher checks, if provided type is a string.

import { expectTypeOf } from 'vitest'

expectTypeOf(42).not.toBeString()
expectTypeOf('').toBeString()
expectTypeOf('a').toBeString()

toBeBoolean

Type: () => void

This matcher checks, if provided type is boolean.

import { expectTypeOf } from 'vitest'

expectTypeOf(42).not.toBeBoolean()
expectTypeOf(true).toBeBoolean()
expectTypeOf<boolean>().toBeBoolean()

toBeVoid

Type: () => void

This matcher checks, if provided type is void.

import { expectTypeOf } from 'vitest'

expectTypeOf(() => {}).returns.toBeVoid()
expectTypeOf<void>().toBeVoid()

toBeSymbol

Type: () => void

This matcher checks, if provided type is a symbol.

import { expectTypeOf } from 'vitest'

expectTypeOf(Symbol(1)).toBeSymbol()
expectTypeOf<symbol>().toBeSymbol()

toBeNull

Type: () => void

This matcher checks, if provided type is null.

import { expectTypeOf } from 'vitest'

expectTypeOf(null).toBeNull()
expectTypeOf<null>().toBeNull()
expectTypeOf(undefined).not.toBeNull()

toBeUndefined

Type: () => void

This matcher checks, if provided type is undefined.

import { expectTypeOf } from 'vitest'

expectTypeOf(undefined).toBeUndefined()
expectTypeOf<undefined>().toBeUndefined()
expectTypeOf(null).not.toBeUndefined()

toBeNullable

Type: () => void

This matcher checks, if you can use null or undefined with provided type.

import { expectTypeOf } from 'vitest'

expectTypeOf<1 | undefined>().toBeNullable()
expectTypeOf<1 | null>().toBeNullable()
expectTypeOf<1 | undefined | null>().toBeNullable()

toBeCallableWith

Type: () => void

This matcher ensures you can call provided function with a set of parameters.

import { expectTypeOf } from 'vitest'

type NoParam = () => void
type HasParam = (s: string) => void

expectTypeOf<NoParam>().toBeCallableWith()
expectTypeOf<HasParam>().toBeCallableWith('some string')

::: warning If used on a non-function type, it will return never, so you won't be able to chain it with other matchers. :::

toBeConstructibleWith

Type: () => void

This matcher ensures you can create a new instance with a set of constructor parameters.

import { expectTypeOf } from 'vitest'

expectTypeOf(Date).toBeConstructibleWith(new Date())
expectTypeOf(Date).toBeConstructibleWith('01-01-2000')

::: warning If used on a non-function type, it will return never, so you won't be able to chain it with other matchers. :::

toHaveProperty

Type: <K extends keyof T>(property: K) => ExpectTypeOf<T[K>

This matcher checks if a property exists on provided object. If it exists, it also returns the same set of matchers for the type of this property, so you can chain assertions one after another.

import { expectTypeOf } from 'vitest'

const obj = { a: 1, b: '' }

expectTypeOf(obj).toHaveProperty('a')
expectTypeOf(obj).not.toHaveProperty('c')

expectTypeOf(obj).toHaveProperty('a').toBeNumber()
expectTypeOf(obj).toHaveProperty('b').toBeString()
expectTypeOf(obj).toHaveProperty('a').not.toBeString()

assertType

Type: <T>(value: T): void

You can use this function as an alternative for expectTypeOf to easily assert that argument type is equal to provided generic.

import { assertType } from 'vitest'

function concat(a: string, b: string): string
function concat(a: number, b: number): number
function concat(a: string | number, b: string | number): string | number

assertType<string>(concat('a', 'b'))
assertType<number>(concat(1, 2))
// @ts-expect-error wrong types
assertType(concat('a', 2))

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. These hooks are not called, when you are running Vitest as a type checker.

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 each test run
  await prepareSomething()

  // clean up function, called once after each test 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 provide 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.getTimerCount

Type: () => number

Get the number of waiting timers.

vi.clearAllMocks

Will call .mockClear() on all spies. This will clear mock history, but not reset its implementation to the default one.

vi.clearAllTimers

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

vi.dynamicImportSettled

Wait for all imports to load. Useful, if you have a synchronous call that starts importing a module, that you cannot wait otherwise.

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

Substitutes all imported modules from provided path with another module. You can use configured Vite aliases inside a path. The call to vi.mock is hoisted, so it doesn't matter where you call it. It will always be executed before all imports.

::: warning vi.mock works only for modules that were imported with the import keyword. It doesn't work with require.

Vitest statically analyzes your files to hoist vi.mock. It means that you cannot use vi that was not imported directly from vitest package (for example, from some utility file). To fix this, always use vi.mock with vi imported from vitest, or enable globals config option. :::

If factory is defined, all imports will return its result. Vitest calls factory only once and caches result for all subsequent imports until vi.unmock or vi.doUnmock is called.

Unlike in jest, the factory can be asynchronous, so you can use vi.importActual or a helper, received as the first argument, inside to get the original module.
```
vi.mock('./path/to/module.js', async (importOriginal) => {
  const mod = await importOriginal()
  return {
    ...mod,
    // replace some exports
    namedExport: vi.fn(),
  }
})
```
::: warning vi.mock is hoisted (in other words, moved) to top of the file. It means that whenever you write it (be it inside beforeEach or test), it will actually be called before that.

This also means that you cannot use any variables inside the factory that are defined outside the factory.

If you need to use variables inside the factory, try vi.doMock. It works the same way but isn't hoisted. Beware that it only mocks subsequent imports. :::

::: warning If you are mocking a module with default export, you will 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. For example,
```
vi.mock('./path/to/module.js', () => {
  return {
    default: { myDefaultKey: vi.fn() },
    namedExport: vi.fn(),
    // etc...
  }
})
```
:::

If there is a __mocks__ folder alongside a file that you are mocking, and the factory is not provided, Vitest will try to find a file with the same name in the __mocks__ subfolder and use it as an actual module. If you are mocking a dependency, Vitest will try to find a __mocks__ folder in the root of the project (default is process.cwd()).

For example, you have this file structure:
```
- __mocks__
  - axios.js
- src
  __mocks__
    - increment.js
  - increment.js
- tests
  - increment.test.js
```
If you call vi.mock in a test file without a factory provided, it will find a file in the __mocks__ folder to use as a module:
```
// increment.test.js
import { vi } from 'vitest'
// axios is a default export from `__mocks__/axios.js`
import axios from 'axios'
// increment is a named export from `src/__mocks__/increment.js`
import { increment } from '../increment.js'

vi.mock('axios')
vi.mock('../increment.js')

axios.get(`/apples/${increment(1)}`)
```
::: warning Beware that if you don't call vi.mock, modules are not mocked automatically. :::

If there is no __mocks__ folder or a factory provided, Vitest will import the original module and auto-mock all its exports. For the rules applied, see algorithm.

vi.doMock

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

The same as vi.mock, but it's not hoisted at the top of the file, so you can reference variables in the global file scope. The next import of the module will be mocked. This will not mock modules that were imported before this was called.

// ./increment.js
export const increment = number => number + 1

import { beforeEach, test } from 'vitest'
import { increment } from './increment.js'

// the module is not mocked, because vi.doMock is not called yet
increment(1) === 2

let mockedIncrement = 100

beforeEach(() => {
  // simple doMock doesn't clear the previous cache, so we need to clear it manually here
  vi.doUnmock('./increment.js')
  // you can access variables inside a factory
  vi.doMock('./increment.js', () => ({ increment: () => mockedIncrement++ }))
})

test('importing the next module imports mocked one', () => {
  // original import WAS NOT MOCKED, because vi.doMock is evaluated AFTER imports
  expect(increment(1)).toBe(2)
  const { increment: mockedIncrement } = await import('./increment.js')
  // new import returns mocked module
  expect(mockedIncrement(1)).toBe(101)
  expect(mockedIncrement(1)).toBe(102)
  expect(mockedIncrement(1)).toBe(103)
})

vi.mocked

Type: <T>(obj: T, deep?: boolean) => MaybeMockedDeep<T>
Type: <T>(obj: T, options?: { partial?: boolean; deep?: boolean }) => MaybePartiallyMockedDeep<T>

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

When partial is true it will expect a Partial<T> as a return value.
```
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.resetAllMocks

Will call .mockReset() on all spies. This will clear mock history and reset its implementation to an empty function (will return undefined).

vi.resetConfig

Type: RuntimeConfig

If vi.setConfig was called before, this will reset config to the original state.

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')
})

::: warning Does not reset mocks registry. To clear mocks registry, use vi.unmock or vi.doUnmock. :::

vi.restoreAllMocks

Will call .mockRestore() on all spies. This will clear mock history and reset its implementation to the original one.

vi.restoreCurrentDate

Type: () => void

Restores Date back to its native implementation.

vi.stubEnv

Type: (name: string, value: string) => Vitest
Version: Since Vitest 0.26.0

Changes the value of environmental variable on process.env and import.meta.env. You can restore its value by calling vi.unstubAllEnvs.

import { vi } from 'vitest'

// `process.env.NODE_ENV` and `import.meta.env.NODE_ENV`
// are "development" before calling "vi.stubEnv"

vi.stubEnv('NODE_ENV', 'production')

process.env.NODE_ENV === 'production'
import.meta.env.NODE_ENV === 'production'
// doesn't change other envs
import.meta.env.MODE === 'development'

:::tip You can also change the value by simply assigning it, but you won't be able to use vi.unstubAllEnvs to restore previous value:

import.meta.env.MODE = 'test'

:::

:::warning Vitest transforms all import.meta.env calls into process.env, so they can be easily changed at runtime. Node.js only supports string values as env parameters, while Vite supports several built-in envs as boolean (namely, SSR, DEV, PROD). To mimic Vite, set "truthy" values as env: '' instead of false, and '1' instead of true.

But beware that you cannot rely on import.meta.env.DEV === false in this case. Use !import.meta.env.DEV. This also affects simple assigning, not just vi.stubEnv method. :::

vi.unstubAllEnvs

Type: () => Vitest
Version: Since Vitest 0.26.0

Restores all import.meta.env and process.env values that were changed with vi.stubEnv. When it's called for the first time, Vitest remembers the original value and will store it, until unstubAllEnvs is called again.

import { vi } from 'vitest'

// `process.env.NODE_ENV` and `import.meta.env.NODE_ENV`
// are "development" before calling stubEnv

vi.stubEnv('NODE_ENV', 'production')

process.env.NODE_ENV === 'production'
import.meta.env.NODE_ENV === 'production'

vi.stubEnv('NODE_ENV', 'staging')

process.env.NODE_ENV === 'staging'
import.meta.env.NODE_ENV === 'staging'

vi.unstubAllEnvs()

// restores to the value that were stored before the first "stubEnv" call
process.env.NODE_ENV === 'development'
import.meta.env.NODE_ENV === 'development'

vi.stubGlobal

Type: (name: stirng | number | symbol, value: uknown) => Vitest

Changes the value of global variable. You can restore its original value by calling vi.unstubAllGlobals.

import { vi } from 'vitest'

// `innerWidth` is "0" before callling stubGlobal

vi.stubGlobal('innerWidth', 100)

innerWidth === 100
globalThis.innerWidth === 100
// if you are using jsdom or happy-dom
window.innerWidth === 100

:::tip You can also change the value by simply assigning it to globalThis or window (if you are using jsdom or happy-dom environment), but you won't be able to use vi.unstubAllGlobals to restore original value:

globalThis.innerWidth = 100
// if you are using jsdom or happy-dom
window.innerWidth = 100

:::

vi.unstubAllGlobals

Type: () => Vitest
Version: Since Vitest 0.26.0

Restores all global values on globalThis/global (and window/top/self/parent, if you are using jsdom or happy-dom environment) that were changed with vi.stubGlobal. When it's called for the first time, Vitest remembers the original value and will store it, until unstubAllGlobals is called again.

import { vi } from 'vitest'

const Mock = vi.fn()

// IntersectionObserver is "undefined" before calling "stubGlobal"

vi.stubGlobal('IntersectionObserver', Mock)

IntersectionObserver === Mock
global.IntersectionObserver === Mock
globalThis.IntersectionObserver === Mock
// if you are using jsdom or happy-dom
window.IntersectionObserver === Mock

vi.unstubAllGlobals()

globalThis.IntersectionObserver === undefined
'IntersectionObserver' in globalThis === false
// throws ReferenceError, because it's not defined
IntersectionObserver === undefined

vi.runAllTicks

Type: () => Vitest

Calls every microtask that was 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 === 3)
    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.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.setConfig

Type: RuntimeConfig

Updates config for the current test file. You can only affect values that are used, when executing tests.

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 the mocked registry. All calls to import will return the original module even if it was mocked before. This call is hoisted (moved) to the top of the file, so it will only unmock modules that were defined in setupFiles, for example.

vi.doUnmock

Type: (path: string) => void

The same as vi.unmock, but is not hoisted to the top of the file. The next import of the module will import the original module instead of the mock. This will not unmock previously imported modules.

// ./increment.js
export const increment = number => number + 1

import { increment } from './increment.js'

// increment is already mocked, because vi.mock is hoisted
increment(1) === 100

// this is hoisted, and factory is called before the import on line 1
vi.mock('./increment.js', () => ({ increment: () => 100 }))

// all calls are mocked, and `increment` always returns 100
increment(1) === 100
increment(30) === 100

// this is not hoisted, so other import will return unmocked module
vi.doUnmock('./increment.js')

// this STILL returns 100, because `vi.doUnmock` doesn't reevaluate a module
increment(1) === 100
increment(30) === 100

// the next import is unmocked, now `increment` is the original function that returns count + 1
const { increment: unmockedIncrement } = await import('./increment.js')

unmockedIncrement(1) === 2
unmockedIncrement(30) === 31

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 mockReset does and restores inner implementation to the original function.

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

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

mockResolvedValue

Type: (value: any) => MockInstance

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

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)

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API Reference

test

test.skip

test.skipIf

test.runIf

test.only

test.concurrent

test.todo

test.fails

test.each

bench

bench.skip

bench.only

bench.todo

describe

describe.skip

describe.only

describe.concurrent

describe.shuffle

describe.todo

describe.each

expect

not

toBe

toBeCloseTo

toBeDefined

toBeUndefined

toBeTruthy

toBeFalsy

toBeNull

toBeNaN

toBeTypeOf

toBeInstanceOf

toBeGreaterThan

toBeGreaterThanOrEqual

toBeLessThan

toBeLessThanOrEqual

toEqual

toStrictEqual

toContain

toContainEqual

toHaveLength

toHaveProperty

toMatch

toMatchObject

toThrowError

toMatchSnapshot

toMatchInlineSnapshot

toThrowErrorMatchingSnapshot

toThrowErrorMatchingInlineSnapshot

toHaveBeenCalled

toHaveBeenCalledTimes

toHaveBeenCalledWith

toHaveBeenLastCalledWith

toHaveBeenNthCalledWith

toHaveReturned

toHaveReturnedTimes

toHaveReturnedWith

toHaveLastReturnedWith

toHaveNthReturnedWith

toSatisfy

resolves

rejects

expect.assertions

expect.hasAssertions

expect.anything

expect.any

expect.arrayContaining

expect.objectContaining

expect.stringContaining

expect.stringMatching

expect.addSnapshotSerializer

expect.extend

expectTypeOf

not

toEqualTypeOf

toMatchTypeOf

extract

exclude

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