/vitest

Expect API

Vitest uses Chai assertions with Jest-compatible API.

Basic Assertions

import { expect, test } from 'vitest'

test('assertions', () => {
  // Equality
  expect(1 + 1).toBe(2)              // Strict equality (===)
  expect({ a: 1 }).toEqual({ a: 1 }) // Deep equality

  // Truthiness
  expect(true).toBeTruthy()
  expect(false).toBeFalsy()
  expect(null).toBeNull()
  expect(undefined).toBeUndefined()
  expect('value').toBeDefined()

  // Numbers
  expect(10).toBeGreaterThan(5)
  expect(10).toBeGreaterThanOrEqual(10)
  expect(5).toBeLessThan(10)
  expect(0.1 + 0.2).toBeCloseTo(0.3, 5)

  // Strings
  expect('hello world').toMatch(/world/)
  expect('hello').toContain('ell')

  // Arrays
  expect([1, 2, 3]).toContain(2)
  expect([{ a: 1 }]).toContainEqual({ a: 1 })
  expect([1, 2, 3]).toHaveLength(3)

  // Objects
  expect({ a: 1, b: 2 }).toHaveProperty('a')
  expect({ a: 1, b: 2 }).toHaveProperty('a', 1)
  expect({ a: { b: 1 } }).toHaveProperty('a.b', 1)
  expect({ a: 1 }).toMatchObject({ a: 1 })

  // Types
  expect('string').toBeTypeOf('string')
  expect(new Date()).toBeInstanceOf(Date)
})

Negation

expect(1).not.toBe(2)
expect({ a: 1 }).not.toEqual({ a: 2 })

Error Assertions

// Sync errors - wrap in function
expect(() => throwError()).toThrow()
expect(() => throwError()).toThrow('message')
expect(() => throwError()).toThrow(/pattern/)
expect(() => throwError()).toThrow(CustomError)

// Async errors - use rejects
await expect(asyncThrow()).rejects.toThrow('error')

Promise Assertions

// Resolves
await expect(Promise.resolve(1)).resolves.toBe(1)
await expect(fetchData()).resolves.toEqual({ data: true })

// Rejects
await expect(Promise.reject('error')).rejects.toBe('error')
await expect(failingFetch()).rejects.toThrow()

Spy/Mock Assertions

const fn = vi.fn()
fn('arg1', 'arg2')
fn('arg3')

expect(fn).toHaveBeenCalled()
expect(fn).toHaveBeenCalledTimes(2)
expect(fn).toHaveBeenCalledWith('arg1', 'arg2')
expect(fn).toHaveBeenLastCalledWith('arg3')
expect(fn).toHaveBeenNthCalledWith(1, 'arg1', 'arg2')

expect(fn).toHaveReturned()
expect(fn).toHaveReturnedWith(value)

// v4 additions
expect(fn).toHaveBeenCalledExactlyOnceWith('arg1', 'arg2')
expect(fnA).toHaveBeenCalledBefore(fnB)
expect(fnA).toHaveBeenCalledAfter(fnB)

Chai-Style Spy Assertions (4.1+)

Sinon-chai-compatible aliases, useful when migrating from Sinon:

expect(spy).to.have.been.called
expect(spy).to.have.been.calledOnce
expect(spy).to.have.been.calledWith('arg1', 'arg2')
expect(spy).to.have.been.calledOnceWith('arg')

Conditional Mock Exhaustion (v5)

Assert every vi.when behavior was consumed:

const w = vi.when(spy).calledWith(1).thenReturnOnce('a')
spy(1)
expect(w).toHaveBeenExhausted()

Asymmetric Matchers

Use inside toEqual, toHaveBeenCalledWith, etc:

expect({ id: 1, name: 'test' }).toEqual({
  id: expect.any(Number),
  name: expect.any(String),
})

expect({ a: 1, b: 2, c: 3 }).toEqual(
  expect.objectContaining({ a: 1 })
)

expect([1, 2, 3, 4]).toEqual(
  expect.arrayContaining([1, 3])
)

expect('hello world').toEqual(
  expect.stringContaining('world')
)

expect('hello world').toEqual(
  expect.stringMatching(/world$/)
)

expect({ value: null }).toEqual({
  value: expect.anything() // Matches anything except null/undefined
})

// Negate with expect.not
expect([1, 2]).toEqual(
  expect.not.arrayContaining([3])
)

// toBeOneOf - value matches any option (great for optional props)
expect(user).toEqual({
  name: expect.any(String),
  middleName: expect.toBeOneOf([expect.any(String), undefined]),
})

// schemaMatching (4.0+) - matches any Standard Schema (Zod, Valibot, ArkType)
import { z } from 'zod'
expect(payload).toEqual({
  email: expect.schemaMatching(z.string().email()),
})
expect(repo.save).toHaveBeenCalledWith(expect.schemaMatching(UserSchema))

Soft Assertions

Prefer expect.soft for non-critical assertions — it marks the test failed but continues so all failures are reported together:

expect.soft(response.status).toBe(200) // non-critical, keeps going
expect.soft(response.headers.get('x-id')).toBeTruthy()
expect(response.body).toBeDefined() // critical: hard expect stops on failure

Type-Narrowing Assertions (4.0+)

expect.assert throws at runtime and narrows the TypeScript type (unlike toBeTruthy/toBeDefined, which return void):

const user = cache.get('alice') // { id, name } | undefined
expect.assert(user)             // throws if undefined, narrows below
expect(user.name).toBe('Alice') // no `!`, no `as`

// Narrows typeof / instanceof too
expect.assert(typeof input === 'string')
input.toUpperCase()

// Chai assert helpers via the same namespace
expect.assert.isDefined(maybeUser)
expect.assert.instanceOf(error, MyError)

Poll Assertions

Retry until passes:

await expect.poll(() => fetchStatus()).toBe('ready')

await expect.poll(
  () => document.querySelector('.element'),
  { interval: 100, timeout: 5000 }
).toBeTruthy()

Assertion Count

test('async assertions', async () => {
  expect.assertions(2) // Exactly 2 assertions must run

  await doAsync((data) => {
    expect(data).toBeDefined()
    expect(data.id).toBe(1)
  })
})

test('at least one', () => {
  expect.hasAssertions() // At least 1 assertion must run
})

Extending Matchers

expect.extend({
  toBeWithinRange(received, floor, ceiling) {
    const pass = received >= floor && received <= ceiling
    return {
      pass,
      message: () =>
        `expected ${received} to be within range ${floor} - ${ceiling}`,
    }
  },
})

test('custom matcher', () => {
  expect(100).toBeWithinRange(90, 110)
})

Snapshot Assertions

expect(data).toMatchSnapshot()
expect(data).toMatchInlineSnapshot(`{ "id": 1 }`)
await expect(result).toMatchFileSnapshot('./expected.json')

expect(() => throw new Error('fail')).toThrowErrorMatchingSnapshot()

Key Points

• Use toBe for primitives, toEqual for objects/arrays
• toStrictEqual checks undefined properties and array sparseness
• Always await async assertions (resolves, rejects, poll)
• Use context's expect in concurrent tests for correct tracking
• toThrow requires wrapping sync code in a function
• Use expect.soft for non-critical assertions; reserve hard expect for must-pass conditions
• Use expect.assert (not toBeTruthy) when you also need TypeScript narrowing