Vitest

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)
})

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

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

// v5: toThrow('') matches ANY message (empty string is a substring of all).
// To assert an empty message, match the pattern: .toThrow(/^$/)

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

// 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()

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)

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')

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

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))

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

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)

const result = v.safeParse(UserSchema, input) // { success: true, output } | { success: false, ... }
if (!result.success)
  expect.unreachable('should parse')
expect(result.output.age).toBe(25) // narrowed to the success branch

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

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

await expect.poll(async ({ signal }) => {
  const res = await fetch('/api/status', { signal })
  return res.status
}, { timeout: 1000 }).toBe(200)

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
})

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)
})

import 'vitest'

declare module 'vitest' {
  interface Matchers<R, T> {
    toBeFoo: () => R
    toEqualTyped: (expected: T) => R // T mirrors the received value's type
  }
}

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

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