Vitest Testing

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name: vitest-testing description: Vitest testing framework patterns and best practices. Use when writing unit tests, integration tests, configuring vitest.config, mocking with vi.mock/vi.fn, using snapshots, or setting up test coverage. Triggers on describe, it, expect, vi.mock, vi.fn, beforeEach, afterEach, vitest.

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Vitest Best Practices

Quick Reference

import { describe, it, expect, beforeEach, vi } from 'vitest'
describe('feature name', () => {
  beforeEach(() => {
    vi.clearAllMocks()
  })
  it('should do something specific', () => {
    expect(actual).toBe(expected)
  })
  it.todo('planned test')
  it.skip('temporarily disabled')
  it.only('run only this during dev')
})

Common Assertions

// Equality
expect(value).toBe(42)                    // Strict (===)
expect(obj).toEqual({ a: 1 })             // Deep equality
expect(obj).toStrictEqual({ a: 1 })       // Strict deep (checks types)
// Truthiness
expect(value).toBeTruthy()
expect(value).toBeFalsy()
expect(value).toBeNull()
expect(value).toBeUndefined()
// Numbers
expect(0.1 + 0.2).toBeCloseTo(0.3)
expect(value).toBeGreaterThan(5)
// Strings/Arrays
expect(str).toMatch(/pattern/)
expect(str).toContain('substring')
expect(array).toContain(item)
expect(array).toHaveLength(3)
// Objects
expect(obj).toHaveProperty('key')
expect(obj).toHaveProperty('nested.key', 'value')
expect(obj).toMatchObject({ subset: 'of properties' })
// Exceptions
expect(() => fn()).toThrow()
expect(() => fn()).toThrow('error message')
expect(() => fn()).toThrow(/pattern/)

Async Testing

// Async/await (preferred)
it('fetches data', async () => {
  const data = await fetchData()
  expect(data).toEqual({ id: 1 })
})
// Promise matchers - ALWAYS await these
await expect(fetchData()).resolves.toEqual({ id: 1 })
await expect(fetchData()).rejects.toThrow('Error')
// Wrong - creates false positive
expect(promise).resolves.toBe(value)  // Missing await!

Quick Mock Reference

const mockFn = vi.fn()
mockFn.mockReturnValue(42)
mockFn.mockResolvedValue({ data: 'value' })
expect(mockFn).toHaveBeenCalled()
expect(mockFn).toHaveBeenCalledWith('arg1', 'arg2')
expect(mockFn).toHaveBeenCalledTimes(2)

Additional Documentation

Mocking: See references/mocking.md for module mocking, spying, cleanup

Configuration: See references/config.md for vitest.config, setup files, coverage

Patterns: See references/patterns.md for timers, snapshots, anti-patterns

Test Methods Quick Reference

| Method | Purpose | |--------|---------| | it() / test() | Define test | | describe() | Group tests | | beforeEach() / afterEach() | Per-test hooks | | beforeAll() / afterAll() | Per-suite hooks | | .skip | Skip test/suite | | .only | Run only this | | .todo | Placeholder | | .concurrent | Parallel execution | | .each([...]) | Parameterized tests |