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)

Verification gates

Use this sequence when you add or change tests; each step has an objective pass condition.

1. Run the test suite — From the package or workspace root, run the same command CI uses (check package.json scripts; often vitest run, pnpm test, or npm test). Pass: exit code is 0 and the report shows zero failing tests.
2. Async matchers — Pass: every expect(…).resolves and expect(…).rejects is prefixed with await (await expect(...)), as in Async Testing. A line where resolves or rejects appears without await fails this gate.

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