Vitest Testing

A solid reference for Vitest that covers the essentials without the fluff. You get common assertions, async testing patterns with the critical await reminders, and a quick mock reference. The verification gates are actually useful, especially the async matcher check since forgetting await on resolves/rejects creates silent false positives. It splits deeper topics like module mocking and configuration into separate references, which keeps the main doc scannable. Best used when you're writing tests and need to confirm syntax or catch yourself before making the usual mistakes. Not a tutorial, just the patterns you'd otherwise grep the docs for.

skills.sh installs: 471
GitHub stars: 87
last updated: Aug 21, 2026

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Install

$ npx -y skills add existential-birds/beagle --skill vitest-testing --agent claude-code

Installs into .claude/skills of the current project.

Files

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' })

SKILL.md on GitHub