Vitest testing

Skill secondsky/claude-skills/plugins/vitest-testing/skills/vitest-testing

Production-ready skills for Claude Code CLI - Cloudflare, React, Tailwind v4, and AI integrations

Install

npx -y skills add secondsky/claude-skills --skill vitest-testing

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Modern TypeScript/JavaScript testing with Vitest. Fast unit and integration tests, native ESM support, Vite-powered HMR, and comprehensive mocking. Use for testing TS/JS projects.

The file declares its own license as MIT. That is the author’s claim about this one file, and it is not the same thing as the license GitHub reports for the repository, which is listed with the other numbers below.

SKILL.md

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Vitest Testing

Expert knowledge for testing JavaScript/TypeScript projects using Vitest - a blazingly fast testing framework powered by Vite.

Quick Start

Installation

# Using Bun (recommended)
bun add -d vitest

# Using npm
npm install -D vitest

Configuration

// vitest.config.ts
import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    globals: true,
    environment: 'node', // or 'jsdom'
    coverage: {
      provider: 'v8',
      reporter: ['text', 'json', 'html'],
      thresholds: { lines: 80, functions: 80, branches: 80 },
    },
    include: ['**/*.{test,spec}.{js,ts,jsx,tsx}'],
  },
})

Running Tests

# Run all tests (prefer bun)
bun test

# Watch mode (default)
bun test --watch

# Run once (CI mode)
bun test --run

# With coverage
bun test --coverage

# Specific file
bun test src/utils/math.test.ts

# Pattern matching
bun test --grep="calculates sum"

# UI mode (interactive)
bun test --ui

# Verbose output
bun test --reporter=verbose

Writing Tests

Basic Structure

import { describe, it, expect, beforeEach, afterEach } from 'vitest'
import { add, subtract } from './math'

describe('Math utilities', () => {
  beforeEach(() => {
    // Setup before each test
  })

  it('adds two numbers correctly', () => {
    expect(add(2, 3)).toBe(5)
  })

  it('subtracts two numbers correctly', () => {
    expect(subtract(5, 3)).toBe(2)
  })
})

Parametrized Tests

describe.each([
  { input: 2, expected: 4 },
  { input: 3, expected: 9 },
])('square function', ({ input, expected }) => {
  it(`squares ${input} to ${expected}`, () => {
    expect(square(input)).toBe(expected)
  })
})

Assertions

// Equality
expect(value).toBe(expected)
expect(value).toEqual(expected)

// Truthiness
expect(value).toBeTruthy()
expect(value).toBeNull()
expect(value).toBeDefined()

// Numbers
expect(number).toBeGreaterThan(3)
expect(number).toBeCloseTo(0.3, 1)

// Strings/Arrays
expect(string).toMatch(/pattern/)
expect(array).toContain(item)

// Objects
expect(object).toHaveProperty('key')
expect(object).toMatchObject({ a: 1 })

// Exceptions
expect(() => throwError()).toThrow('message')

// Promises
await expect(promise).resolves.toBe(value)
await expect(promise).rejects.toThrow()

Mocking

Function Mocks

import { vi } from 'vitest'

const mockFn = vi.fn()
mockFn.mockReturnValue(42)
mockFn.mockResolvedValue('async result')
mockFn.mockImplementation((x) => x * 2)

expect(mockFn).toHaveBeenCalled()
expect(mockFn).toHaveBeenCalledWith('arg')

Module Mocking

vi.mock('./api', () => ({
  fetchUser: vi.fn(() => ({ id: 1, name: 'Test User' })),
}))

import { fetchUser } from './api'

beforeEach(() => {
  vi.clearAllMocks()
})

Timers

beforeEach(() => vi.useFakeTimers())
afterEach(() => vi.restoreAllMocks())

it('advances timers', () => {
  const callback = vi.fn()
  setTimeout(callback, 1000)
  vi.advanceTimersByTime(1000)
  expect(callback).toHaveBeenCalledOnce()
})

it('mocks dates', () => {
  const date = new Date('2024-01-01')
  vi.setSystemTime(date)
  expect(Date.now()).toBe(date.getTime())
})

Coverage

# Generate coverage report
bun test --coverage

# HTML report
bun test --coverage --coverage.reporter=html
open coverage/index.html

# Check against thresholds
bun test --coverage --coverage.thresholds.lines=90

Integration Testing

import request from 'supertest'
import { app } from './app'

describe('API endpoints', () => {
  it('creates a user', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({ name: 'John' })
      .expect(201)

    expect(response.body).toMatchObject({
      id: expect.any(Number),
      name: 'John',
    })
  })
})

Best Practices

One test file per source file: math.ts → math.test.ts
Group related tests with describe() blocks
Use descriptive test names
Mock only external dependencies
Use concurrent tests for independent async tests
Share expensive fixtures with beforeAll()
Aim for 80%+ coverage but don't chase 100%