vitest-testing

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Vitest test runner for JavaScript and TypeScript. Fast, modern alternative to Jest. Vite-native, ESM support, watch mode, UI mode, coverage, mocking, snapshot testing. Use when setting up tests for Vite projects, migrating from Jest, or needing fast test execution.

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Sourcelaurigates/claude-plugins

Added on2026-02-20

NPX Install

npx skill4agent add laurigates/claude-plugins vitest-testing

SKILL.md Content

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Vitest Testing

Vitest is a modern test runner designed for Vite projects. It's fast, ESM-native, and provides a Jest-compatible API with better TypeScript support and instant HMR-powered watch mode.

When to Use This Skill

Use this skill when...	Use another skill instead when...
Setting up or configuring Vitest	Writing E2E browser tests (use playwright-testing)
Writing unit/integration tests in TS/JS	Testing Python code (use python-testing)
Migrating from Jest to Vitest	Analyzing test quality (use test-quality-analysis)
Configuring coverage thresholds	Generating property-based tests (use property-based-testing)
Using mocks, spies, or fake timers	Validating test effectiveness (use mutation-testing)

Core Expertise

Vite-native: Reuses Vite config, transforms, and plugins
Fast: Instant feedback with HMR-powered watch mode
Jest-compatible: Drop-in replacement with similar API
TypeScript: First-class TypeScript support
ESM: Native ESM support, no transpilation needed

Installation

bash

bun add --dev vitest
bun add --dev @vitest/coverage-v8      # Coverage (recommended)
bun add --dev happy-dom                # DOM testing (optional)
bunx vitest --version                  # Verify

Configuration (vitest.config.ts)

typescript

import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    globals: true,
    environment: 'node',
  },
});

Essential Commands

bash

bunx vitest                            # Watch mode (default)
bunx vitest run                        # Run once (CI mode)
bunx vitest --coverage                 # With coverage
bunx vitest src/utils.test.ts          # Specific file
bunx vitest -t "should add numbers"    # Filter by name
bunx vitest related src/utils.ts       # Related tests
bunx vitest -u                         # Update snapshots
bunx vitest bench                      # Benchmarks
bunx vitest --ui                       # UI mode

Writing Tests

Basic Test Structure

typescript

import { describe, it, expect } from 'vitest';
import { add, multiply } from './math';

describe('math utils', () => {
  it('should add two numbers', () => {
    expect(add(2, 3)).toBe(5);
  });

  it('should multiply two numbers', () => {
    expect(multiply(2, 3)).toBe(6);
  });
});

Key Assertions

Assertion	Description
`toBe(value)`	Strict equality
`toEqual(value)`	Deep equality
`toStrictEqual(value)`	Deep strict equality
`toBeTruthy()` / `toBeFalsy()`	Truthiness
`toBeNull()` / `toBeUndefined()`	Null checks
`toBeGreaterThan(n)` / `toBeLessThan(n)`	Numeric comparison
`toBeCloseTo(n)`	Float comparison
`toMatch(regex)` / `toContain(str)`	String matching
`toHaveLength(n)`	Array/string length
`toHaveProperty(key)`	Object property
`toMatchObject(obj)`	Partial object match
`toThrow(msg)`	Error throwing

Async Tests

typescript

test('async test', async () => {
  const data = await fetchData();
  expect(data).toBe('expected');
});

test('promise resolves', async () => {
  await expect(fetchData()).resolves.toBe('expected');
});

test('promise rejects', async () => {
  await expect(fetchBadData()).rejects.toThrow('error');
});

Mocking (Essential Patterns)

typescript

import { vi, test, expect } from 'vitest';

// Mock function
const mockFn = vi.fn();
mockFn.mockReturnValue(42);

// Mock module
vi.mock('./api', () => ({
  fetchUser: vi.fn(() => Promise.resolve({ id: 1, name: 'John' })),
}));

// Mock timers
vi.useFakeTimers();
vi.advanceTimersByTime(1000);
vi.restoreAllMocks();

// Spy on method
const spy = vi.spyOn(object, 'method');

Snapshot Testing

typescript

test('snapshot test', () => {
  expect(data).toMatchSnapshot();
});

test('inline snapshot', () => {
  expect(result).toMatchInlineSnapshot('5');
});
// Update snapshots: bunx vitest -u

Coverage

bash

bun add --dev @vitest/coverage-v8
bunx vitest --coverage

Key config options:

provider

,

reporter

,

include

,

exclude

,

thresholds

.

Agentic Optimizations

Context	Command
Quick test	`bunx vitest --reporter=dot --bail=1`
CI test	`bunx vitest run --reporter=junit`
Coverage check	`bunx vitest --coverage --reporter=dot`
Single file	`bunx vitest run src/utils.test.ts --reporter=dot`
Failed only	`bunx vitest --changed --bail=1`

For detailed examples, advanced patterns, and best practices, see REFERENCE.md.

References

Official docs: https://vitest.dev
Configuration: https://vitest.dev/config/
API reference: https://vitest.dev/api/
Migration from Jest: https://vitest.dev/guide/migration.html
Coverage: https://vitest.dev/guide/coverage.html

vitest-testing

NPX Install

Tags

SKILL.md Content

Vitest Testing

When to Use This Skill

Core Expertise

Installation

Configuration (vitest.config.ts)

Essential Commands

Writing Tests

Basic Test Structure

Key Assertions

Async Tests

Mocking (Essential Patterns)

Snapshot Testing

Coverage

Agentic Optimizations

References