vitest

This skill is a comprehensive best-practices reference for the Vitest testing framework, the Vite-powered test runner. It packages 44 rules across 8 categories prioritized by impact, aiming to solve the recurring problems of flaky, slow, and unreliable JavaScript/TypeScript test suites. It gives an AI agent or a developer a structured checklist for writing new tests, debugging failures, configuring test environments, and reviewing test code in pull requests.

The content is organized so the highest-leverage areas come first: Async Patterns and Test Setup & Isolation are marked CRITICAL, followed by Mocking Patterns and Performance (HIGH), then Snapshot Testing, Environment configuration, Assertions, and Test Organization. Each rule has a short slug (for example async-await-assertions, setup-restore-mocks, mock-vi-mock-hoisting, perf-happy-dom) and a dedicated reference file with explanations and incorrect-vs-correct code examples. The material covers awaiting async assertions, fake timers, vi.waitFor polling, cleanup in afterEach, restoring mocks, choosing vi.spyOn versus vi.mock, pool selection, sharding, happy-dom over jsdom, inline snapshots, and matcher selection. A companion AGENTS.md compiles everything into one automation-friendly guide.

The target users are frontend and full-stack engineers writing Vitest tests, teams migrating from Jest, and AI coding agents maintaining test suites. Typical use cases include speeding up CI, eliminating flaky async tests, and enforcing consistent testing conventions during code review. It explicitly excludes TDD methodology, MSW-based API mocking, and Jest-specific APIs, deferring those to sibling skills.