Vitest Best Practices

Comprehensive performance optimization and best practices guide for Vitest testing framework. Contains 44 rules across 8 categories, prioritized by impact to guide test writing, refactoring, and code review.

When to Apply

Reference these guidelines when:

• •Writing new Vitest tests
• •Debugging flaky or slow tests
• •Setting up test configuration
• •Reviewing test code in PRs
• •Migrating from Jest to Vitest
• •Optimizing CI/CD test performance

Rule Categories by Priority

Quick Reference

1. Async Patterns (CRITICAL)

• •async-await-assertions - Await async assertions to prevent false positives
• •async-return-promises - Return promises from test functions
• •async-fake-timers - Use fake timers for time-dependent code
• •async-waitfor-polling - Use vi.waitFor for async conditions
• •async-concurrent-expect - Use test context expect in concurrent tests
• •async-act-wrapper - Await user events to avoid act warnings
• •async-error-handling - Test async error handling properly

2. Test Setup & Isolation (CRITICAL)

• •setup-beforeeach-cleanup - Clean up state in afterEach hooks
• •setup-restore-mocks - Restore mocks after each test
• •setup-avoid-shared-state - Avoid shared mutable state between tests
• •setup-beforeall-expensive - Use beforeAll for expensive one-time setup
• •setup-reset-modules - Reset modules when testing module state
• •setup-test-factories - Use test factories for complex test data

3. Mocking Patterns (HIGH)

• •mock-vi-mock-hoisting - Understand vi.mock hoisting behavior
• •mock-spyon-vs-mock - Choose vi.spyOn vs vi.mock appropriately
• •mock-implementation-not-value - Use mockImplementation for dynamic mocks
• •mock-msw-network - Use MSW for network request mocking
• •mock-avoid-overmocking - Avoid over-mocking
• •mock-type-safety - Maintain type safety in mocks
• •mock-clear-between-tests - Clear mock state between tests

4. Performance (HIGH)

• •perf-pool-selection - Choose the right pool for performance
• •perf-disable-isolation - Disable test isolation when safe
• •perf-happy-dom - Use happy-dom over jsdom when possible
• •perf-sharding - Use sharding for CI parallelization
• •perf-run-mode-ci - Use run mode in CI environments
• •perf-bail-fast-fail - Use bail for fast failure in CI

5. Snapshot Testing (MEDIUM)

• •snap-inline-over-file - Prefer inline snapshots for small values
• •snap-avoid-large - Avoid large snapshots
• •snap-stable-serialization - Ensure stable snapshot serialization
• •snap-review-updates - Review snapshot updates before committing
• •snap-describe-intent - Name snapshot tests descriptively

6. Environment (MEDIUM)

• •env-per-file-override - Override environment per file when needed
• •env-setup-files - Use setup files for global configuration
• •env-globals-config - Configure globals consistently
• •env-browser-api-mocking - Mock browser APIs not available in test environment

7. Assertions (LOW-MEDIUM)

• •assert-specific-matchers - Use specific matchers over generic ones
• •assert-edge-cases - Test edge cases and boundaries
• •assert-one-assertion-concept - Test one concept per test
• •assert-expect-assertions - Use expect.assertions for async tests
• •assert-toequal-vs-tobe - Choose toBe vs toEqual correctly

8. Test Organization (LOW)

• •org-file-colocation - Colocate test files with source files
• •org-describe-nesting - Use describe blocks for logical grouping
• •org-test-naming - Write descriptive test names
• •org-test-skip-only - Use skip and only appropriately

How to Use

Read individual reference files for detailed explanations and code examples:

• •Section definitions - Category structure and impact levels
• •Rule template - Template for adding new rules
• •async-await-assertions - Example rule file
• •mock-vi-mock-hoisting - Example rule file

Related Skills

• •For TDD methodology, see test-tdd skill
• •For API mocking with MSW, see test-msw skill
• •For TypeScript testing patterns, see typescript skill

Full Compiled Document

For the complete guide with all rules expanded: AGENTS.md