vitest

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Vitest testing framework patterns for test setup, async testing, mocking with vi.*, snapshots, and test performance (formerly test-vitest). This skill should be used when writing or debugging Vitest tests. This skill does NOT cover TDD methodology (use test-tdd skill), API mocking with MSW (use test-msw skill), or Jest-specific APIs.

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Sourcethongdn-it/react-agent-skills

Added on2026-03-10

NPX Install

npx skill4agent add thongdn-it/react-agent-skills vitest

SKILL.md Content

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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


async-
setup-
mock-
perf-
snap-
env-
assert-
org-

Priority	Category	Impact	Prefix
1	Async Patterns	CRITICAL	`async-`
2	Test Setup & Isolation	CRITICAL	`setup-`
3	Mocking Patterns	HIGH	`mock-`
4	Performance	HIGH	`perf-`
5	Snapshot Testing	MEDIUM	`snap-`
6	Environment	MEDIUM	`env-`
7	Assertions	LOW-MEDIUM	`assert-`
8	Test Organization	LOW	`org-`

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