Vitest Test Runner Patterns

Quick Guide: Vitest is a fast, Vite-native test runner. Use describe/it/expect for test structure, vi.fn() for mocks, vi.mock() for module mocking, vi.spyOn() for spying. Co-locate tests with code. Use network-level API mocking over module-level mocks. Vitest v4 is current stable (requires Vite 6+, Node 20+).

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<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST co-locate tests with code in feature-based structure - NOT in separate test directories)

(You MUST use network-level API mocking - NOT module-level mocks where possible)

(You MUST use named constants for test data - no magic strings or numbers in test files)

(You MUST use vi.fn() for mocks and vi.spyOn() for spying - NEVER manually replace functions)

(You MUST use the v3+ test options syntax: test("name", { timeout: 10_000 }, () => {}) - NOT as third argument)

</critical_requirements>

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Auto-detection: Vitest, vi.fn, vi.mock, vi.spyOn, describe, it, expect, test, beforeEach, afterEach, vi.useFakeTimers, vitest.config, defineConfig, coverage, snapshot, mockResolvedValue, mockRejectedValue

When to use:

• Configuring and running tests with Vitest
• Writing unit tests for pure functions
• Writing integration tests with network-level mocking
• Mocking modules, functions, and timers
• Snapshot testing
• Configuring coverage, workspaces, and projects

When NOT to use:

• E2E browser testing (use your E2E testing tool)
• Component rendering and querying (use your component testing library)
• Testing third-party library behavior (library already has tests)
• Testing TypeScript compile-time guarantees (TypeScript already enforces)

Key patterns covered:

• Test structure and organization (describe, it, expect)
• Mocking (vi.fn, vi.mock, vi.spyOn, vi.mockObject)
• Fake timers (vi.useFakeTimers, vi.advanceTimersByTime)
• Network-level API mocking for integration tests
• Feature-based test organization (co-located with code)
• Configuration (vitest.config, projects, coverage)
• Vitest v3/v4 migration and breaking changes

Detailed Resources:

• examples/core.md - Unit test examples, pure functions
• examples/integration.md - Integration tests with network-level mocking
• examples/anti-patterns.md - What NOT to test
• reference.md - Vitest v3/v4 migration notes, decision frameworks

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Philosophy

Vitest is a Vite-native test runner designed for speed and developer experience. It provides Jest-compatible APIs with native ESM support, TypeScript out of the box, and Vite's transform pipeline.

Core Principles:

1. Co-locate tests with code - Tests live next to the code they test for discoverability and maintenance
2. Network-level mocking over module mocking - Mock at the HTTP boundary, not at import boundaries
3. Named constants for all test data - No magic strings or numbers in test files
4. Test behavior, not implementation - Focus on inputs and outputs, not internal state
5. Fast feedback loops - Leverage Vitest's HMR-based watch mode for instant re-runs

When to use Vitest:

• Unit testing pure functions (business logic, utilities, formatters)
• Integration testing with network-level API mocking
• Snapshot testing for serializable outputs
• Any test that doesn't require a real browser

When NOT to use Vitest:

• Browser-based E2E testing (use your E2E testing tool)
• Visual regression testing (use your visual testing tool)

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Core Patterns

Pattern 1: Unit Testing Pure Functions

Write unit tests for pure functions with no side effects. Focus on clear input/output behavior.

What to test:

• Pure functions with clear input -> output
• Business logic calculations (pricing, taxes, discounts)
• Data transformations and formatters
• Edge cases and boundary conditions

See examples/core.md for pure function test examples.

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Pattern 2: Integration Testing with Network-Level Mocking

Use Vitest with network-level API mocking to test components with their API integration layer.

When Integration Tests Make Sense:

• Component behavior in isolation (form validation, UI state)
• Testing edge cases that are hard to reproduce in E2E
• Development workflow (faster than spinning up full stack)

Key Pattern:

• Tests in __tests__/ directories co-located with code
• Network-level API mocking (intercepts HTTP requests)
• Centralized mock data in shared package
• Test all states: loading, empty, error, success

See examples/integration.md for integration test examples.

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Pattern 3: Feature-Based Test Organization

Co-locate tests with code in feature-based structure. Tests live next to what they test.

Direct Co-location (Recommended):

src/
  features/
    auth/
      components/
        login-form.tsx
        login-form.test.tsx        # Test next to component
      hooks/
        use-auth.ts
        use-auth.test.ts           # Test next to hook

Alternative: __tests__/ Subdirectories:

src/features/auth/
  components/
    login-form.tsx
    __tests__/
      login-form.test.tsx

E2E Test Organization:

tests/
  e2e/
    auth/
      login-flow.spec.ts
      register-flow.spec.ts
    checkout/
      checkout-flow.spec.ts

File Naming Convention:

• *.test.tsx / *.test.ts for unit and integration tests

Choose one pattern and be consistent across the codebase.

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<red_flags>

RED FLAGS

High Priority Issues:

• Module-level mocking (vi.mock) instead of network-level - breaks when import structure changes, doesn't test serialization
• Only testing happy paths - error states go untested until users report them
• Not using named constants for test data - magic strings and numbers make tests unreadable

Medium Priority Issues:

• Mocks that don't match real API - tests pass but production fails because mocks drifted
• Complex mocking setup - simplify or test at a higher level
• Not resetting mocks between tests - causes cross-test pollution

Gotchas & Edge Cases:

• Network mock handlers are typically global - reset handlers after each test to prevent pollution
• Vitest v3+: Test options must be second argument: test("name", { timeout: 10_000 }, () => {}) NOT test("name", () => {}, { timeout: 10_000 })
• Vitest v4: Multiple mock behavior changes (getMockName, restoreAllMocks, automocked getters) - see reference.md for full v4 migration notes
• vi.fn().mock.invocationCallOrder starts at 1 in v4 instead of 0
• vi.restoreAllMocks() only affects manual spies in v4, not automocks - use vi.resetAllMocks() for full reset

</red_flags>

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<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md

(You MUST co-locate tests with code in feature-based structure - NOT in separate test directories)

(You MUST use network-level API mocking - NOT module-level mocks where possible)

(You MUST use named constants for test data - no magic strings or numbers in test files)

(You MUST use vi.fn() for mocks and vi.spyOn() for spying - NEVER manually replace functions)

(You MUST use the v3+ test options syntax: test("name", { timeout: 10_000 }, () => {}) - NOT as third argument)

Failure to follow these rules will result in fragile tests that break on refactoring and false confidence from poorly structured test suites.

</critical_reminders>