testing-vitest

name	testing-vitest
description	Use when writing or configuring Vitest unit tests in TypeScript or TSX files. Covers mocking patterns, setup files, snapshots, coverage, and DOM testing.

Vitest Guidelines

• Use vi.fn() for function mocks, vi.spyOn() to monitor existing functions, vi.stubGlobal() for global mocks
• Prefer spies over mocks when you only need to verify interactions without changing behavior
• Place vi.mock() factory functions at the top level — the factory runs before imports are processed
• Use mockImplementation() or mockReturnValue() for dynamic control during tests
• Define global mocks, custom matchers, and environment setup in setup files referenced in vitest.config.ts
• Use inline snapshots (toMatchInlineSnapshot()) for readable assertions visible in code review
• Configure coverage thresholds in vitest.config.ts only when asked; focus on meaningful tests over percentages
• Run vitest --watch during development for instant feedback; use -t to filter specific tests
• Use vitest --ui to navigate large test suites visually during development
• Set environment: 'jsdom' for frontend component tests; combine with testing-library for user interaction simulation
• Group related tests with describe blocks; follow Arrange-Act-Assert pattern
• Use expectTypeOf() for type-level assertions; ensure mocks preserve original type signatures