Vitest Testing Skill - Master Reference

Vitest tests follow F.I.R.S.T principles—Fast, Isolated, Repeatable, Self-Checking, Timely—to ensure reliability. Use the AAA pattern (Arrange-Act-Assert) to structure tests clearly, isolate each test with mocks and spies, and keep assertions focused on a single behavior per test for maintainability.

AAA (Arrange-Act-Assert) structures tests in three explicit phases: set up data, execute the function, verify results. BDD (Given-When-Then) expresses the same logic in behavior-focused language. Both are supported in Vitest; choose AAA for unit tests and BDD for integration or acceptance tests to match your team's vocabulary.

Extract pure logic into separate functions, inject dependencies rather than hardcoding them, and avoid side effects in core functions. Vitest's mocking and spying capabilities then let you test each unit in isolation, making refactored code more modular and easier to verify across unit, integration, and API scenarios.

Yes. Vitest supports all five test types with ready-to-use templates for authentication, CRUD, state management, and API testing. The same F.I.R.S.T principles and AAA patterns apply across scenarios, so you maintain consistent test design whether testing a function, component, or full workflow.

Vitest provides built-in patterns for testing async code—await promises, use async/await in test functions, and test error handling by asserting exceptions or rejection states. The Skill includes templates for common async and error scenarios so tests remain isolated and self-checking.

Describe a feature or bug to Claude Code and request Vitest tests using AAA and black-box strategies following F.I.R.S.T principles. The Skill provides agent integration guidance so Claude Code can generate production-ready tests, maintain consistency across your project, and automate test coverage.