vitest-testing-patterns

Vitest unit tests in TypeScript use describe and test blocks to organize assertions. Write test cases by importing your code, setting up inputs, executing the function, and asserting expected outputs. Vitest runs tests in parallel and provides built-in matchers for common assertions.

Mock dependencies in Vitest using vi.mock() to replace modules or vi.fn() to create mock functions. Mocks isolate the unit under test by replacing external calls with controlled stubs, letting you verify interactions and control return values without executing real code.

Spies in Vitest wrap existing functions with vi.spyOn() to monitor calls without replacing implementation. Track how many times a function was called, what arguments it received, and what it returned—useful for verifying side effects and integration points.

Vitest browser mode simulates a real browser environment for testing DOM interactions and browser APIs. Configure browser mode in your Vitest setup to test code that depends on window, document, or other browser globals without a full browser instance.

Configure Vitest by creating a vitest.config.ts file specifying test environment, include/exclude patterns, and module resolution. TypeScript projects typically need environment set to 'node' or 'jsdom', resolver configuration, and transpiler setup to handle .ts files.

Use mocks when you need complete replacement and control over a dependency's behavior; use spies when you want to observe an existing function without changing it. Mocks suit external services; spies suit internal functions where you verify side effects without stubbing the implementation.