Writing and organizing tests

Tests are defined with the test function and assertions with expect, both imported from vitest; it is an alias of test and behaves identically, so teams can pick whichever reads better. Group related tests with describe into named suites — nesting is supported, but keep it shallow because deeply nested tests are harder to read, and a flat list is often enough for simple modules. By default Vitest discovers any file containing .test. or .spec. in its name (the patterns */.test.{ts,js,mjs,cjs,tsx,jsx} and */.spec.{ts,js,mjs,cjs,tsx,jsx}), searching all subdirectories, so tests can live co-located with the source or in a dedicated directory; the include and exclude config options customize discovery. TypeScript works out of the box through Vite's on-the-fly transform with no extra compiler to install, but Vitest does not type-check during the test run — run tsc or vitest typecheck separately when full type checking is needed. Scope: Vitest. Reviewed October 4, 2026.