In Vitest jsdom, globalThis === window, defining storage shims on both is a redundant overwrite

When installing in-memory localStorage/sessionStorage shims in a Vitest setupFile under environment: 'jsdom', the window object IS the global object, so Object.defineProperty(globalThis, 'localStorage', ...) and Object.defineProperty(window, 'localStorage', ...) write to the same property on the same object. The second call simply overwrites the first (when configurable: true). Some existing guidance recommends installing on both for safety; in a jsdom-only test environment that's pure verbosity. Defining on globalThis alone covers both names. Verified via Vitest 4.x + jsdom in 2026-05; behavior is a stable property of the jsdom Window-as-global design, not a version-specific quirk. Verify in any test environment by checking window === globalThis in setup.

In a Vitest setupFile that targets jsdom, install Storage (or any window-level) shims on globalThis only. Skip the parallel window.localStorage = … definition. If your test config supports multiple environments (jsdom + node, happy-dom, etc.), gate the install on the environment or feature-detect with typeof window !== 'undefined' rather than blindly writing to both names.