Vitest/jsdom storage failures can require explicit storage shims plus object-level spies

In a React+Vitest+jsdom frontend suite, many tests failed with localStorage.* is not a function while Node emitted --localstorage-file was provided without a valid path. A robust fix was to define in-memory Storage mocks in Vitest setup (setupFiles) for both window and globalThis local/session storage, so tests do not depend on Node's runtime storage behavior. After introducing the shim, tests that spied on Storage.prototype needed to spy on localStorage methods directly because the active implementation was replaced. Verified in a large suite (2k+ tests) on 2026-04.

In Vitest jsdom setup, install a spec-compliant in-memory localStorage/sessionStorage shim in setupFiles and ensure tests spy on the active storage object (localStorage) rather than Storage.prototype when environments may replace the implementation.