React Performance: JS Perf — Cache Storage API. Keywords: localstorage cache read. Platform: React/Next.js. Description: Cache localStorage/sessionStorage reads in memory. Do: Cache storage reads in Map. Don't: Read storage on every call. Good Example: if (!cache.has(key)) cache.set(key, localStorage.getItem(key)). Bad Example: localStorage.getItem('theme') // every call. Severity: Low-Medium.