/frontend-js-best-practices

Cache Storage API Calls

localStorage, sessionStorage, and document.cookie are synchronous and expensive. Cache reads in memory.

Incorrect (reads storage on every call):

function getTheme() {
  return localStorage.getItem("theme") ?? "light";
}
// Called 10 times = 10 storage reads

Correct (Map cache):

const storageCache = new Map<string, string | null>();

function getLocalStorage(key: string) {
  if (!storageCache.has(key)) {
    storageCache.set(key, localStorage.getItem(key));
  }
  return storageCache.get(key);
}

function setLocalStorage(key: string, value: string) {
  localStorage.setItem(key, value);
  storageCache.set(key, value); // keep cache in sync
}

Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.

Cookie caching:

let cookieCache: Record<string, string> | null = null;

function getCookie(name: string) {
  if (!cookieCache) {
    cookieCache = Object.fromEntries(
      document.cookie.split("; ").map((c) => c.split("=")),
    );
  }
  return cookieCache[name];
}

Important (invalidate on external changes):

If storage can change externally (another tab, server-set cookies), invalidate cache:

window.addEventListener("storage", (e) => {
  if (e.key) storageCache.delete(e.key);
});

document.addEventListener("visibilitychange", () => {
  if (document.visibilityState === "visible") {
    storageCache.clear();
  }
});