Js Cache Storage

---
name: js-cache-storage
description: Cache Storage API Calls. Use when you need help with js cache storage.
license: CC-BY-NC-SA-4.0
metadata:
  risk: unknown
  source: community
  kind: mode
  category: rules
---

## Cache Storage API Calls

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

**Incorrect (reads storage on every call):**

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

**Correct (Map cache):**

```typescript
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:**

```typescript
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:

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

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