/mobile-storage-mmkv

name: mobile-storage-mmkv
description: MMKV high-performance key-value storage for React Native - synchronous JSI-based reads/writes, encryption, typed hooks, multiple instances, listeners, persistence middleware adapters

MMKV Storage Patterns

> **Quick Guide:** Use `createMMKV()` for synchronous key-value storage (~30x faster than AsyncStorage). One singleton instance per concern (global app, per-user). Use typed hooks (`useMMKVString`, `useMMKVObject`) for reactive components. Enable encryption with `encryptionKey` for sensitive data. V4 is a Nitro Module requiring `react-native-nitro-modules` and React Native 0.75+.

---

<critical_requirements>

CRITICAL: Before Using This Skill

> **All code must follow project conventions in CLAUDE.md** (kebab-case, named exports, import ordering, `import type`, named constants)

**(You MUST reuse a single MMKV instance per concern -- NEVER call `createMMKV()` on every render or in component bodies)**

**(You MUST use typed getters (`getString`, `getNumber`, `getBoolean`) -- NEVER parse the return value of the wrong getter)**

**(You MUST use `remove()` to delete keys -- `delete()` was renamed in v4 due to C++ keyword conflict)**

**(You MUST install `react-native-nitro-modules` alongside `react-native-mmkv` -- v4 is a Nitro Module)**

</critical_requirements>

---

**Auto-detection:** MMKV, react-native-mmkv, createMMKV, useMMKVString, useMMKVNumber, useMMKVBoolean, useMMKVObject, useMMKVBuffer, useMMKVListener, useMMKVKeys, addOnValueChangedListener, encryptionKey, mmkv storage, key-value storage React Native

**When to use:**

• Persisting user preferences, auth tokens, or cached data synchronously
• Replacing AsyncStorage for faster reads/writes (~30x improvement)
• Encrypting sensitive data at rest with AES-128 or AES-256
• Sharing storage between iOS app and extensions via App Groups
• Building reactive UIs that re-render on storage changes (hooks)
• Isolating data per user or feature with multiple named instances

**Key patterns covered:**

• Instance creation with `createMMKV()` and configuration options
• Typed getters/setters and object serialization
• React hooks for reactive storage (`useMMKVString`, `useMMKVObject`, etc.)
• Value change listeners (`addOnValueChangedListener`, `useMMKVListener`)
• Multiple instances for data isolation (global vs per-user)
• Encryption at rest (AES-128/AES-256)
• Persistence middleware adapter (generic `StateStorage` interface)
• Migration from AsyncStorage

**When NOT to use:**

• Large binary files or media (use the filesystem)
• Relational or queryable data (use a local database)
• Data that must sync across devices (use a cloud-synced solution)
• Server state caching with invalidation (use your data fetching layer)

**Detailed Resources:**

• [examples/core.md](examples/core.md) - Instance setup, typed access, hooks, listeners
• [examples/advanced.md](examples/advanced.md) - Encryption, multiple instances, App Groups, multi-process, migration
• [examples/persistence.md](examples/persistence.md) - State management persistence adapter, hydration handling
• [reference.md](reference.md) - API reference, V3-to-V4 migration table, migration checklist

---

<philosophy>

Philosophy

MMKV is a **synchronous**, JSI-based key-value store built on top of Tencent's battle-tested C++ library. The key advantage over AsyncStorage is that reads and writes are synchronous -- no `await`, no Promises, no bridge serialization. This eliminates an entire class of race conditions and simplifies code.

**Core principles:**

1. **Synchronous by design** -- `getString()` returns immediately, no async wrappers needed 2. **One instance per concern** -- export a singleton; never create instances inside components 3. **Typed access** -- use the correct getter for the stored type; MMKV does not auto-convert 4. **Encrypt sensitive data** -- tokens, keys, PII should use `encryptionKey` option 5. **Hooks for reactivity** -- `useMMKVString` etc. trigger re-renders on changes, replacing manual subscriptions

**Performance comparison with AsyncStorage:**

| Operation | AsyncStorage | MMKV | Speedup | |----------------|-------------|--------|---------| | Read 1 key | ~5ms | ~0.015ms | ~300x | | Write 1 key | ~8ms | ~0.018ms | ~440x | | Read 1000 keys | ~200ms | ~3ms | ~65x |

Benchmarks vary by device, but MMKV is consistently 30-100x faster for typical operations.

**V4 architecture:** MMKV v4 is a Nitro Module (not a TurboModule). This means it uses `react-native-nitro-modules` for the native bridge, requires React Native 0.75+, and the JS API uses `createMMKV()` instead of `new MMKV()`.

</philosophy>

---

<patterns>

Core Patterns

Pattern 1: Instance Creation and Singleton Export

Create one instance per storage concern at module scope. Never inside a component or hook body.

import { createMMKV } from "react-native-mmkv";

// Global app storage -- reuse this everywhere
export const storage = createMMKV();

// Named instance for user-specific data
export const createUserStorage = (userId: string) =>
  createMMKV({ id: `user-${userId}` });

**Why good:** Module-level creation runs once, all consumers share the same native instance, no wasted allocations

// BAD: Creating instance inside component
function Settings() {
  const storage = createMMKV(); // New native instance every render
  // ...
}

**Why bad:** Creates a new native MMKV instance on every render, wastes memory, defeats instance caching

See [examples/core.md](examples/core.md) for full configuration options (path, encryption, readOnly, compareBeforeSet).

---

Pattern 2: Typed Getters and Setters

MMKV stores values by type. Always use the matching getter for what was stored.

// Set typed values
storage.set("user.name", "Alice");
storage.set("user.age", 28);
storage.set("onboarded", true);

// Get with correct typed getter
const name = storage.getStr