react-native-mmkv

# react-native-mmkv A focused reference for AI coding assistants working in a project that uses `react-native-mmkv`. Answer using the real APIs from this library — not invented ones — by routing to the matching `references/*.md` file below. ## Mental model `react-native-mmkv` is a high-performance, synchronous key-value storage library for React Native, backed by Tencent's [MMKV](https://github.com/Tencent/MMKV) C++ engine via Nitro Modules (JSI). It is ~30x faster than AsyncStorage. The three things that trip people up: 1. **Everything is synchronous.** No `await`, no Promises. `storage.getString('key')` returns the value immediately. This is the whole point — MMKV memory-maps files and reads/writes in-process. 2. **It's a key-value store, not a database.** All data is cached in memory. Keep total size under ~100 MB; for larger datasets use [react-native-nitro-sqlite](https://github.com/margelo/react-native-nitro-sqlite) or WatermelonDB. There is no fixed storage limit — but memory warnings will fire if you go too large. 3. **V4 is a breaking rewrite.** The constructor changed from `new MMKV()` to `createMMKV()`, `.delete()` became `.remove()`, and `react-native-nitro-modules` is now a required peer dependency. See the [upgrade guide](./references/upgrade-v4.md). > **V4 requires React Native 0.75+.** It works on both the New Architecture and the old architecture (Nitro is backwards compatible). ## Routing table — problem to reference Load the matching file from `references/` before writing code. Each reference cites real APIs from the library. | User is asking about… | Read | |---|---| | Creating an MMKV instance, configuration options (`id`, `path`, `encryptionKey`, `mode`, `readOnly`, `compareBeforeSet`), the `createMMKV()` factory | [`references/create-instance.md`](./references/create-instance.md) | | Getting/setting values, all data types (string, number, boolean, buffer, objects via JSON), key management (`contains`, `getAllKeys`, `remove`, `clearAll`), `size`, `trim`, `importAllFrom` | [`references/crud-operations.md`](./references/crud-operations.md) | | React hooks (`useMMKVString`, `useMMKVNumber`, `useMMKVBoolean`, `useMMKVBuffer`, `useMMKVObject`, `useMMKV`, `useMMKVListener`, `useMMKVKeys`), reactive UI | [`references/hooks.md`](./references/hooks.md) | | Listening for value changes outside of React, `addOnValueChangedListener`, cleaning up listeners | [`references/listeners.md`](./references/listeners.md) | | Encryption (`encrypt`, `decrypt`, `recrypt`), AES-128 vs AES-256, `encryptionKey` config | [`references/encryption.md`](./references/encryption.md) | | Migrating from AsyncStorage, data transfer pattern, completion flag | [`references/migrate-from-async-storage.md`](./references/migrate-from-async-storage.md) | | zustand persist middleware, redux-persist, jotai `atomWithMMKV`, react-query persister, MobX | [`references/state-management.md`](./references/state-management.md) | | V4 upgrade, breaking changes from V3, `new MMKV()` → `createMMKV()`, `.delete()` → `.remove()` | [`references/upgrade-v4.md`](./references/upgrade-v4.md) | | Storage size limits, memory warnings, when to use a database instead | [`references/limits-and-gotchas.md`](./references/limits-and-gotchas.md) | If the question doesn't match any row, read [`references/create-instance.md`](./references/create-instance.md) first — most setup questions start there. ## Installation ```bash npm install react-native-mmkv react-native-nitro-modules cd ios && pod install ``` Expo: ```bash npx expo install react-native-mmkv react-native-nitro-modules npx expo prebuild ``` After install, rebuild the app. No additional native wiring is required. ## Quick reference — full MMKV instance API ```ts import { createMMKV, existsMMKV, deleteMMKV } from 'react-native-mmkv' const storage = createMMKV({ id: 'my-store' }) // Properties (read-only) storage.id // string storage.length // number of keys storage.byteSize // total file size in bytes storage.size // (deprecated — use byteSize) storage.isReadOnly // boolean storage.isEncrypted // boolean // Write storage.set('key', 'string' | 42 | true | arrayBuffer) // Read storage.getString('key') // string | undefined storage.getNumber('key') // number | undefined storage.getBoolean('key') // boolean | undefined storage.getBuffer('key') // ArrayBuffer | undefined // Keys storage.contains('key') // boolean storage.getAllKeys() // string[] storage.remove('key') // boolean (NOTE: was .delete() in V3) storage.clearAll() // void // Encryption storage.encrypt('password') storage.encrypt('password', 'AES-256') storage.decrypt() storage.recrypt('newPassword' | undefined) // (deprecated — use encrypt()/decrypt()) // Maintenance storage.trim() // free unused space storage.importAllFrom(otherStorage) // returns count imported // Listeners const sub = storage.addOnValueChangedListener((key) => { ... }) sub.remove() // Static helpers existsMMKV('my-store') // boolean deleteMMKV('my-store') // boolean ``` ## Testing A mocked MMKV instance is automatically used when testing with Jest or Vitest. Use `createMMKV()` as normal in tests — no manual mocking needed. ## Verifying the skill is loaded Good test questions: - *"How do I persist zustand state with MMKV?"* → should produce a `StateStorage` adapter using `storage.set`, `storage.getString`, `storage.remove` — not AsyncStorage wrappers. - *"How do I migrate from AsyncStorage to MMKV?"* → should show the batch-copy pattern with a completion flag, not a drop-in replacement. - *"What's the storage limit?"* → should explain there's no fixed limit, but data is memory-mapped so keep it under ~100 MB; use SQLite for larger datasets. ## References | File | Description | |------|-------------| | [create-instance.md](./references/create-instance.md) | `createMMKV()` factory, all configuration options | | [crud-operations.md](./references/crud-operations.md) | Set/get all types, key management, objects, buffers | | [hooks.md](./references/hooks.md) | All React hooks for reactive MMKV usage | | [listeners.md](./references/listeners.md) | `addOnValueChangedListener`, cleanup | | [encryption.md](./references/encryption.md) | Encrypt/decrypt instances, AES options | | [migrate-from-async-storage.md](./references/migrate-from-async-storage.md) | Full migration pattern from AsyncStorage | | [state-management.md](./references/state-management.md) | zustand, redux-persist, jotai, react-query, MobX | | [upgrade-v4.md](./references/upgrade-v4.md) | V3 → V4 breaking changes and migration | | [limits-and-gotchas.md](./references/limits-and-gotchas.md) | Storage limits, multi-process, logging, common pitfalls | ## Problem → Reference Mapping | Problem | Reference | Action | |---------|-----------|--------| | Don't know where to start | [create-instance.md](./references/create-instance.md) | Create instance with `createMMKV()` | | Need reactive UI updates | [hooks.md](./references/hooks.md) | Use `useMMKVString` / `useMMKVObject` etc. | | Encrypting sensitive data | [encryption.md](./references/encryption.md) | Call `storage.encrypt(key)` | | App uses AsyncStorage, want to switch | [migrate-from-async-storage.md](./references/migrate-from-async-storage.md) | Batch migrate then swap calls | | Zustand/Redux/Jotai persistence | [state-management.md](./references/state-management.md) | Wire adapter for your state lib | | Upgrading from V3 to V4 | [upgrade-v4.md](./references/upgrade-v4.md) | `new MMKV()` → `createMMKV()`, `.delete()` → `.remove()` | | Storage growing too large | [limits-and-gotchas.md](./references/limits-and-gotchas.md) | Trim or move to SQLite | | Multi-process data sharing | [limits-and-gotchas.md](./references/limits-and-gotchas.md) | Set `mode: 'multi-process'` |