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 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 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.

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
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
React hooks (useMMKVString, useMMKVNumber, useMMKVBoolean, useMMKVBuffer, useMMKVObject, useMMKV, useMMKVListener, useMMKVKeys), reactive UI	references/hooks.md
Listening for value changes outside of React, addOnValueChangedListener, cleaning up listeners	references/listeners.md
Encryption (encrypt, decrypt, recrypt), AES-128 vs AES-256, encryptionKey config	references/encryption.md
Migrating from AsyncStorage, data transfer pattern, completion flag	references/migrate-from-async-storage.md
zustand persist middleware, redux-persist, jotai atomWithMMKV, react-query persister, MobX	references/state-management.md
V4 upgrade, breaking changes from V3, new MMKV() → createMMKV(), .delete() → .remove()	references/upgrade-v4.md
Storage size limits, memory warnings, when to use a database instead	references/limits-and-gotchas.md

If the question doesn't match any row, read references/create-instance.md first — most setup questions start there.

Installation

npm install react-native-mmkv react-native-nitro-modules
cd ios && pod install

Expo:

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

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	createMMKV() factory, all configuration options
crud-operations.md	Set/get all types, key management, objects, buffers
hooks.md	All React hooks for reactive MMKV usage
listeners.md	addOnValueChangedListener, cleanup
encryption.md	Encrypt/decrypt instances, AES options
migrate-from-async-storage.md	Full migration pattern from AsyncStorage
state-management.md	zustand, redux-persist, jotai, react-query, MobX
upgrade-v4.md	V3 → V4 breaking changes and migration
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	Create instance with createMMKV()
Need reactive UI updates	hooks.md	Use useMMKVString / useMMKVObject etc.
Encrypting sensitive data	encryption.md	Call storage.encrypt(key)
App uses AsyncStorage, want to switch	migrate-from-async-storage.md	Batch migrate then swap calls
Zustand/Redux/Jotai persistence	state-management.md	Wire adapter for your state lib
Upgrading from V3 to V4	upgrade-v4.md	new MMKV() → createMMKV(), .delete() → .remove()
Storage growing too large	limits-and-gotchas.md	Trim or move to SQLite
Multi-process data sharing	limits-and-gotchas.md	Set mode: 'multi-process'