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 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.