What is MMKV and how does it compare to AsyncStorage?

react-native-mmkv is a high-performance key-value storage library based on WeChat's MMKV framework (Memory-Mapped Key-Value). It's synchronous, extremely fast, and supports encryption — making it a superior replacement for AsyncStorage in most cases. Why MMKV over AsyncStorage: MMKV reads and writes synchronously (no async/await); uses memory-mapped files — data written to memory, OS maps to disk; 30-100x faster than AsyncStorage in benchmarks; supports encryption (AES-256); no size limits per key. Installation: npm install react-native-mmkv. Usage: import { MMKV } from "react-native-mmkv"; const storage = new MMKV(); // Synchronous reads and writes: storage.set("user.name", "Alice"); storage.set("user.age", 30); storage.set("user.premium", true); const name = storage.getString("user.name"); // "Alice" const age = storage.getNumber("user.age"); // 30 const isPremium = storage.getBoolean("user.premium"); // true // Delete: storage.delete("user.name"); // Check existence: storage.contains("user.name"); // Objects -- must JSON encode: storage.set("user.profile", JSON.stringify(profile)); const profile = JSON.parse(storage.getString("user.profile") ?? "{}"); // Clear all: storage.clearAll(); // Encrypted storage: const secureStorage = new MMKV({ id: "secure-storage", encryptionKey: "my-encryption-key-32-chars-long" });. With Zustand (popular pattern): const useStore = create(persist(storeDefinition, { name: "app-store", storage: createJSONStorage(() => zustandMMKVStorage), }));. When to use AsyncStorage: when synchronous reads are problematic (large stored values blocking JS thread). Use MMKV for almost everything else.