What is AsyncStorage and when should you use it?

AsyncStorage stores strings under string keys. Values must be serialised, usually with JSON. Reads and writes return promises, and the API is global to the app rather than scoped to a screen.

import AsyncStorage from "@react-native-async-storage/async-storage";

const KEY = "settings.v1";

export async function saveSettings(value: { theme: "light" | "dark" }) {
  await AsyncStorage.setItem(KEY, JSON.stringify(value));
}

export async function loadSettings(): Promise<{ theme: "light" | "dark" } | null> {
  const raw = await AsyncStorage.getItem(KEY);
  return raw ? JSON.parse(raw) : null;
}

Good and bad fits

• Good: theme choice, onboarding flag, last selected tab, a small cached profile
• Bad: passwords, tokens, health data or anything requiring encryption
• Bad: large lists or binary blobs, which are slow and memory-heavy
• Better for secrets: Keychain on iOS and Keystore-backed storage on Android
• Better for large structured data: SQLite, MMKV or a file-backed database

AsyncStorage is not transactional. If you write a related pair of keys and the app is killed between writes, you can end up with inconsistent state. Store related values under one key or use a database with transactions.