Data types

Five JavaScript data types can be stored. Each one has a dedicated setter and getter, plus an async variant of both.

Type	Setter	Getter	Notes
string	setString(key, value)	getString(key)	Stored as-is
number	setInt(key, value)	getInt(key)	Any JS number, integer or float
boolean	setBool(key, value)	getBool(key)
object	setMap(key, value)	getMap(key)	Plain object only, JSON serialized
array	setArray(key, value)	getArray(key)	JSON serialized

The JS Map type is not supported — despite the name, setMap takes a plain object.

Setters return boolean | undefined. Passing null or undefined as the value removes the key instead of storing it. Passing a value of the wrong type throws, for example setString('key', 1) throws Trying to set number as a string.

null vs undefined

Getters distinguish two kinds of empty result:

• null — there is no value for this key, or the stored JSON could not be parsed.
• undefined — the instance is not loaded yet, so the read never reached storage.

storage.getString('never-written'); // => null

Strings

import { MMKVLoader } from 'react-native-mmkv-storage';

const storage = new MMKVLoader().initialize();

storage.setString('user.name', 'robert');
storage.getString('user.name'); // => "robert"

Numbers

storage.setInt('user.age', 24);
storage.getInt('user.age'); // => 24

Booleans

storage.setBool('user.isPremium', true);
storage.getBool('user.isPremium'); // => true

Objects

setMap accepts a plain object and stores its JSON representation, so only JSON-serializable values survive the round trip — functions, Symbols, Dates and undefined properties do not.

storage.setMap('user', { name: 'robert', age: 24 });
storage.getMap('user'); // => { name: "robert", age: 24 }

In TypeScript the getter is generic:

type User = { name: string; age: number };

const user = storage.getMap<User>('user');

Arrays

storage.setArray('user.roles', ['admin', 'editor']);
storage.getArray('user.roles'); // => ["admin", "editor"]

const roles = storage.getArray<string>('user.roles');

Async variants

Every setter and getter also has an async form returning a promise: setStringAsync, getStringAsync, setIntAsync, getIntAsync, setBoolAsync, getBoolAsync, setMapAsync, getMapAsync, setArrayAsync, getArrayAsync.

await storage.setStringAsync('user.name', 'robert');
await storage.getStringAsync('user.name'); // => "robert"

MMKV reads and writes are fast enough that the sync API is a good default; the async variants exist mainly so the library can drop into code written against AsyncStorage.