Querying and indexing

MMKV provides a simple querying system: you can check whether a key exists and you can list all keys in an instance. On top of that, this library maintains a separate index per data type for every storage instance, so you can also list only the string keys, only the object keys, and so on, or read every value of a given type in one call.

All examples on this page assume an initialized instance:

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

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

WARNING

Indexing can be turned off with new MMKVLoader().disableIndexing().initialize(). With indexing disabled, the type indexers (strings, numbers, booleans, maps, arrays) have nothing to read from and return empty or undefined results. indexer.getKeys() and indexer.hasKey() keep working because they query the storage itself, not the index.

indexer

Available on every instance as MMKV.indexer. It covers all keys in the instance regardless of their data type.

getKeys

Get every key stored in the instance.

const keys = await MMKV.indexer.getKeys();
// => ["user", "settings", "posts"]

Returns: Promise<string[] | undefined | null>

hasKey

Check whether a key exists in the instance.

Name	Type	Required
key	string	yes

if (MMKV.indexer.hasKey('user')) {
  console.log(MMKV.getMap('user'));
}

Returns: boolean | undefined

WARNING

hasKey is synchronous. It returns a boolean, not a promise, so MMKV.indexer.hasKey('user').then(...) throws TypeError: .then is not a function. The same applies to hasKey on every type indexer below.

Type indexers

Each data type has its own indexer, reachable from MMKV.indexer:

Indexer	Data type	Values returned by getAll
MMKV.indexer.strings	string	string
MMKV.indexer.numbers	number	number
MMKV.indexer.booleans	boolean	boolean
MMKV.indexer.maps	object	parsed object, or null
MMKV.indexer.arrays	array	parsed array, or null

They all expose the same three methods.

getKeys

Get all keys of that type.

const stringKeys = await MMKV.indexer.strings.getKeys();
// => ["username", "token"]

const objectKeys = await MMKV.indexer.maps.getKeys();
// => ["user", "settings"]

Returns: Promise<string[] | undefined | null>

hasKey

Check whether a key exists in that type's index. This is a synchronous linear scan over the index, so prefer MMKV.indexer.hasKey when you do not care about the type.

if (MMKV.indexer.maps.hasKey('user')) {
  console.log('user is stored as an object');
}

Returns: boolean | undefined

getAll

Read every key/value pair of that type.

const users = await MMKV.indexer.maps.getAll();
// => [["user", { name: "robert" }], ["admin", { name: "andrew" }]]

Returns: Promise<[key: string, value: T | null | undefined][]>

Each entry is a [key, value] pair: the first item is the key, the second is the value.

const numbers = await MMKV.indexer.numbers.getAll();

for (const [key, value] of numbers) {
  console.log(key, value);
}

TIP

maps.getAll() and arrays.getAll() JSON.parse each stored value. If a value cannot be parsed the entry's value is null, so guard for it before using the result.

getAll on maps and arrays accepts a type parameter so the resulting pairs are typed:

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

const users = await MMKV.indexer.maps.getAll<User>();
// users: [key: string, value: User | null | undefined][]

Listing every value of every type

const dump = async () => {
  const [strings, numbers, booleans, maps, arrays] = await Promise.all([
    MMKV.indexer.strings.getAll(),
    MMKV.indexer.numbers.getAll(),
    MMKV.indexer.booleans.getAll(),
    MMKV.indexer.maps.getAll(),
    MMKV.indexer.arrays.getAll()
  ]);

  return { strings, numbers, booleans, maps, arrays };
};

See also

• Transactions for building your own custom indexes.
• useIndex for loading the values of an index inside a component.