MMKV for fast storage in React Native - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when you store and retrieve a value using MMKV?

Consider this React Native code using MMKV storage. What will be the output shown on the screen after pressing the button?

React Native

import React, { useState } from 'react';
import { View, Text, Button } from 'react-native';
import { MMKV } from 'react-native-mmkv';

const storage = new MMKV();

export default function App() {
  const [value, setValue] = useState('');

  const saveAndLoad = () => {
    storage.set('greeting', 'Hello MMKV');
    const stored = storage.getString('greeting');
    setValue(stored ?? '');
  };

  return (
    <View>
      <Button title="Save & Load" onPress={saveAndLoad} />
      <Text>{value}</Text>
    </View>
  );
}

A The text remains empty because MMKV does not save strings.

B The text 'Hello MMKV' appears below the button after pressing it.

C An error occurs because MMKV requires async/await for set and get.

D The app crashes because MMKV is not initialized properly.

💡 Hint

MMKV stores data synchronously and getString returns the saved string immediately.

MMKV's set and getString methods are synchronous. After saving 'Hello MMKV' with set, getString returns it immediately, so the text updates correctly.

❓ lifecycle

intermediate

When is MMKV storage data persisted in React Native?

In a React Native app using MMKV, when is the data saved by MMKV guaranteed to be persisted?

A Immediately after calling storage.set, data is saved to disk synchronously.

B Data is saved only when you call a special save method explicitly.

C Only when the app goes to background, MMKV flushes data to disk asynchronously.

D Data is saved only when the app is closed by the user.

💡 Hint

Think about synchronous vs asynchronous storage methods.

MMKV writes data synchronously to disk when you call set, so data is immediately persisted without waiting for app lifecycle events.

🔧 Debug

advanced

Why does this MMKV getString call return null unexpectedly?

Look at this code snippet: const storage = new MMKV(); storage.set('key', 'value'); const val = storage.getString('Key'); Why does val return null?

A Because MMKV keys are case-sensitive and 'Key' is different from 'key'.

B Because getString requires an async call and this is synchronous.

C Because MMKV does not support string values, only numbers.

D Because the storage instance was not initialized with a namespace.

💡 Hint

Check the spelling and casing of keys carefully.

MMKV keys are case-sensitive. 'key' and 'Key' are different keys, so getString('Key') returns null if only 'key' was set.

🧠 Conceptual

advanced

What is a key advantage of MMKV over AsyncStorage in React Native?

Choose the main advantage MMKV provides compared to AsyncStorage for React Native apps.

A MMKV automatically encrypts all data without configuration.

B MMKV requires no native setup unlike AsyncStorage.

C MMKV offers faster synchronous read/write operations with less CPU usage.

D MMKV supports only JSON objects while AsyncStorage supports strings.

💡 Hint

Think about performance and how data is accessed.

MMKV uses mmap and native code for very fast synchronous storage, making it faster and more efficient than AsyncStorage which is async and slower.

📝 Syntax

expert

Which MMKV usage snippet correctly stores and retrieves a boolean value?

Select the code snippet that correctly saves a boolean and retrieves it using MMKV in React Native.

A

storage.set('flag', 'true');
const val = storage.getBoolean('flag');

B

storage.setBoolean('flag', true);
const val = storage.get('flag');

C

storage.setBool('flag', true);
const val = storage.getBool('flag');

D

storage.set('flag', true);
const val = storage.getBoolean('flag');

💡 Hint

Check the MMKV API method names carefully.

The MMKV API uses set(key, value) for all types and getBoolean(key) to retrieve booleans. Option D uses correct method names and types.