MMKV for fast storage in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is MMKV in React Native?

MMKV is a fast, efficient key-value storage library for React Native apps, designed to store data quickly and persistently on the device.

intermediate

Why use MMKV instead of AsyncStorage?

MMKV is faster and more efficient than AsyncStorage because it uses mmap for memory mapping, reducing read/write latency and improving app performance.

beginner

How do you store a string value using MMKV in React Native?

Use the set method: mmkv.set('key', 'value'); This saves the string 'value' under the key 'key' in fast storage.

beginner

How do you retrieve a stored value from MMKV?

Use the getString method: const value = mmkv.getString('key'); This fetches the string stored under 'key'.

intermediate

What is a key benefit of MMKV's storage method?

MMKV uses memory mapping (mmap) which allows fast access to data without loading the entire file into memory, saving time and battery.

What type of storage does MMKV provide in React Native?

A Key-value storage

B Relational database

C File system storage

D Cloud storage

MMKV provides fast key-value storage for storing small pieces of data efficiently.

Which method stores a string value in MMKV?

A mmkv.saveString()

B mmkv.set()

C mmkv.put()

D mmkv.store()

The set() method is used to store values in MMKV.

How do you retrieve a string value from MMKV?

A mmkv.getString()

B mmkv.fetch()

C mmkv.read()

D mmkv.loadString()

The getString() method fetches a stored string value from MMKV.

What makes MMKV faster than AsyncStorage?

A It uses SQLite internally

B It stores data in the cloud

C It uses memory mapping (mmap)

D It compresses data before saving

MMKV uses mmap to access data quickly without loading entire files.

Which of these is NOT a feature of MMKV?

A Fast read/write

B Key-value API

C Persistent storage

D Supports relational queries

MMKV does not support relational queries; it is a key-value store.

Explain how MMKV improves storage speed in React Native apps.

Think about how MMKV accesses data differently than traditional storage.

Describe the basic steps to save and retrieve a string using MMKV in React Native.

Focus on the methods used for storing and fetching data.

Practice

1. What is the main purpose of using MMKV in a React Native app?

easy

A. To design app layouts

B. To store small data quickly and keep it after app closes

C. To handle network requests

D. To create complex animations

Solution

1. Step 1: Understand MMKV's role

   MMKV is designed for fast storage of small data in apps.

2. Step 2: Compare options

   Options A, B, and C relate to app layouts, animations, and networking, which are not MMKV's purpose.

3. Final Answer:

   To store small data quickly and keep it after app closes -> Option B

4. Quick Check:

   MMKV = fast persistent storage [OK]

Hint: MMKV is for fast data storage, not UI or network [OK]

Common Mistakes:

• Confusing MMKV with UI or animation libraries
• Thinking MMKV handles network calls
• Assuming MMKV is for large file storage

2. Which of the following is the correct way to save a string value using MMKV in React Native?

easy

A. MMKV.set('key', 'value')

B. MMKV.save('key', 'value')

C. MMKV.store('key', 'value')

D. MMKV.put('key', 'value')

Solution

1. Step 1: Recall MMKV save method

   The correct method to save data is set.

2. Step 2: Check other options

   Methods like save, store, and put are not valid MMKV methods.

3. Final Answer:

   MMKV.set('key', 'value') -> Option A

4. Quick Check:

   Use set() to save data [OK]

Hint: Use set() to save data in MMKV [OK]

Common Mistakes:

• Using incorrect method names like save or put
• Confusing MMKV with AsyncStorage syntax
• Forgetting to pass both key and value

3. What will be the output of this code snippet?

const MMKV = require('react-native-mmkv').MMKV;
const storage = new MMKV();
storage.set('name', 'Alice');
const result = storage.getString('name');
console.log(result);

medium

A. undefined

B. null

C. 'Alice'

D. Error: getString is not a function

Solution

1. Step 1: Analyze storage.set call

   The code saves the string 'Alice' with key 'name'.

2. Step 2: Analyze storage.getString call

   Retrieves the string stored under 'name', which is 'Alice'.

3. Final Answer:

   'Alice' -> Option C

4. Quick Check:

   getString('name') returns 'Alice' [OK]

Hint: getString returns saved string value [OK]

Common Mistakes:

• Expecting undefined if not awaited
• Confusing getString with get() method
• Assuming getString returns null if key exists

4. Identify the error in this MMKV usage code:

const storage = new MMKV();
storage.set('count', 5);
const value = storage.getString('count');
console.log(value);

medium

A. Using getString to read a number value

B. Missing await before storage.set

C. MMKV constructor requires parameters

D. set method only accepts strings

Solution

1. Step 1: Check data type saved

   The code saves a number 5 using set.

2. Step 2: Check retrieval method

   getString expects a string value, but 'count' stores a number, causing mismatch.

3. Final Answer:

   Using getString to read a number value -> Option A

4. Quick Check:

   getString expects string data [OK]

Hint: Match getString with string data only [OK]

Common Mistakes:

• Trying to getString for non-string data
• Assuming set is async and needs await
• Thinking MMKV constructor needs arguments

5. You want to save a user's theme preference ('dark' or 'light') and retrieve it safely after app restart using MMKV. Which code snippet correctly does this?

hard

A. const storage = new MMKV();
storage.set('theme', 'dark');
const theme = storage.getString('theme');

B. const storage = new MMKV();
storage.set('theme', 'dark');
const theme = storage.get('theme');

C. const storage = new MMKV();
storage.save('theme', 'dark');
const theme = storage.getString('theme');

D. const storage = new MMKV();
storage.set('theme', 'dark');
const theme = storage.getString('theme') ?? 'dark';

Solution

1. Step 1: Save theme preference correctly

   Use set to save the string 'dark' under 'theme'.

2. Step 2: Retrieve with fallback

   Use getString('theme') and fallback to 'dark' if null or undefined using ??.

3. Final Answer:

   const storage = new MMKV(); storage.set('theme', 'dark'); const theme = storage.getString('theme') ?? 'dark'; -> Option D

4. Quick Check:

   Use set() and getString() with nullish fallback [OK]

Hint: Use ?? for safe fallback when reading MMKV data [OK]

Common Mistakes:

• Using non-existent save() method
• Not providing fallback value on getString
• Using get() instead of getString for strings