Native modules concept in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a native module in React Native?

A native module is a piece of code written in native platform languages (like Java for Android or Swift/Objective-C for iOS) that can be called from React Native JavaScript code to access platform-specific features.

beginner

Why do we use native modules in React Native apps?

We use native modules to access device features or APIs that are not available in React Native's JavaScript environment, like sensors, camera controls, or advanced system functions.

beginner

Which languages are used to write native modules for Android and iOS?

Android native modules are usually written in Java or Kotlin, while iOS native modules are written in Objective-C or Swift.

intermediate

How does React Native communicate with native modules?

React Native uses a bridge to send messages between JavaScript and native code asynchronously, allowing JavaScript to call native functions and receive results.

beginner

What is a common use case for creating a native module?

A common use case is when you need to use a device feature not supported by React Native, like accessing a custom Bluetooth API or advanced camera controls.

What language is typically used to write native modules for iOS in React Native?

A Java

B JavaScript

C Swift

D Kotlin

Swift (or Objective-C) is used for iOS native modules, while Java or Kotlin is used for Android.

Why might you create a native module in a React Native app?

A To replace React Native entirely

B To improve JavaScript performance

C To write UI components in native code

D To access platform-specific features not available in React Native

Native modules let you access device features or APIs that React Native does not support directly.

How does React Native communicate with native modules?

A Using a bridge asynchronously

B Direct synchronous calls

C Through REST API calls

D By embedding native code in JavaScript

React Native uses a bridge to communicate asynchronously between JavaScript and native code.

Which of these is NOT a typical language for writing native modules?

A Python

B Kotlin

C Objective-C

D Java

Python is not used for native modules in React Native; native modules use platform languages like Java, Kotlin, Objective-C, or Swift.

What is the main benefit of using native modules?

A Faster JavaScript execution

B Access to native device features

C Simpler app design

D No need for native code knowledge

Native modules allow React Native apps to access native device features not available in JavaScript.

Explain what a native module is and why it is important in React Native development.

Think about how JavaScript talks to native code to use device features.

Describe the process of creating and using a native module in a React Native app.

Consider the steps from native code to JavaScript usage.

Practice

1. What is the main purpose of native modules in React Native?

easy

A. To access device features not available in JavaScript

B. To style components using CSS

C. To manage app navigation

D. To write UI components in JavaScript

Solution

1. Step 1: Understand React Native limitations

   JavaScript in React Native cannot access all device hardware or OS features directly.

2. Step 2: Role of native modules

   Native modules are written in native code to bridge this gap and provide access to those features.

3. Final Answer:

   To access device features not available in JavaScript -> Option A

4. Quick Check:

   Native modules = access device features [OK]

Hint: Native modules bridge JavaScript to device features [OK]

Common Mistakes:

• Thinking native modules are for styling
• Confusing native modules with navigation libraries
• Believing native modules are UI components

2. Which syntax correctly imports a native module named CameraModule in React Native?

easy

A. import CameraModule from 'react-native';

B. import { NativeModules } from 'react-native'; const { CameraModule } = NativeModules;

C. const CameraModule = require('CameraModule');

D. import CameraModule from 'NativeModules';

Solution

1. Step 1: Recall native module import pattern

   Native modules are accessed via the NativeModules object from 'react-native'.

2. Step 2: Correct destructuring syntax

   We import NativeModules, then extract CameraModule from it using const { CameraModule } = NativeModules.

3. Final Answer:

   import { NativeModules } from 'react-native'; const { CameraModule } = NativeModules; -> Option B

4. Quick Check:

   NativeModules destructuring = correct import [OK]

Hint: Always import NativeModules then destructure [OK]

Common Mistakes:

• Importing native module directly from 'react-native'
• Using require without NativeModules
• Wrong import source string

3. Given this code snippet, what will be logged if the native module method getBatteryLevel returns 85 asynchronously?

import { NativeModules } from 'react-native';
const { BatteryModule } = NativeModules;

BatteryModule.getBatteryLevel().then(level => {
  console.log('Battery level is: ' + level);
});

medium

A. Error: getBatteryLevel is not a function

B. Battery level is: undefined

C. Battery level is: [object Promise]

D. Battery level is: 85

Solution

1. Step 1: Understand asynchronous method call

   getBatteryLevel returns a Promise that resolves to 85.

2. Step 2: Analyze the .then callback

   The callback receives the resolved value 85 and logs 'Battery level is: 85'.

3. Final Answer:

   Battery level is: 85 -> Option D

4. Quick Check:

   Promise resolves to 85, logged correctly [OK]

Hint: Promises log resolved value inside .then [OK]

Common Mistakes:

• Logging the Promise object instead of resolved value
• Assuming synchronous return
• Confusing undefined with error

4. What is wrong with this native module usage code?

import { NativeModules } from 'react-native';
const { LocationModule } = NativeModules;

const location = LocationModule.getCurrentLocation();
console.log(location);

medium

A. getCurrentLocation likely returns a Promise but is used synchronously

B. getCurrentLocation is not imported correctly

C. NativeModules cannot be destructured

D. console.log cannot print location objects

Solution

1. Step 1: Identify asynchronous method usage

   Native module methods like getCurrentLocation usually return Promises.

2. Step 2: Check synchronous usage

   The code assigns the Promise to location and logs it immediately, not waiting for resolution.

3. Final Answer:

   getCurrentLocation likely returns a Promise but is used synchronously -> Option A

4. Quick Check:

   Async method used without await or .then [OK]

Hint: Async native methods need await or .then [OK]

Common Mistakes:

• Ignoring Promise nature of native methods
• Trying to destructure NativeModules incorrectly
• Assuming console.log can't print objects

5. You want to create a native module to access the phone's flashlight. Which steps are necessary to correctly use this native module in your React Native app?

hard

A. Write native code, but no need to register module or import it in JavaScript

B. Write JavaScript code only, import flashlight from 'react-native', call methods synchronously

C. Write native code for flashlight, register module, import NativeModules, call methods asynchronously

D. Use only React Native UI components, no native code needed

Solution

1. Step 1: Develop native code and register module

   You must write native code (Java/Swift/Obj-C) to control flashlight and register it with React Native.

2. Step 2: Import and call from JavaScript

   Import NativeModules, extract your flashlight module, and call its methods asynchronously.

3. Final Answer:

   Write native code for flashlight, register module, import NativeModules, call methods asynchronously -> Option C

4. Quick Check:

   Native code + registration + async calls = correct usage [OK]

Hint: Native module needs native code, registration, async JS calls [OK]

Common Mistakes:

• Skipping native code or registration
• Calling native methods synchronously
• Assuming React Native UI covers hardware access