Native modules concept in React Native

Introduction

Native modules let your React Native app use features that are only available in the phone's system. This helps your app do more things by talking directly to the phone's special parts.

You want to use the phone's camera in a way React Native doesn't support by default.

You need to access the phone's sensors like the accelerometer or gyroscope.

You want to use a special feature from iOS or Android that React Native can't do alone.

You want to improve app speed by running some code directly on the phone's system.

You want to reuse existing native code from another app inside your React Native app.

Syntax

React Native

// Example of a native module in JavaScript
import { NativeModules } from 'react-native';
const { MyNativeModule } = NativeModules;

MyNativeModule.myMethod('Hello', (result) => {
  console.log(result);
});

You import native modules from NativeModules in React Native.

Native modules are written in native code (Java/Kotlin for Android, Objective-C/Swift for iOS) and linked to JavaScript.

Examples

This calls a native method to show a toast message on the phone.

React Native

// Calling a native method
MyNativeModule.showToast('Hi!');

This asks the native code for the device name and prints it.

React Native

// Getting a value from native code
MyNativeModule.getDeviceName((name) => {
  console.log('Device name is ' + name);
});

Sample App

This React Native app has a button. When you press it, it calls a native module method to get a welcome message from the phone's native code. Then it shows that message on the screen.

React Native

import React from 'react';
import { View, Button, NativeModules, Text } from 'react-native';

export default function App() {
  const [message, setMessage] = React.useState('');

  const showNativeMessage = () => {
    NativeModules.MyNativeModule.getWelcomeMessage((msg) => {
      setMessage(msg);
    });
  };

  return (
    <View style={{ flex: 1, justifyContent: 'center', alignItems: 'center' }}>
      <Button title="Get Native Message" onPress={showNativeMessage} />
      <Text>{message}</Text>
    </View>
  );
}

Important Notes

Native modules require writing code in native languages and linking it to React Native.

Always test native modules on real devices or emulators because they depend on platform features.

Use native modules only when React Native does not provide the needed feature.

Summary

Native modules connect React Native apps to phone features not available in JavaScript.

They are written in native code and called from JavaScript.

Use them to access special hardware or improve performance.

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