React Native Global Scope Variables | Initialize once use Everywhere

Global Scope Variables

We all know either we are developing any web or any mobile app, We always need Global Scope Variables. If you are not familiar with the global variable just think about the variables which can be accessed from any screen/activity after first initialization.

They can be used on any screen as local variables but the difference is their scope is global. In React Native we can make any variable Global Scope Variables by just putting global as prefix.

Example of Global Scope Variables

If we have myvar which is a local variable and accessible only from the same class. and if we have global.myvar then it becomes global and can be accessed from any class of the application.

Here is an example to understand it. We have a global.MyVar variable initialized in the first screen which can be accessed from First Screen as well as from the Second Screen.

To Make a React Native App

Getting started with React Native will help you to know more about the way you can make a React Native project. We are going to use react native command line interface to make our React Native App.

If you have previously installed a global react-native-cli package, please remove it as it may cause unexpected issues:

npm uninstall -g react-native-cli @react-native-community/cli

Run the following commands to create a new React Native project

npx react-native init ProjectName

If you want to start a new project with a specific React Native version, you can use the --version argument:

npx react-native init ProjectName --version X.XX.X

Note If the above command is failing, you may have old version of react-native or react-native-cli installed globally on your pc. Try uninstalling the cli and run the cli using npx.

This will make a project structure with an index file named App.js in your project directory.

We are going to use the react-navigation library to switch our screen so you need to install the same.

Installation of Dependencies

To switch between the screens we need to add react-navigation in our application. To install the dependencies open the terminal and jump into your project

cd ProjectName

Now run the following commands to install the dependencies

npm install @react-navigation/native --save

npm install @react-navigation/native-stack --save

npm install react-native-screens react-native-safe-area-context --save

react-native-screens package requires one additional configuration step to properly work on Android devices. Edit MainActivity.java file which is located in android/app/src/main/java/<your package name>/MainActivity.java.

Add the following code to the body of MainActivity class:

@Override
protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(null);
}

and make sure to add the following import statement at the top of this file below your package statement:

import android.os.Bundle;

This change is required to avoid crashes related to View state being not persisted consistently across Activity restarts.
These commands will copy all the dependencies into your node_module directory.

CocoaPods Installation

Please use the following command to install CocoaPods

npx pod-install

Project File Structure

To start with this Example you need to create a directory named pages in your project and create two files FirstPage.js and SecondPage.js

Code

Now Open App.js in any code editor and replace the code with the following code.

App.js

// React Native Global Scope Variables
// https://aboutreact.com/react-native-global-scope-variables/

import React from 'react';

import { NavigationContainer } from '@react-navigation/native';
import { createNativeStackNavigator } from '@react-navigation/native-stack';

import FirstPage from './pages/FirstPage';
import SecondPage from './pages/SecondPage';

const Stack = createNativeStackNavigator();

function App() {
  return (
    <NavigationContainer>
      <Stack.Navigator
        initialRouteName="FirstPage"
        screenOptions = {{
          headerStyle: {backgroundColor: '#f4511e'},
          headerTintColor: '#fff',
        }}
      >
        <Stack.Screen
          name="FirstPage"
          component={FirstPage}
          options={{title: 'First Page'}}
        />
        <Stack.Screen
          name="SecondPage"
          component={SecondPage}
          options={{title: 'Second Page'}}
        />
      </Stack.Navigator>
    </NavigationContainer>
  );
}

export default App;

Open pages/FirstPage.js in any code editor and replace the code with the following code.

FirstPage.js

// React Native Global Scope Variables
// https://aboutreact.com/react-native-global-scope-variables/

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

const FirstPage = ({navigation}) => {
  global.MyVar = 'https://aboutreact.com';

  return (
    <SafeAreaView style={{flex: 1}}>
      <View style={{flex: 1, padding: 16}}>
        <View
          style={{
            flex: 1,
            alignItems: 'center',
            justifyContent: 'center',
          }}>
          <Text
            style={{
              fontSize: 25,
              textAlign: 'center',
              marginBottom: 16,
            }}>
            This is the First Page of the App
          </Text>
          <Text
            style={{
              fontSize: 18,
              textAlign: 'center',
              marginBottom: 16,
              color: 'red',
            }}>
            Value of Global Variable is: {global.MyVar}
          </Text>
          <Button
            onPress={
              () => navigation.navigate('SecondPage')
            }
            title="Go to Second Page"
          />
        </View>
        <Text
          style={{
            fontSize: 18,
            textAlign: 'center',
            color: 'grey'
          }}>
          React Native Global Scope Variables
        </Text>
        <Text
          style={{
            fontSize: 16,
            textAlign: 'center',
            color: 'grey'
          }}>
          www.aboutreact.com
        </Text>
      </View>
    </SafeAreaView>
  );
};

export default FirstPage;

Open pages/SecondPage.js in any code editor and the Replace the code with the following code.

SecondPage.js

// React Native Global Scope Variables
// https://aboutreact.com/react-native-global-scope-variables/

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

const SecondPage = () => {
  return (
    <SafeAreaView style={{flex: 1}}>
      <View style={{flex: 1, padding: 16}}>
        <View
          style={{
            flex: 1,
            alignItems: 'center',
            justifyContent: 'center',
          }}>
          <Text
            style={{
              fontSize: 25,
              textAlign: 'center',
              marginBottom: 16,
            }}>
            This is Second Page of the App
          </Text>
          <Text
            style={{
              fontSize: 18,
              textAlign: 'center',
              marginBottom: 16,
              color: 'red',
            }}>
            Value of Global Variable is: {global.MyVar}
          </Text>
        </View>
        <Text
          style={{
            fontSize: 18,
            textAlign: 'center',
            color: 'grey'
          }}>
          React Native Global Scope Variables
        </Text>
        <Text
          style={{
            fontSize: 16,
            textAlign: 'center',
            color: 'grey'
          }}>
          www.aboutreact.com
        </Text>
      </View>
    </SafeAreaView>
  );
};

export default SecondPage;

To Run the React Native App

Open the terminal again and jump into your project using.

cd ProjectName

1. Start Metro Bundler

First, you will need to start Metro, the JavaScript bundler that ships with React Native. To start Metro bundler run following command

npx react-native start

Once you start Metro Bundler it will run forever on your terminal until you close it. Let Metro Bundler run in its own terminal. Open a new terminal and run the application.

2. Start React Native Application

To run the project on an Android Virtual Device or on real debugging device

npx react-native run-android

or on the iOS Simulator by running (macOS only)

npx react-native run-ios

Output Screenshot

This is how you can make a Global Scope Variables. If you have any doubts or you want to share something about the topic you can comment below or contact us here. There will be more posts coming soon. Stay tuned!

Hope you liked it. 🙂