React Native Interview Questions and Answers

The Stylesheet.create function in React Native is used for the following:

• It validates the keys and registers them to React
• It creates a StyleSheet style reference from the specified object.
• It allows you to send the style only once through the bridge while referring to all subsequent users through ID.

Short form for “reference”, Refs are used to access underlying DOM nodes or React elements within a React component.

The ReactJs is a JavaScript library to develop apps in HTML5 while using JavaScript as the developing language. Whereas, React Native is a JavaScript framework used to create native mobile applications while using JavaScript as the development language.

Higher-Order-Component, shortly known as HOC is an advanced React Native technique to reuse the component logic. The function obtains a component and returns a new element.

Example

                                                    
function HOC(Comp) {

     return class NewComp extends Component {

         render() {

            return 

         }

    }

}
                                                

const AppNavigator = createStackNavigator({
      Home: {
          screen: HomeScreen,
      },
});

To install a specific version of React Native, use this following command:

$ react-native init newproject --version [email protected]

Note: In the above, replace VersionNumber with the version of React you want to install.

The map function is used to show a list of elements in an array. It can be used in React Native like the following example:

Example

                                                    
import React, { Component } from "react";

import { Text, View } from "react-native";



export default class mapFunction extends Component {

constructor(props) {

super(props);

this.state = {

array: [

{

title: "example title 1",

subtitle: "example subtitle 1"

},

{

title: "example title 2",

subtitle: "example subtitle 2"

},

{

title: "example title 3",

subtitle: "example subtitle 3"

}

]

};

}



list = () => {

return this.state.array.map(element => {

return (

<View style={{ margin: 10 }}>

<Text>{element.title}</Text>

<Text>{element.subtitle}</Text>

</View>

);

});

};



render() {

return <View>{this.list()}</View>;

}

} 
                                                

To use the camera to React Native, follow these steps carefully:

• On Android, camera permission must be asked:
  <uses-permission android:name="android.permission.CAMERA" />
• Now, to enable video recording feature add the following code to the AndroidManifest.xml:
  <uses-permission android:name="android.permission.RECORD_AUDIO"/> <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" /> <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
• For iOS, you must update the Info.plist with a usage description for camera
  <key>NSCameraUsageDescription</key> <string>Your own description of the purpose</string>

To pass the state from one component to another, you need to call an API, and then you would need to setup redux. After this, you can call an action where you will call an API and set its response in your reducer. The response can then be accessed in the component through the props.

Example

                                                    
//component

handleSubmit=()=>{

   const {data}=this.state;

   this.props.callAPI(data)

}



Action.js

   export function callAPI(data){

   return (dispatch)=>{

      dispatch(getData(data))

   }

}



function getData(data){

   //use fetch/axios to call the api

}

//in your component, use connect({mapStateToProps},{mapDispatchToProps})

(nameofYourComponent) and you can use your data as props.

 
                                                

The use of Arrow functions in React Native is to help reduce your application's memory consumption. It is done by reducing the CPU time required to iterate the over loops to generate the components necessary for your lists.

Here’s an example of Arrow function in render:
class Foo extends Component {
   handleClick() {
   console.log('Click happened');
}
render() {
   return <button onClick={() => this.handleClick()}>Click Me</button>;
}
}

Navigation is done in react native by this.props.navigation.navigate('Page Name on which you want to navigate where')

Example

                                                    
import React from 'react';

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



class HomePage extends React.Component{

     render(){

         return (

             <view>

                 <text>Home Page</text>

                 <button onpress="{()=" title="Go to Detail page">this.props.navigation.navigate('Details')} ></button>

           </view>

        )

    }

}
                                                

Few significant advantages of React native are as following:

• Conveniently uses the client as well as server side
• Superbly cost effective and code reuse
• Better code readability because of JSX use
• Easy to integrate with other significant frameworks
• Easy to write UI test cases because of React

• Add this code in the component.js file destination file
  import Title from './src/components/importcomponentdemo';
• Here’s the complete code:
  import React from 'react';
import { AppRegistry } from 'react-native';
import App from './src/components/importcomponentdemo';
const App = () => (
   <Title />
);
AppRegistry.registerComponent('ComponentDemo', () => App);

The stylesheet is a React native module that allows developers to create immutable stylesheet references. App developers can pass natural style object into the create() method, which will freeze the objects and assign each with an ID. It will allow developers to avoid creating a new style object every render pass and only once send the purpose across the asynchronous bridge.

Redux is a standalone state management library present in React Native and can be used combined with any framework or library. With the use of Redux, app developers can use one application state as a global state and interact with the state from any react component will be easy.

Here’s an example to call a web service to React Native.

fetch('http://**someurl**', {
method: 'POST',
headers: {
   'Accept': 'application/json',
   'Content-Type': 'application/json',
},
body: JSON.stringify({
   username : "admin1",
   password: "admin13",
})
})

To install React Native, we will have to follow these steps:

• Start with installing node and watchman
• Then, install React native CLI with npm
• Install Xcode and its Command line tools
• Create a React Native project by using the following command

react-native init MyNewProject

cd MyNewProject

react-native run-ios

props

In Native React, props are utilized to customize the components by giving them different parameters. Each element here has its rare ability to customize the item.

state

In English, the 'state of a being' refers to the physical circumstance of a person, and it is a mere state, which changes over time. Well, in a similar fashion state in React/React Native is used inside factors to maintain music of information. Keep in idea now not to replace the country without delay the use of this.state.

React Native easily handles multiple platforms. As the majority of React Native APIs are cross-platform, app developers have to write one React Native component, and it will work on both iOS and Android seamlessly. Facebook, the creator of React native, claims that the Ad Manager application has 87% code reuse across these two platforms. React Native also allows users to specify platform-specific versions of each component which they can then integrate into rest of the React Native application.

The interaction manager is a native module present in React Native with the workability of deferring the execution of a function until an ‘interaction’ has finished. This module is crucial because React Native has only one thread for making UI updates which can be overloaded and result in drop frames. Developers use interaction manager to ensure that the function is only executed after the animations occurred so that any frame drops may not result.

import React from 'react';
import { View, Text } from 'react-native';
import { createAppContainer } from 'react-navigation';
import { createStackNavigator } from 'react-navigation-stack';
class HomePage extends React.Component {
render() {
    return (
       <view>
         <text>Home Page</text></view>
    );
}
}
class DetailPage extends React.Component{
    render(){
        return (
           <view>
              <text>Detail Page</text></view>

        )
    }
}
const AppNavigator = createStackNavigator({
   Home: HomePage,
   Details: DetailPage,
},
{
    initialRouteName: 'Home',
});
export default createAppContainer(AppNavigator);

It is important to upgrade the existing react-native to the latest version in order to access more featural aspects that involve views, APIs, classes, arrow functions, template string, import and export modules, array destructing and development tools.

Following steps should need to be performed to update the react-native to the latest version:

• Update react native, react and Expo package versions for upgrading the expo projects in package.json
• Install the latest SDK version which is compatible with the latest version of react-native in the app.json file.
• Upgrade React Native CLI using the command: react-native upgrade
• Install upgrade helper web tool for upgrading the applications
• Upgrade existing file using the command react-native init
• Perform troubleshoot activity to upgrade with react native CLI

React Native 0.61 is the latest version of react native which is proficiently able to optimize the features of react-native version 0.60. the latest version of react-native comes with the latest and improved features.

The react native version 0.61 is incorporated with four advanced level features such as:

• React native 0.61 is provided with improved CocoaPods Support
• For dimension updates, new useWindowDimensions Hook feature is provided
• Fest processing and refreshing of the application
• React native is upgraded to 16.9 as it opposes old names for unsafe methods

API is the acronym for Application Programming Interface, which is a software intermediary that lets in two applications to talk to every other. Each time you use an app like Facebook, send an on the spot message or take a look at the weather on your phone, you’re the usage of an API.

In React Native, we can use the Fetch to swimsuit our needs. You can absolutely name the URL thru Fetch, and make requests to the server as needed.

Lifecycle approach in React Native

There are several lifecycle methods to React Native. We’ll use three of these lifecycle strategies in this article; constructor, componentDidMount and Render.

With the help of Axios, the user can send GET, POST, PUT, and DELETE requests to the REST API and render a response to the application. With the help of Axios, users can interact with the REST API. In general, Axios is a promise-based HTTP client which used by the react-native.

Axios is associated with a number of features which are enlisted below:

• It makes XMLHttpRequests from the browser
• From react native framework, it makes Http requests
• It supports react-native API’s
• It provides a client-side feature that protects the application from XSRF.
• It automatically transforms response and request data.

The main components of react-native are-

• OS-dependent components
• React Native CLI
• Android studio
• Android SDK
• Xcode

Firebase is a mobile platform that helps you quickly develop high-quality apps, grow your user base and earn more money. This is a tool and infrastructure that users need to build better apps and grow a successful business. In addition, firebase is made up of a complementary feature that users can mix-and-match to fit your needs. There are total give key features of firebase namely authentication, Realtime database, cloud messaging, crash reporting and analytics.

Getting started with firebase by using following steps-

• Create a firebase project in the Firebase console
• Retrieve apikey, authDomian, DatabaseURL and storage bucket from firebase console
• Create a new react-native project
• Install firebase from npm
• Add it into the react-native project

npm install -g react-native-cli

When a developer is working on a project and wants to maintain the project for a long period of time then the user should need TypeScript. In order to use the TypeScript user should need to configure the TypeScript because it gives an option to configure the compiler. Moreover, users can configure the TypeScript as per their requirements and can effectively prioritize the type of errors. Given below are the steps that will enable the developer to use TypeScript in react native-

• Creating a project in react native using-
  react-native init myapp --template typescript && node myapp/setup.js && cd myapp
• Set up and configure TSlint
  npm install --save-dev tslint tslint-eslint-rules tslint-react tslint-config-prettier
• Setting up Jest
  npm install --save-dev ts-jest
• Setting up an enzyme
  npm install --save-dev enzyme enzyme-adapter-react-16 react-dom @types/enzyme @types/enzyme-adapter-react-16
• Setting up a test library of react-native
  npm install --save-dev react-native-testing-library

Here you will discover a list of fundamental instructions to start creating iOS and Android apps the use of React Native.

• react-native init PROJECTNAME
• react-native run-android
• react-native run-ios
• watchman watch-del-all
• react-native link LIBRARYNAME

React Native is better than native because of React native as it is a faster framework and allows the developers to create mobile applications that further support iOS and Android. The react native increases the speed of mobile development effectively and it is quite easier for the developers to maintain the mobile applications which are developed to react-native. It is easier for the developer to identify the bug in the applications which are developed in react native.

Yes, we can use react native for the web as it makes possible to run react-native APIs and component on the web platform.

For example-

Web interfaces with high quality: It is easier to create web applications and user interfaces in JavaScript. It provides native quality of interactions and it supports multiple input modes also.

Write once, render anywhere: It interoperates with a number of React- Native DOM components. Moreover, it is compatible with all of the React Native API.

Gesture Responder System is a system that manages the lifecycle of the applications which are built in React- Native framework. The applications automatically determine the intention of the users through touch. For example, the application determines slide on the widget, touch is scrolling or tapping. The Gesture Responder System allows the components to negotiate these touch interactions without integrating any sought of knowledge regarding their parent component and child component. React Native gesture responder system is the most powerful system, as it uses standard library to detect gestures such as scroll distance, single tap, double-tap, single tap confirmed and pinch distance.

When developers develop applications, they need to store information permanently for an application. The react native applications use the database to remember all the information which is related to the applications and their respective users. React Native uses AsyncStorage for data storing purpose. AsyncStorage is a simple, unencrypted synchronous, persistent, key-value storage system that is global to the application. On iOS, AsyncStorage is backed by native code that stores small values in the serialized dictionary and large values in separate files. On Android, AsyncStorage will use either RocksDB or SQLite based on which is available.

Four steps are needed to understand how to use AsyncStorage to React Native-

• Don’t need to install any extra library. By default, it comes with React Native. Import {AsyncStorage} from 'react-native';
• AsyncStorage uses key-value pairs for saving the data for example- AsyncStorage.setItem ('myKey', myValue);
• To load the saved data run given the command- AsyncStorage.getItem('myKey'). then((myValue)=> {this.setState ({'myKey': myValue});});
• Since loading data is a time-consuming task, it is designed to be an asynchronous operation. So getItem returned the promise, which will invoke the call back function when the read operation is completed.

SQLite database is best for react native.

Given steps are used to create SQLite database in react native-

• create a folder which is called as “www’ through the finder in the project folder
• create a database file
• add the created file to the project
• Choose a file to add
• Verify the structure of the project

React Native gives the Fetch API which offers networking needs. React Native makes use of componentDidMount lifecycle method to load the records from server.
fetch('https://bestinterviewquestion.com/menu.json')

React Component: A class or function which optionally accepts inputs and returns a React element via JSX.

React Element: It describes what the user wants to be displayed on the screen, Simplifying it, the react element is an object representation for some UI.

Yes, it’s entirely possible. App developers use the same code base for iOS and Android as Reacts entirely takes care of the native components translations part. For example, a React Native ScrollView uses ScrollView on Android and native UiScrollView on IOS.

React Native is a next-generation JavaScript code library developed by Facebook for native app creation. The feature of reusable code across the web and mobile devices makes React Native highly demanding among developers. With the reusable codes, app developers don’t have to build the same app for different platforms from scratch.

This also helps businesses especially startups and medium enterprises to better sustain their workforce as now they can hire fewer individuals to perform the same task but in different operating systems.

The React Native has following limitations or cons

• Security risks
• Single threaded interface
• Poor memory management
• Doesn’t support all native APIs
• Dependable on third-party libraries
• Not equivalent to true native apps written entirely on Java or Swift/Obj-C.

Today hundreds and thousands of mobile applications are using React native as their base. Many multi-billion companies are also using this framework for their business apps, such as Facebook, Uber, Instagram, Wal-Mart, Skype, Pinterest, Tencent QQ, wix.com, delivery.com, Adidas GLITCH, JD, Baidu Mobile and more.

Yes, React Native complies with ALL the requires of a native mobile app using native app components. It's neither a mobile web app nor a hybrid mobile app that uses WebView to run the HTML5 app successfully. It builds a real mobile app, utterly indistinguishable from apps built using Java or Objective-C.

To run React Native in Android, we have to follow these:

• First, enable the USB Debugging option inside the Developer Options.
• Plug the device via USB to the development machine.
• Run adb devices command to check that the device is correctly connected to ADB
• Now install and launch your app by using the below-given command.

$ react-native run-android

Create a simple "Hello World" app by using modifying App.js file of FirstApp. Save the utility and reload by way of in reality pressing twice "R" or Ctrl+M (Reload).

Example

                                                    
import React, {Component} from 'react';  

import {Platform, StyleSheet, Text, View} from 'react-native';  

type Props = {};  

export default class App extends Component<Props> {  

render() {  

return (  

      <View>  

        <Text>Hello World</Text>  

     </View>  

    );  

  }  

}  
                                                

Dispatch is a feature of the Redux store. You call store.dispatch to dispatch an action. This is the only way to set off a kingdom change. With React Redux, your factors never get admission to the keep without delay - connect does it for you.

Virtual DOM is a copy of the real DOM. This node tree lists the elements, their attributions, contents, and properties also. Whenever any underlying data changes in React Native, the entire UI will be re-rendered in Virtual DOM representation. Then, the difference between previous DOM representation and virtual DOM will be counted. After that, the real DOM will be updated.

Here’s an example to call a function from another component in React Native:

Create a function, sayHello and then use it as a named-function

const hellos = ['Hola', 'Salut', 'Ciao', 'Ahoj', 'Annyeong-haseyo', 'Aloha', 'Howdy', 'Ni Hao', 'Konnichiwa'];
const sayHello = function() {
return hellos[Math.floor((Math.random()*hellos.length))];
};

export { sayHello };

Now, you can import into whichever component you wish to share the functionality like this:

import { sayHello } from './hello';
class CompA extends React.Component {
    render() {
       return <span>{sayHello()}</span>;
    }
}
class CompB extends React.Component {
    render() {
       return <span>{sayHello()}</span>;
   }
}

render(<span>
<CompA />
<CompB />
</span>, document.querySelector('#app'));

• In React native we use react navigation instead of routes.
• In React we use React Router and In React Native we use React Navigation.

You can install react navigation by the following command:

yarn add react-navigation
or npm install react-navigation

You have to install following modules in addition to react-navigation:

• react-native-gesture-handler
• react-native-reanimated
• react-native-screens
• react-native-safe-area-context
• react-navigation-stack