React.js library is all about splitting the app into several components. Each Component has its own lifecycle. React provides us some in-built methods that we can override at particular stages in the life-cycle of the component.
In class-based components, the render() method is the only required and most important method of all in-built life-cycle hooks/methods. In the render() method, we can read props and state and return our JSX code to the root component of our app. In the render() method, we cannot change the state, and we cannot cause side effects( such as making an HTTP request to the webserver).
Creating React Application And Installing Module:
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Step 1: Create a React application using the following command.
npx create-react-app foldername
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Step 2: After creating your project folder i.e. foldername, move to it using the following command.
cd foldername
Project Structure: It will look like the following.
Project Structure
Example: Now write down the following code in the App.js file. Here, App is our default component where we have written our code.Â
App.js
import React, { Component } from 'react'; export default class App extends Component {   state = {     PawriDays: [       { id: '123s', Day: 'Monday' },       { id: '234r', Day: 'Saturday' },       { id: '12d5', Day: 'Sunday' }     ]   }     render() {     const PartyDays = this.state.PawriDays.length     const style = {       'textAlign': 'center',       'color': 'green'    }       // Return JSX code     return (       <div style={style}>         <h1>I am User</h1>         <p> We party: {PartyDays} days a week </p>       </div>     );   } } |
Step to Run Application: Run the application  from the root directory of the project, using the following command
npm start
Output:
