How to add Stateful component without constructor class in React?

Generally, we set the initial state of the component inside the constructor class and change the state using the setState method. In React basically, we write HTML-looking code called JSX. JSX is not a valid JavaScript code but to make the developer’s life easier BABEL takes all the responsibility to convert JSX into valid JavaScript code and allow the developers to write code in HTML-looking syntax. Similarly, the state can’t be initialized without constructor class, when we initialize state outside constructor class again Babel read the syntax and understands there is a need to create constructor inside the class and do all the required hard works behind the scene. This is called a class property proposal.

Syntax: Initialize state outside the constructor class using syntax.

state = {stateName1:stateValue1, 
         stateName2:stateName2, 
         ....... 
         stateNamek:stateValuek}

Example 1: This example illustrates how to class property proposal to initialize state without constructor 

index.js:

Javascript

import React from 'react'
import ReactDOM from 'react-dom'
import App from './App'

ReactDOM.render(<App />, document.querySelector('#root'))

Filename – App.js

Javascript

import React, { Component } from 'react'

class App extends Component {

// The class property proposal
// The state initialization without
// constructor class
state = {msg : 'Hi, There!'}

handleClick(){
// Changing state
this.setState({msg : 'Welcome to the React world!'})
}

render(){
return (
<div>
    <h2>Message :</h2>
    <p>{this.state.msg}</p>
    {/* Set click handler */}
    <button onClick={()=> this.handleClick()}>
        Click here!
    </button>
</div>
)
}
}

export default App

Output:

Example 2: This example illustrates how to class property proposal to initialize state without constructor 

index.js:

Javascript

import React from 'react'
import ReactDOM from 'react-dom'
import App from './App'

ReactDOM.render(<App />, document.querySelector('#root'))

App.js:

Javascript

import React, { Component } from 'react'

class App extends Component {
  static defaultProps = {
    courseContent : [
      'JSX', 'React Props', 'React State',
      'React Lifecycle Methods', 'React Event Handlers',
      'React Router', 'React Hooks', 'Readux',
      'React Context'
    ]
  }
  // The class property proposal
  // The state initialization without
  // constructor class
  state = {msg : 'React Course', content:''}

  renderContent(){
    return (
      <ul>
        {this.props.courseContent.map(content => (
          <li>{content}</li>
        ))}
      </ul>
    )
  }
  handleClick(){
    //changing state
    this.setState({
      msg : 'Course Content',
      content : this.renderContent()
    })
  }
  render(){
    return (
      <div>
        <h2>Message :</h2>
<p>{this.state.msg}</p>

<p>{this.state.content}</p>
        {/* set click handler */}
        <button onClick={() => this.handleClick()}>
          Click here to know contents!
        </button>
      </div>
    )
  }
}

export default App

Output: