Class Based Counter Example

class_counter.js

import React from 'react';
import ReactDOM from 'react-dom';

class App extends React.Component {
  state = {
    counter: 0
  };

  handleOnClick = () => {
    this.setState(prevState => ({
      counter: prevState.counter + 1
    }));
  };

  render() {
    return (
      <div>
        <p>Counter value is: {this.state.counter} </p>
        <button onClick={this.handleOnClick}>Increment</button>
      </div>
    );
  }
}

ReactDOM.render(<App />, document.getElementById('root'));