Counter Example with class properties syntax

Counter.js

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

class Counter extends React.Component {
  state = {
    counter: 0
  };
  handleAddOne = () => {
    this.setState(prevState => {
      return {
        counter: prevState.counter + 1
      };
    });
  };
  render() {
    return (
      <div>
        <div>Counter: {this.state.counter}</div>
        <button onClick={this.handleAddOne}>Add One</button>
      </div>
    );
  }
}

const rootElement = document.getElementById("root");
ReactDOM.render(<Counter />, rootElement);