ReactCounter.jsx

import React, { Component } from 'react';
import { render } from 'react-dom';
import './style.css';

class App extends Component {
  constructor() {
    super();
    this.state = {
      count: 0,
    };
  }

  inc = () => {
    this.setState({
      count: this.state.count + 1,
    })
  }

  dec = () => {
    this.setState({
      count: this.state.count - 1,
    })
  }

  render() {
    return (
      <div>
        <div>{this.state.count}</div>
        <button onClick={this.inc}>+</button>  
        <button onClick={this.dec}>-</button>  
      </div>
    );
  }
}

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

https://stackblitz.com/edit/react-jmx3a7?file=index.js