App.js

import React, { Component } from 'react'
import logo from './logo.svg'
import './App.css'

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

  handleClick() {
    this.setState({
      value: this.state.value + 1
    })
  }

  render() {
    return (
      <div>
        {this.state.value}
        <button
          onClick={() => {
            this.handleClick()
          }}
        >
          Click me to add one
        </button>
      </div>
    )
  }
}

export default App