Button.js

// App.js
import React, { Component } from 'react';
import Button from './Button';
import './App.css';

class App extends Component {
  state = {
    num: 0,
  }
  handleclick = () => {
    const { num } = this.state;
    console.log(num);
    this.setState({
      num: num + 1
    }, () => alert('update'));
  }
  render() {
    const { num } = this.state;
    const { handleclick } = this;
    return (
      <Button
        number={num}
        handleclick={handleclick}
      />
    );
  }
}

export default App;

// Button.js

import React from 'react';

const Button = ({number, handleclick}) => {
  return (
    <div>
      <div
        onClick={handleclick}
      >
        button
      </div>
      <div>
        {number}
      </div>
    </div>
  )
}

export default Button;