React Hooksを触ってみた - State Hooks ref: https://qiita.com/Naturalclar/items/706d10e4d3a442f4e513

app.jsx

import React, { Component } from "react";
import ReactDOM from "react-dom";

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

  setCount(val) {
    this.setState({ count: val });
  }

  render() {
    const { count } = this.state;
    return (
      <div>
        <p>You clicked {count} times </p>
        <button onClick={() => this.setCount(count + 1)}>Click me</button>
      </div>
    );
  }
}

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

file0.sh

yarn add react@next react-dom@next

file1.txt

const [count, setCount] = useState(0)

file2.txt

const [count, setCount] = useState(0)

file3.txt

<button onClick={() => setCount(count + 1)}>Click me</button>