useEffect可以相当于三个生命周期钩子 页面加载 组件更新 页面卸载

import './App.css';
import React, { useState } from 'react'
import ReactDOM from 'react-dom'
function App() {
  const [count, setcount] = useState({
    a: 0,
    b: 3
  })
React.useEffect(()=>{
  console.log("11111");
  return()=>{
    console.log("222");
  }
},[])
function add(){
  setcount({
    ...count,
    b:count.b+1
  })
}
function unmount(){
  ReactDOM.unmountComponentAtNode(document.getElementById("root"))
}
  return (
    <div id="App">
      <div></div>
      <p id="a">{count.b}</p>
      <button onClick={add}>按钮</button>
      <button onClick={unmount}>按钮</button>
    </div>
  );
}
export default App;