class

import React, { Component } from 'react';
import './App.css';
class App extends Component {  
  constructor(props) {
    super(props);
    this.state = {bg:'red'}
  }
  changeClass(e){
    this.setState(pre=>{
      return ({bg:pre.bg==='red'?'blue':'red'});
    })
  }
  render() {     
    return (
      

        按钮
      
    );
  }
}

export default App;