react.js gives the identifier ref and gets the content

import React,{Component} from 'react'import ReactDOM from 'react-dom'class App extends Component{ //给一个初始化状态 constructor(){ super(); this.state={ result:0 } } handChange=()=>{ //获取input a的类容，这个a就是我们给的标识ref="a" let a=this.refs.a.value||0; let b=this.refs.b.value||0; //改变状态，更新视图 this.setState({result:parseFloat(a)+parseFloat(b)}) } render(){ return ( 
  +  
{this.state.result}
 
 ) } } ReactDOM.render(,document.querySelector("#root"))