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 (            <p>
                <input type="text" ref="a" onChange={this.handChange}/>
                +
                <input type="text" ref="b" onChange={this.handChange}/>
                <p>{this.state.result}</p>
            </p>        )
    }
}
ReactDOM.render(<App/>,document.querySelector("#root"))

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