function One() {
            return (
                <div>
                    <h1>One</h1>
                </div>
            )
        }
        function Two() {
            return (
                <h1>Two</h1>
            )
        }
        function Three() {
            return (<h1>Three</h1>)
        }
        var num = 2;
        ReactDOM.render(<div>
            {
                num == 1? <One> </One> :null
            }
            {
                num == 2? <Two/> :null
            } {
                num == 3? <Three/> :null
            }
            </div>, app)

function App(props) {
            // let obj = { 
            //     x:One
            // }
            // return  <obj.x> </obj.x>
            //判断 有点 是对象的属性
            return <props.is> </props.is>
        }
     var num = 3
        var x ;
        if(num ==1){
             x = One
        }else if(num==2){
            x = Two
        }else {
            x = Three
        }
        // ReactDOM.render(<App is={One}/>, app)
        ReactDOM.render(<App is={x}/>, app)