在JSX中如果有元素变量，在使用的时候用{}包裹起来使用

function UserGreeting(props) {
    return <h1>Welcome back!</h1>;
}

function GuestGreeting(props) {
    return <h1>Please sign up.</h1>;
}

function Greeting(props) {
    const isLoggedIn = props.isLoggedIn;
    if (isLoggedIn) {
        return <UserGreeting />;
    }
    return <GuestGreeting />;
}

class LoginControl extends React.Component {
    constructor(props) {
        super(props);
        this.handleLoginClick = this.handleLoginClick.bind(this);
        this.handleLogoutClick = this.handleLogoutClick.bind(this);
        this.state = { isLoggedIn: false };
    }

    handleLoginClick() {
        this.setState({ isLoggedIn: true });
    }

    handleLogoutClick() {
        this.setState({ isLoggedIn: false });
    }

    render() {
        const isLoggedIn = this.state.isLoggedIn;
         //元素变量
        let button = null;
        if (isLoggedIn) {
            button = <LogoutButton onClick={this.handleLogoutClick} />;
        } else {
            button = <LoginButton onClick={this.handleLoginClick} />;
        }

        return (
            <div>
                <Greeting isLoggedIn={isLoggedIn} />
                //通过{ }包裹使用
                {button}
            </div>
        );
    }
}
ReactDOM.render(
    <LoginControl />,
    document.getElementById('root')
);