SOURCE

class Timer extends React.Component {
    constructor(props) {
        super(props)
    }
    render() {
        return (
            <b>{this.props.date}</b>
        )
    }
}

class Clock extends React.Component {
    constructor(props) {
        super(props);
        this.state = { date: new Date() };
    }

    componentDidMount() {
        this.timerID = setInterval(
            () => this.tick(),
            1000
        );
    }

    componentWillUnmount() {
        clearInterval(this.timerID);
    }

    tick() {
        this.setState({
            date: new Date()
        });
    }

    render() {
        return (
            <div>
                <h1>Hello, world!</h1>
                <h2>
                    现在是
                    <Timer date={this.state.date.toLocaleTimeString()}/>
                </h2>
            </div>
        );
    }
}

ReactDOM.render(
    <Clock />,
    document.getElementById('root')
);

<div id="root">
    <!-- This element's contents will be replaced with your component. -->
</div>