浅义理解React

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class Test 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>It is {this.state.date.toLocaleTimeString()}.</h2>
            </div>
        );
    }
}

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