1  componentDidMount() {
2    this.intervalID = setInterval(
3      () => this.tick(),
4      1000
5    );
6  }
7  componentWillUnmount() {
8    clearInterval(this.intervalID);
9  }
10

1class Clock extends React.Component {
2  constructor(props) {
3    super(props);
4    this.state = {
5      time: new Date().toLocaleString()
6    };
7  }
8  componentDidMount() {
9    this.intervalID = setInterval(
10      () => this.tick(),
11      1000
12    );
13  }
14  componentWillUnmount() {
15    clearInterval(this.intervalID);
16  }
17  tick() {
18    this.setState({
19      time: new Date().toLocaleString()
20    });
21  }
22  render() {
23    return (
24      <p className="App-clock">
25        The time is {this.state.time}.
26      </p>
27    );
28  }
29}
30