1class Clock extends React.Component {
2  constructor(props) {
3    super(props);
4    this.state = {date: new Date()};
5  }
6
7  componentDidMount() {  }
8  componentWillUnmount() {  }
9  render() {
10    return (
11      <div>
12        <h1>Hello, world!</h1>
13        <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
14      </div>
15    );
16  }
17}

1shouldComponentUpdate(nextProps, nextState) {
2  return true;
3}

1class Test extends React.Component {
2  constructor() {
3    console.log('Constructor')
4    super();
5    this.state = {
6      count: 0
7    };
8  }
9
10  componentDidMount() {
11    console.log("component did mount");
12  }
13  componentDidUpdate() {
14    console.log("component did update");
15  }
16
17  onClick = () => {
18    this.setState({ count: this.state.count + 1 });
19  };
20  render() {
21    console.log("render");
22    return (
23      <div>
24        Hello Test
25        <button onClick={this.onClick}>
26      		{this.state.count}
27		</button>
28      </div>
29    );
30  }
31}
32
33
34//--for first time
35//Constructor
36//render
37//component did mount
38//--for any update
39//render
40//component did update

1componentDidUpdate(prevProps, prevState, snapshot)