class Demo extends React.Component {
    constructor(props) {
      super(props);
      this.state = {x:10};
      console.log(1);
   }
   componentDidMount() {
      console.log(4,this.state.x);
      this.setState({x:30});
   }
   shouldComponentUpdate() {
      console.log(5);
      return true;
   }
   getSnapshotBeforeUpdate(prevProps, prevState){
      console.log(6,prevState.x,this.state.x);
      return 999;
   }
   componentDidUpdate(prevProps, prevState, snapshot) {
      console.log(7,snapshot);
   }
   componentWillUnmount() {
      console.log(8);
   }
   static getDerivedStateFromProps(props, state){  // always called before render()
      console.log(2,state.x);
      return {x:20};    // the new state to be updated
   }
   render() {
      console.log(3);
      return null;
   }}ReactDOM.render(
   <Demo />,
   document.querySelector('div'));ReactDOM.unmountComponentAtNode(document.querySelector('div'));