class App extends React.Component {
      state = {
        age: 29,
      }
      constructor(props) {
        super(props);
        console.log('constructor', props);
      }
      render() {
        console.log('render');
        return (
          <div>
            <h2>
              Hello {this.props.name} - {this.state.age}
            </h2>
          </div>
        );
      }
      componentWillMount() {
        console.log("componentWillMount");
      }
      componentDidMount() {
        console.log("componentDidMount");

        // state 를 update 시켜주니까 render 가 다시 호출 되고 있다.
        setInterval(() => {
          console.log('setIn')
          this.setState(state => ({
            ...state,
            age : state.age + 1
          }))
        }, 1000);
      }
    }