(1)-react-component-prop-state.js

// Create Component and Function
const Subheading = (props) => {
  return (
    <h3>{props.title}</h3>
  );
};

class SubheadingWithState extends React.Component {
  state = { numOfDay: 99 };

  handleClickEvent = () => {
  	this.setState((prevState) => ({ numOfDay: prevState.numOfDay - 1 }));
  };

  render() {
    return (
      <h3 onClick={this.handleClickEvent}>
        Counting down {this.state.numOfDay} days to Annabelle birthday
      </h3>
    )
  };
}

//ReactDOM.render(<Subheading title="Hi Annabelle" />, mountNode);
ReactDOM.render(<SubheadingWithState/>, mountNode);