class HelloMessage extends React.Component {
  handlePress = () => {
    alert('Hello')
  }
  render() {
    return (
      <div>
        <p>Hello, my name is {this.props.name}</p>
        <button onClick={this.handlePress}>Say Hello</button>
      </div>
    );
  }
}

ReactDOM.render(
  <HelloMessage name="null" />, 
  mountNode 
);