class Parent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      value: 0
    };
  }

  incrementValue() {
    this.setState({ value: this.state.value + 1 });
  }

  render() {
    return (
      <div>
        <Button handleClick={() => this.incrementValue()} />
        <Display value={this.state.value} />
      </div>
    );
  }
}

function Button(props) {
  return <button onClick={props.handleClick}>Click me!</button>;
}

function Display(props) {
  return <div>{props.value}</div>;
}

ReactDOM.render(<Parent />, document.getElementById("root"));