class Parent extends React.Component {
  state = { message: "parent message" };
  callbackFunction = (childData) => {
    this.setState({ message: childData });
  };
  render() {
    return (
      <div>
        <Child parentCallback={this.callbackFunction} />
        <p> {this.state.message} </p>
      </div>
    );
  }
}

class Child extends React.Component {
  sendBackData = () => {
    this.props.parentCallback("child message");
  };
  render() {
    return (
      <button onClick={this.sendBackData.bind(this)}>
        click me to send back
      </button>
    );
  }
}

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