reference

index.js

class Parent extends Component {
  methodFromChild = () => null;

  getMethodForMe = method => {
    this.methodFromChild = method;
  }
 
  render() {
    return <Child setFunctionReference={this.getMethodForMe} />
  }
}

class Child extends Component {
  componentDidMount() {
    this.props.setFunctionReference(this.methodForParent);
  }
  
  methodForParent = () => {
    // do something
  }
}