how to get ref of parent in react code example

Example 1: use ref call parent component to child react

const { forwardRef, useRef, useImperativeHandle } = React;

// We need to wrap component in `forwardRef` in order to gain
// access to the ref object that is assigned using the `ref` prop.
// This ref is passed as the second parameter to the function component.
const Child = forwardRef((props, ref) => {

  // The component instance will be extended
  // with whatever you return from the callback passed
  // as the second argument
  useImperativeHandle(ref, () => ({

    getAlert() {
      alert("getAlert from Child");
    }

  }));

  return <h1>Hi</h1>;
});

const Parent = () => {
  // In order to gain access to the child component instance,
  // you need to assign it to a `ref`, so we call `useRef()` to get one
  const childRef = useRef();

  return (
    <div>
      <Child ref={childRef} />
      <button onClick={() => childRef.current.getAlert()}>Click</button>
    </div>
  );
};

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

Example 2: how to get parent's ref react

constructor(props) {
    super(props);
    this.domElem = React.createRef();
}

getElem = () => {
    return this.domElem;
}

render() {
    return (
       <div>
           <div id="elem" ref={this.domElem}>Test Elem</div>
           <Child getParentElem={this.getElem}/>
       </div>
    )
}