REACT How to call child component method from parent

getChildInstance.js

/* Child.js */
import React from 'react'

class Child extends React.Component {
  componentDidMount() {
    this.props.onRef(this)
  }
  componentWillUnmount() {
    this.props.onRef(undefined)
  }
  method() {
    window.alert('do stuff')
  }
  render() {
    return <h1>Hello World!</h1>
  }
}
export default Child;



/* Parent.js */
import React from 'react'
import Child from './Child'

class Parent extends React.Component {
  onClick = () => {
    this.child.method() // do stuff
  }
  render() {
    return (
      <div>
        <Child onRef={ref => (this.child = ref)} />
        <button onClick={this.onClick}>Child.method()</button>
      </div>
    );
  }
}