// EmailInput wraps an HTML `input` and adds some app-specific styling.
const EmailInput = React.forwardRef((props, ref) => (
  <input ref={ref} {...props} type="email" className="AppEmailInput" />
));

class App extends Component {
  emailRef = React.createRef();

  render() {
    return (
      <div>
        <EmailInput ref={this.emailRef} />
        <button onClick={() => this.onClickButton()}>
          Click me to focus email
        </button>
      </div>
    );
  }

  // `this.emailRef.current` points to the `input` component inside of EmailInput,
  // because EmailInput is forwarding its ref via its `React.forwardRef` callback.
  onClickButton() {
    this.emailRef.current.focus();
  }
}