demo07/index.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8" />
    <script src="../build/react.development.js"></script>
    <script src="../build/react-dom.development.js"></script>
    <script src="../build/babel.min.js"></script>
  </head>
  <body>
    <div id="example"></div>
    <!-- https://reactjs.org/docs/refs-and-the-dom.html#callback-refs -->
    <script type="text/babel">
      class MyComponent extends React.Component {
        constructor(props) {
          super(props);
          this.myTextInput = React.createRef();
          this.handleClick = this.handleClick.bind(this)
        }
        handleClick() {
          this.myTextInput.current.focus();
        }
        render() {
          return (
            <div>
              <input type="text" ref={this.myTextInput} />
              <input type="button" value="Focus the text input" onClick={this.handleClick} />
            </div>
          );
        }
      }

      ReactDOM.render(
        <MyComponent />,
        document.getElementById('example')
      );
    </script>
  </body>
</html>