function TextInputWithFocusButton() {
   const inputEl = React.useRef(null);   // initialized to null
   const onButtonClick = () => {
      inputEl.current.focus();
   };
   return (<div><input ref={inputEl} type="text" />
                       <button onClick={onButtonClick}>Focus the input</button>
              </div>);}ReactDOM.render(<TextInputWithFocusButton/>,document.querySelector("div"));