useRef-example.ts

function TextInputWithFocusButton() {
  // The type of our ref is an input element
  const inputEl = useRef<HTMLInputElement>(null);
  const onButtonClick = () => {
    // `current` points to the mounted text input element
    inputEl.current.focus();
  };

  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

With type script 3.5.3 :
Had to change
inputEl.current!.focus();

non-null assertion operator !