Get real height of element react

Question

My div is not visible on the screen and obviously the height is 0px. However, the clientHeight returns 154. What is my mistake? I would want to get the real height of the div that I see on the screen.

function TestComponent() {
   const iframeRef = React.useRef<HTMLDivElement>(null);

   useEffect(() => {
     console.log('HEIGHT HERE: ',iframeRef.current?.getBoundingClientRect().height); // return 154
  }, [iframeRef.current]);

return <div className='test'>...</div>

}