useResizeObserver

Watch size of an HTML element. ResizeObserver MDN

useResizeObserver wraps the native ResizeObserver API to watch for size changes on a DOM element. It invokes a callback with the full ResizeObserverEntry array whenever the element’s dimensions change. The hook returns a stop function to disconnect the observer. You can also pass ResizeObserverOptions to control which box model (content-box, border-box, or device-pixel-content-box) is observed.

When to Use

• Executing custom logic (e.g., recalculating layouts or redrawing canvases) in response to element size changes
• Building wrapper components that need raw ResizeObserver entries for advanced measurement logic
• Observing size changes on elements that resize independently of the window (e.g., resizable panels, flex items)

Notes

• Performance: ResizeObserver is much more efficient than polling element dimensions or listening to window resize events. The callback fires only when the observed element actually changes size.
• Cleanup: The observer is automatically disconnected on unmount. Call the returned stop function to disconnect manually at any time.
• See also useElementSize for a simple [width, height] return value, and useMeasure for a full DOMRect return value — both built on top of ResizeObserver.

Usage

function Demo() {
  const ref = useRef<HTMLTextAreaElement>(null);
  const [width, setWidth] = useState(0);
  const [height, setHeight] = useState(0);
  const stop = useResizeObserver(ref, (entries) => {
    const [entry] = entries;
    const { width, height } = entry.contentRect;
    setWidth(width);
    setHeight(height);
  });

  return (
    <div>
      <div>Resize the box to see changes</div>
      <div>
        <button onClick={() => stop()}>stop observe</button>
      </div>
      <br />
      <textarea
        ref={ref}
        disabled
        style={{ width: 286, height: 166 }}
        value={`width: ${width}\nheight: ${height}`}
      />
    </div>
  );
};

API

useResizeObserver

Returns

() => void

Arguments

BasicTarget

export type BasicTarget<T extends TargetType = Element> = (() => TargetValue<T>) | TargetValue<T> | MutableRefObject<TargetValue<T>>;

TargetValue

type TargetValue<T> = T | undefined | null;

TargetType

type TargetType = HTMLElement | Element | Window | Document | EventTarget;