The ResizeObserverSize interface of the Resize Observer API is used by the ResizeObserverEntry interface to access the box sizing properties of the element being observed.

ResizeObserverSize.blockSize Read only

The length of the observed element's border box in the block dimension. For boxes with a horizontal writing-mode, this is the vertical dimension, or height; if the writing-mode is vertical, this is the horizontal dimension, or width.

ResizeObserverSize.inlineSize Read only

The length of the observed element's border box in the inline dimension. For boxes with a horizontal writing-mode, this is the horizontal dimension, or width; if the writing-mode is vertical, this is the vertical dimension, or height.

In this example the ResizeObserverEntry.contentBoxSize property returns a ResizeObserverSize object. This is an array containing the sizing information for the content box of the observed element.

const resizeObserver = new ResizeObserver((entries) => {
  for (const entry of entries) {
    console.log(entry.contentBoxSize[0]); // a ResizeObserverSize
  }
});

resizeObserver.observe(divElem);