The blockSize read-only property of the ResizeObserverSize interface returns 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.

A decimal representing the block size in pixels.

In this example we return an array of sizing information with ResizeObserverEntry.contentBoxSize. The blockSize property returns the block dimension size of the observed element.

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

resizeObserver.observe(divElem);