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

A decimal representing the inline size in pixels.

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

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

resizeObserver.observe(divElem);