The ResizeObserverEntry interface represents the object passed to the ResizeObserver() constructor's callback function, which allows you to access the new dimensions of the Element or SVGElement being observed.

ResizeObserverEntry.borderBoxSize Read only

An object containing the new border box size of the observed element when the callback is run.

ResizeObserverEntry.contentBoxSize Read only

An object containing the new content box size of the observed element when the callback is run.

ResizeObserverEntry.devicePixelContentBoxSize Read only

An object containing the new content box size in device pixels of the observed element when the callback is run.

ResizeObserverEntry.contentRect Read only

A DOMRectReadOnly object containing the new size of the observed element when the callback is run. Note that this is better supported than the above two properties, but it is left over from an earlier implementation of the Resize Observer API, is still included in the spec for web compat reasons, and may be deprecated in future versions.

ResizeObserverEntry.target Read only

A reference to the Element or SVGElement being observed.

None.

The following snippet is taken from the resize-observer-text.html (see source) example. This uses a simple feature detection test to see if the browser supports the newer contentBoxSize property — if so, it uses that to get the sizing data it needs. If not, it uses the older contentRect property.

const resizeObserver = new ResizeObserver((entries) => {
  for (const entry of entries) {
    if (entry.contentBoxSize) {
      h1Elem.style.fontSize = `${Math.max(
        1.5,
        entry.contentBoxSize.inlineSize / 200
      )}rem`;
      pElem.style.fontSize = `${Math.max(
        1,
        entry.contentBoxSize.inlineSize / 600
      )}rem`;
    } else {
      h1Elem.style.fontSize = `${Math.max(
        1.5,
        entry.contentRect.width / 200
      )}rem`;
      pElem.style.fontSize = `${Math.max(1, entry.contentRect.width / 600)}rem`;
    }
  }
});

resizeObserver.observe(divElem);