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 array of objects containing the new border box size of the observed element when the callback is run.

ResizeObserverEntry.contentBoxSize Read only

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

ResizeObserverEntry.devicePixelContentBoxSize Read only

An array of objects 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.

Note that the code covers three different compatibility cases:

• Some old browsers may support contentRect but not contentBoxSize.
• Old versions of Firefox support contentBoxSize, but incorrectly implemented it as a single object rather than an array.
• Modern browsers support contentBoxSize as an array of objects, to enable them to report box sizes for fragmented elements (for example, in a multi-column scenario).

const resizeObserver = new ResizeObserver((entries) => {
  for (let entry of entries) {
    if (entry.contentBoxSize) {
      // The standard makes contentBoxSize an array...
      if (entry.contentBoxSize[0]) {
        h1Elem.style.fontSize =
          Math.max(1.5, entry.contentBoxSize[0].inlineSize / 200) + "rem";
        pElem.style.fontSize =
          Math.max(1, entry.contentBoxSize[0].inlineSize / 600) + "rem";
      } else {
        // ...but old versions of Firefox treat it as a single item
        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";
    }
  }
  console.log("Size changed");
});

resizeObserver.observe(divElem);