ResizeObserverEntry

Baseline Widely available *

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨July 2020⁩.

* Some parts of this feature may have varying levels of support.

TheResizeObserverEntry interface represents the object passed to theResizeObserver() constructor's callback function, which allows you to access the new dimensions of theElement orSVGElement being observed.

Instance properties

ResizeObserverEntry.borderBoxSizeRead only: An array of objects containing the new border box size of the observed element when the callback is run.
ResizeObserverEntry.contentBoxSizeRead only: An array of objects containing the new content box size of the observed element when the callback is run.
ResizeObserverEntry.devicePixelContentBoxSizeRead only: An array of objects containing the new content box size indevice pixels of the observed element when the callback is run.
ResizeObserverEntry.contentRectRead only: ADOMRectReadOnly object containing the new size of the observed element when the callback is run. Note that this is now a legacy property that is retained in the spec for backward-compatibility reasons only.
ResizeObserverEntry.targetRead only: A reference to theElement orSVGElement being observed.

Note:The content box is the box in which content can be placed, meaning the border box minus the padding and border width. The border box encompasses the content, padding, and border. SeeThe box model for further explanation.

Instance methods

None.

Examples

The following snippet is taken from theresize-observer-text.html (see source) example.

Note that the code covers three different compatibility cases:

Some old browsers may supportcontentRect but notcontentBoxSize.
Old versions of Firefox supportcontentBoxSize, but incorrectly implemented it as a single object rather than an array.
Modern browsers supportcontentBoxSize 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 (const 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);