ResizeObserverEntry.borderBoxSize

This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The borderBoxSize read-only property of the ResizeObserverEntry interface returns an array containing the new border box size of the observed element when the callback is run.

Syntax

var myBorderBoxSize = ResizeObserverEntry.borderBoxSize;

Value

An array containing objects with the new border box size of the observed element. The array is necessary to support elements that have multiple fragments, which occur in multi-column scenarios. Each object in the array contains two properties:

blockSize
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.
inlineSize
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.

Note: For more explanation of writing modes and block and inline dimensions, read Handling different text directions.

Examples

const resizeObserver = new ResizeObserver(entries => {
  for (let entry of entries) {
    if(entry.borderBoxSize && entry.borderBoxSize.length > 0) {
      entry.target.style.borderRadius = Math.min(100, (entry.borderBoxSize[0].inlineSize/10) +
                                                      (entry.borderBoxSize[0].blockSize/10)) + 'px';
    } else {
      entry.target.style.borderRadius = Math.min(100, (entry.contentRect.width/10) +
                                                      (entry.contentRect.height/10)) + 'px';
    }
  }
});

resizeObserver.observe(document.querySelector('div'));

Specifications

Specification Status Comment
Resize Observer
The definition of 'target' in that specification.
Editor's Draft Initial definition.

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
borderBoxSize
Experimental
Chrome Full support 84Edge Full support 84Firefox Full support 69IE No support NoOpera Full support YesSafari No support NoWebView Android Full support 84Chrome Android Full support 84Firefox Android No support NoOpera Android Full support YesSafari iOS No support NoSamsung Internet Android No support No

Legend

Full support  
Full support
No support  
No support
Experimental. Expect behavior to change in the future.
Experimental. Expect behavior to change in the future.