ResizeObserverEntry.contentBoxSize

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

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

Syntax

var myContentBoxSize = ResizeObserverEntry.contentBoxSize;

Value

An object containing the new content box size of the observed element. This object contains two properties:

blockSize
The length of the observed element's content 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 content 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

The following snippet is taken from the resize-observer-border-radius.html (see source) example. This example includes a green box, sized as a percentage of the viewport size. When the viewport size is changed, the box's rounded corners change in proportion to the size of the box. We could just implement this using border-radius with a percentage, but that quickly leads to ugly-looking elliptical corners; this solution gives you nice square corners that scale with the box size.

const resizeObserver = new ResizeObserver(entries => {
  for (let entry of entries) {
    if(entry.contentBoxSize) {
      entry.target.style.borderRadius = Math.min(100, (entry.contentBoxSize.inlineSize/10) +
                                                      (entry.contentBoxSize.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
contentBoxSize
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.