Web/API/ResizeObserverEntry/contentBoxSize

This is an experimental technologyCheck 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 [[../../../../../../../mdn.github.io/dom-examples/resize-observer/resize-observer-border-radius|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 ObserverThe definition of 'target' in that specification.	Editor's Draft	Initial definition.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`contentBoxSize` Experimental'	Chrome Full support 84	Edge Full support 84	Firefox Full support 69	IE No support No	Opera Full support Yes	Safari No support No	WebView Android Full support 84	Chrome Android Full support 84	Firefox Android No support No	Opera Android Full support Yes	Safari iOS No support No	Samsung 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.

ResizeObserverEntry.contentBoxSize by Mozilla Contributors is licensed under CC-BY-SA 2.5.