ResizeObserverEntry: contentBoxSize property

{{APIRef("Resize Observer API")}}

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

Value

An array containing objects with the new content 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 content box in the block dimension. For boxes with a horizontal {{cssxref("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 {{cssxref("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 {{cssxref("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 (const entry of entries) {
    if (entry.contentBoxSize) {
      // The standard makes contentBoxSize an array...
      if (entry.contentBoxSize[0]) {
        entry.target.style.borderRadius = `${Math.min(
          100,
          entry.contentBoxSize[0].inlineSize / 10 +
            entry.contentBoxSize[0].blockSize / 10,
        )}px`;
      } else {
        // … but old versions of Firefox treat it as a single item
        entry.target.style.borderRadius = `${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

{{Specifications}}

Browser compatibility

{{Compat}}