ResizeObserverEntry: borderBoxSize property

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.

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.

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) => {
  const calcBorderRadius = (size1, size2) =>
    `${Math.min(100, size1 / 10 + size2 / 10)}px`;

  for (const entry of entries) {
    if (entry.borderBoxSize?.length > 0) {
      entry.target.style.borderRadius = calcBorderRadius(
        entry.borderBoxSize[0].inlineSize,
        entry.borderBoxSize[0].blockSize,
      );
    } else {
      entry.target.style.borderRadius = calcBorderRadius(
        entry.contentRect.width,
        entry.contentRect.height,
      );
    }
  }
});

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

Specifications

Specification
Resize Observer # dom-resizeobserverentry-borderboxsize

Browser compatibility

BCD tables only load in the browser