vant/packages/vant-use/src/useRect/index.ts

import { Ref, unref } from 'vue';

function isWindow(val: unknown): val is Window {
  return val === window;
}

export const useRect = (
  elementRef: (Element | Window) | Ref<Element | Window | undefined>
) => {
  const element = unref(elementRef);

  if (isWindow(element)) {
    const width = element.innerWidth;
    const height = element.innerHeight;

    return {
      top: 0,
      left: 0,
      right: width,
      bottom: height,
      width,
      height,
    };
  }

  if (element && element.getBoundingClientRect) {
    return element.getBoundingClientRect();
  }

  return {
    top: 0,
    left: 0,
    right: 0,
    bottom: 0,
    width: 0,
    height: 0,
  };
};