useElementSize
Reactive size of an HTML element. ResizeObserver MDN
Demo
 Resize the box to see changes 
Usage
html
<template>
  <div ref="el">
    Height: {{ height }}
    Width: {{ width }}
  </div>
</template>
<script>
import { ref } from 'vue'
import { useElementSize } from '@vueuse/core'
export default {
  setup() {
    const el = ref(null)
    const { width, height } = useElementSize(el)
    return {
      el,
      width,
      height,
    }
  }
})
</script>
<template>
  <div ref="el">
    Height: {{ height }}
    Width: {{ width }}
  </div>
</template>
<script>
import { ref } from 'vue'
import { useElementSize } from '@vueuse/core'
export default {
  setup() {
    const el = ref(null)
    const { width, height } = useElementSize(el)
    return {
      el,
      width,
      height,
    }
  }
})
</script>
Component Usage
This function also provides a renderless component version via the
@vueuse/componentspackage. Learn more about the usage.
html
<UseElementSize v-slot="{ width, height }">
  Width: {{ width }}
  Height: {{ height }}
</UseElementSize>
<UseElementSize v-slot="{ width, height }">
  Width: {{ width }}
  Height: {{ height }}
</UseElementSize>
Directive Usage
This function also provides a directive version via the
@vueuse/componentspackage. Learn more about the usage.
html
<script setup lang="ts">
import { vElementSize } from '@vueuse/components'
function onResize({ width, height }: { width: number; height: number }) {
  console.log(width, height)
}
</script>
<template>
  <textarea v-element-size="onResize" />
  <!-- with options -->
  <textarea v-element-size="[onResize, {width:100,height:100}, {'box':'content-box'} ]" />
</template>
<script setup lang="ts">
import { vElementSize } from '@vueuse/components'
function onResize({ width, height }: { width: number; height: number }) {
  console.log(width, height)
}
</script>
<template>
  <textarea v-element-size="onResize" />
  <!-- with options -->
  <textarea v-element-size="[onResize, {width:100,height:100}, {'box':'content-box'} ]" />
</template>
Type Declarations
typescript
export interface ElementSize {
  width: number
  height: number
}
/**
 * Reactive size of an HTML element.
 *
 * @see https://vueuse.org/useElementSize
 * @param target
 * @param callback
 * @param options
 */
export declare function useElementSize(
  target: MaybeComputedElementRef,
  initialSize?: ElementSize,
  options?: UseResizeObserverOptions
): {
  width: Ref<number>
  height: Ref<number>
}
export declare type UseElementSizeReturn = ReturnType<typeof useElementSize>
export interface ElementSize {
  width: number
  height: number
}
/**
 * Reactive size of an HTML element.
 *
 * @see https://vueuse.org/useElementSize
 * @param target
 * @param callback
 * @param options
 */
export declare function useElementSize(
  target: MaybeComputedElementRef,
  initialSize?: ElementSize,
  options?: UseResizeObserverOptions
): {
  width: Ref<number>
  height: Ref<number>
}
export declare type UseElementSizeReturn = ReturnType<typeof useElementSize>