So geben Sie Eigenschaften mit der Vue 3-Kompositions-API zurück firstDigit 的计算值？计算属性中的关键字 this 是 undefined 但是当我将 this 排除在外时，我收到错误 fourDigits is not Defined.

<script setup>
import { computed, reactive } from 'vue'

const input = reactive({
    fourDigits: Array(1,2,3,4),
    firstDigit: computed(() => {
      return this.fourDigits[0] <===== `this` is undefined but if I leave `this` out, then `fourDigits` is undefined.
    })
</script>

<template>
   <div>
     <pre>
       {{JSON.stringify(input.firstDigit, null, 2)}}
     </pre>
   </div>
</template>