store.ts

import { defineStore } from 'pinia'

export const useMainStore = defineStore('main', () => {
  const count = ref(0)

  return { count }
})

HelloWorld.vue

<template>
  <p>Count: {{ mainStroe.count }}</p>
</template>

<script setup lang="ts">
  import { useMainStore } from './store.ts'

  const mainStroe = useMainStore()
</script>