the array inside of the state is always undefind when using pinia

I am using pinia in my vue3 projects and for some reason I am trying to store some temporary data in pinia, here is my store file:

// template.ts
export const useTemplateStore = defineStore('template', {
  state: () => {
    return {
      templates: [] as Template[],
      temporary: {
        id: generateUUID(),
        itemList: []
      } as Template
    }
  },
  actions: {
    reset() {
      this.temporary = {
        id: generateUUID(),
        itemList: []
      }
    },
    addItem(item: TemplateItem) {
      this.temporary.itemList.push(item);
    }
  }
})

But when I tried to push item into temporary.itemList in my vue file, it failed:

<script setup lang="ts">
// ...
const useTemplate = useTemplateStore();
// both of them failed
useTemplate.addItem({ ... });
useTemplate.temporary.itemList.push({ ... });
</script>

when I checked the console, it showed:
directly push
using addItem method

I log the itemList out(console.log(useTemplate.temporary.itemList)), and the result is undefined.

where is the problem? it bothers me very much，so if anyone can help me out I'd appreciate it.

By the way, my projects is coding with typescript.

It could be something caused by TypeScript.

The JavaScript version works well.

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const { createApp } = Vue
const { createPinia, defineStore } = Pinia

const useTemplateStore = defineStore('template', {
  state: () => {
    return {
      templates: [],
      temporary: {
        itemList: []
      } 
    }
  },
  actions: {
    reset() {
      this.temporary = {
        itemList: []
      }
    },
    addItem(item) {
      this.temporary.itemList.push(item);
    }
  }
})

const App = {
  setup() {
    const useTemplate = useTemplateStore();
    // both of them failed
    useTemplate.addItem({ item: 1 });
    const addItem2 = () => { 
      useTemplate.temporary.itemList.push({ item: useTemplate.temporary.itemList.length + 1}) 
    }
    return {
      useTemplate,
      addItem2
    }
  }
}

const app = createApp(App)
const pinia = createPinia()
app.use(pinia)
app.mount('#app')

<!-- language: lang-css -->

#app { line-height: 1.75; }
[v-cloak] { display: none; }

<!-- language: lang-html -->

<div id="app" v-cloak>
  useTemplate.temporary.itemList: {{ useTemplate.temporary.itemList }}<br/>
  <button @click="addItem2()">Add Item</button> 
  <button @click="useTemplate.reset()">Reset</button>
</div>
<script src="https://unpkg.com/vue@3/dist/vue.global.prod.js"></script>
<script src="https://unpkg.com/vue-demi@0.13.11/lib/index.iife.js"></script>
<script src="https://unpkg.com/pinia@2.0.30/dist/pinia.iife.js"></script>

<!-- end snippet -->