app.vue

<template>
  <!-- 子组件 -->
    <button @click="add">add</button>
</template>
<script setup>
import {onMounted, ref} from 'vue'
const  add = () => {
    localStorage.setItem('tokens', '123456')
}
const demo = () => {
    window.addEventListener('beforeunload', () => {
        alert('页面关闭')
        localStorage.removeItem('tokens')
    })
}
onMounted(() => {
    demo()
})
</script>