最简单的写法，引入$route.path 如果报错先引入定义

:default-active="$route.path"

 <el-menu
        class="el-menu-vertical-demo"
        :collapse="homeStore.isCollapse"
        background-color="#001529"
        text-color="white"
        active-text-color="yellowgreen"
        :router="trueValue"
        :default-active="$route.path"
        @select="handleSelect"
      >

vue3

<script setup>
import { appSource } from '../../utils/app.js'
import { Calendar, Timer, TakeawayBox } from '@element-plus/icons-vue'
import { useRouter, useRoute } from 'vue-router'
import { onMounted, ref, watch } from 'vue'
let $route = useRoute()
const trueValue = ref(true)
console.log(appSource)
let activeIndex = ref('')
let activePath = ref('/1