举例

<el-menu
      :default-active="onRoutes"
      class="el-menu-vertical-demo"
      @open="handleOpen"
      @close="handleClose"
    >
      <el-menu-item index="my">
        <i class="el-icon-setting"></i>
        <span slot="title">
          <router-link to="/my" class="web-nav">我的资料</router-link>
        </span>
      </el-menu-item>
      <el-menu-item index="myLoginRecord">
        <i class="el-icon-menu"></i>
        <span slot="title">
          <router-link to="/myLoginRecord" class="web-nav"
            >登录记录</router-link
          >
        </span>
      </el-menu-item>
    </el-menu>       

在计算属性中添加一个计算函数

 computed: {
    onRoutes() {
      let path = this.$route.path.replace("/", "");

      return path ? path : "/";
    },
  },

----------------------------------------------- 摘自—伴侣哟-Blyoo