Header.vue

<template>
  <div>
    <el-menu theme="dark" :default-active="activeIndex" mode="horizontal" @select="handleSelect">
      <el-menu-item index="1">List of Posts</el-menu-item>
      <el-submenu index="2">
        <template slot="title">Documentation</template>
        <el-menu-item index="2-1"><a href="https://nuxtjs.org/guide" target="_blank">NUXT</a></el-menu-item>
        <el-menu-item index="2-2"><a href="https://vuejs.org/v2/guide/" target="_blank">Vue.js</a></el-menu-item>
        <el-menu-item index="2-3"><a href="http://element.eleme.io/#/en-US/component/installation" target="_blank">Element</a></el-menu-item>
      </el-submenu>
      <el-menu-item index="3"><nuxt-link to="/">Back Top</nuxt-link></el-menu-item>
    </el-menu>
  </div>
</template>

<script>
export default {
  data () {
    return {
      activeIndex: '1',
      activeIndex2: '1'
    }
  },
  methods: {
    handleSelect (key, keyPath) {
      console.log(key, keyPath)
    }
  }
}
</script>