axios.defaults.baseUrl = process.env.API_ROOT || "//localhost:3000";这样设置的默认路径，

import Vue from 'vue'
import ElementUI from 'element-ui';
import 'element-ui/lib/theme-chalk/index.css';
import App from './App'
import router from './router'
import axios from 'axios'
import $ from 'jquery';
Vue.use(ElementUI);

library.add(faCoffee)

Vue.component('font-awesome-icon', FontAwesomeIcon)

Vue.config.productionTip = false

axios.defaults.baseUrl = process.env.API_ROOT || "//localhost:3000";
Vue.prototype.$axios = axios;
Vue.config.productionTip = false

/* eslint-disable no-new */
new Vue({
    el: '#app',
    router,
    components: { App },
    template: '<App/>'
})

在页面中其它页面引用，报错post 未定义，这个应该怎么改？

    this.$axios.post('/api/login', {
      username: this.username,
      password: this.password
    }).then( res => {