注意：
配置404页面一定要在路由表的最后一个，放在*的下面路由是不起作用的

定义404错误模版并配置路径

import Vue from 'vue'
import VueRouter from 'vue-router'
Vue.use(VueRouter)

const Home = {
  template:`
    <div>
      <h2>Home</h2>
      <p>this is Home</p>
</div>
  `
}
const Parent = {
  template:`
    <div>
      <h2>Parent</h2>
      <p>this is Parent</p>
</div>
  `
}

const Page404 = {      /*404页面默认跳转，定义时不能以数字开头*/
  template:`
    <div>
      <h2>404</h2>
      <p>error:404  页面走丢了</p>
</div>
  `
}

const router = new VueRouter({
  mode:'history',
  data(){
    return{
      aaa:'fade'
    }
  },
  base: __dirname,
  routes:[
    {path:'/',component:Home},
    {path:'/Parent',component:Parent},
    {path:'*',component:Page404}
  ]
})

new Vue({
  router,
  template:`
  <div id="app">
    <h1>this is transition</h1>
    <ul>
        <li><router-link to="/">/</router-link></li>
        <li><router-link to="/Parent">/Parent</router-link></li>
        <li><router-link to="/asdf">/错误的路由链接，跳转至404页面</router-link></li>
    </ul>
    <transition :name="aaa" mode="out-in">
        <router-view></router-view>
    </transition>
</div>
  `,
  watch:{
      '$route' (to,from){
          console.log(to);
          console.log(from);
          if(from.path == '/Parent'){
            this.aaa = 'fade1';
          }else{
            this.aaa = 'fade2';
          }
      }
  }
}).$mount('#app')

在之后路径出错后便会跳转到这里

运行结果：