elementUI select，option变化，如果option不存在上次的选项，自动清空上次的选择

select，option变化，如果option不存在上次的选项，自动清空上次的选择

<script src="//unpkg.com/vue@2/dist/vue.js"></script>
<script src="//unpkg.com/element-ui@2.15.14/lib/index.js"></script>
<div id="app">
<template>
  <el-select v-model="value" placeholder="请选择">
    <el-option
      v-for="item in arr"
      :key="item.value"
      :label="item.label"
      :value="item.value">
    </el-option>
  </el-select>
  <el-button @click="fun">fun</el-button>
  <el-button @click="fun2">fun2</el-button>
  <el-button @click="fun3">fun3</el-button>
  <el-button @click="fun4">fun4</el-button>
</template>
</div>

var Main = {
    data() {
      return {
        arr:[],
        options: [{value: '黄金糕',label: '黄金糕'}, {
          value: '双皮奶',label: '双皮奶'}, {
          value: '蚵仔煎',label: '蚵仔煎'}],
        options2: [{value: '黄金糕2',label: '黄金糕2'}, {
          value: '双皮奶2',label: '双皮奶2'}, {
          value: '蚵仔煎2',label: '蚵仔煎2'} ,{
          value: '蚵仔煎',label: '蚵仔煎'}],
        value: ''//
      }
    },
    methods:{
      fun(){
        this.arr= this.options
      },
      fun2(){
        this.arr=this.options2
      },
      fun3(){ this.arr=[]},
      fun4(){ alert(this.value)}
    },
    watch:{
      arr(val){
        let data=val.find(i=>i.value==this.value)
        if(!data)this.value=''
      }
    }
  }
var Ctor = Vue.extend(Main)
new Ctor().$mount('#app')