解决vue2.0动态绑定图片src属性值初始化时报错的问题

<template>
  <div>
    <img :src="imgUrl">
  </div>
</template>
<script>
export default {
  data(){
    return {
      captcha_id: "" 
    }
  },
  computed: {
    imgUrl(){
      return ' http://www.demo.com/static/ '+ this.captcha_id + '.png'
    },
  },
  methods: {
    init(){
        // 此处省略一个请求 ，假设成功返回数据为 res
        this.captcha_id = res.data.captcha_id;
    },
  }  
  created(){
    this.init();
  }
}
</script>
<style lang="scss" scoped>
</style>

computed: {
    imgUrl(){
      if(this.captcha_id){
        return this.$store.state.cmnUrl +'/v1/cmn/captcha/new/' + this.captcha_id + '.png'
      }else{
        return null;
      }
    },
  },