vue踩坑记录之数组定义和赋值问题

<el-form-item label="有效日期">
 <div class="block">
  <el-date-picker
   v-model="effective_date"
   type="daterange"
   range-separator="至"
   start-placeholder="开始日期"
   end-placeholder="结束日期">
  </el-date-picker>
 </div>
</el-form-item>

data(){
 return{
  effective_date: []
 }
}

methods: {
 getContractInfo(){
    this.service.hqStore.getContractList(this.$route.query.uid).then((res)=>{
   let detail = res.datalist[this.$route.query.index];
   this.effective_date[0] = this.stamptimeToDate(detail.valid_date);
   this.effective_date[1] = this.stamptimeToDate(detail.invalid_date);
  }
 }
}

created(){
   this.getContractInfo();
   setTimeout(()=>{
    console.log(this.effective_date);
   })
  },

this.effective_date = [this.stamptimeToDate(detail.valid_date),this.stamptimeToDate(detail.invalid_date)];