element Cascader 级联选择器 动态加载省市区

<el-form-item label="居住地址" prop="userArea">
  <el-cascader
     ref="cascader"
     v-model="loanForm.userArea"
     :props="props"
     placeholder="请选择居住地址"
     @change="changeC"
     >
  </el-cascader>
</el-form-item>

data(){
  return {
    loanForm:{
      userArea:''
    },
    props:{
      lazy: true,
      label:'name',
      value:'code',
      lazyLoad:this.lazyLoad
    },
  }
}

methods:{
  lazyLoad(node, resolve){
      const { level,value } = node;

      var requestCode = '';
      // level0, 后端这里需要2147483647获取所有省份列表
      if(level === 0){
        requestCode = '2147483647';
      }
      // level=1，则获取当前城市的code值
      if(level >= 1){
        requestCode = value;
      }
      
      findTreeBook({code:requestCode}).then(res=>{
        if(res.code===200){
          res.data.forEach(item=>{
            item.leaf = level >= 1;  //添加叶子节点作为最后展示的子节点
          })
          resolve(res.data);
        }
      })
  }
}