thinkphp5 返回json后没有length属性？

Question

function findChild(&$arr,$id){ $childs=array(); foreach ($arr as $k => $v){ if($v['pid']== $id){ $childs[]=$v; } } return $childs; } function build_tree($rows,$root_id){ $childs=$this->findChild($rows,$root_id); if(empty($childs)){ return '[]'; } foreach ($childs as $k => $v){ $rescurTree=$this->build_tree($rows,$v['id']); if( null != $rescurTree){ $childs[$k]['data']=$rescurTree; } $childs[$k]['value'] = $v['id']; } return $childs; } public function test(){ $sql = db('auth_rule')->select(); $data = $this->build_tree($sql,0); return json($data) ; } 返回的json： [{ "id": 1, "name": "sys", "title": "系统设置", "type": 1, "status": 1, "condition": "", "pid": 0, "level": 0, "sort": 7, "data": [{ "id": 11, "name": "conf\/lst", "title": "配置列表", "type": 1, "status": 1, "condition": "", "pid": 1, "level": 1, "sort": 50, "data": [{ "id": 12, "name": "conf\/add", "title": "添加配置", "type": 1, "status": 1, "condition": "", "pid": 11, "level": 2, "sort": 50 }, { "id": 13, "name": "conf\/del", "title": "配置删除", "type": 1, "status": 1, "condition": "", "pid": 11, "level": 2, "sort": 50 }, { "id": 14, "name": "conf\/edit", "title": "配置编辑", "type": 1, "status": 1, "condition": "", "pid": 11, "level": 2, "sort": 50 }] }, { "id": 9, "name": "conf\/conf", "title": "配置项", "type": 1, "status": 1, "condition": "", "pid": 1, "level": 1, "sort": 50 }] }, { "id": 15, "name": "admin", "title": "管理员", "type": 1, "status": 1, "condition": "", "pid": 0, "level": 0, "sort": 50, "data": [{ "id": 16, "name": "admin\/lst", "title": "管理员列表", "type": 1, "status": 1, "condition": "", "pid": 15, "level": 1, "sort": 50, "data": [{ "id": 17, "name": "admin\/add", "title": "管理员添加", "type": 1, "status": 1, "condition": "", "pid": 16, "level": 2, "sort": 50 }, { "id": 18, "name": "admin\/del", "title": "管理员删除", "type": 1, "status": 1, "condition": "", "pid": 16, "level": 2, "sort": 50 }, { "id": 19, "name": "admin\/edit", "title": "管理员修改", "type": 1, "status": 1, "condition": "", "pid": 16, "level": 2, "sort": 50 }] }] }] 但是用js去获取res.length的时候，却提示cannot read property 'length' of undefined

Answer 1

你这返回了个数组，不是json对象

Answer 2

cannot read property 'length' of undefined

的意思是undefined没有length属性，说明说你的res是undefined，undefined当然没有length属性了。

为啥res会是undefined？不是在控制台里打印出来了吗？有两种可能，第一种是你打印的根本就不是同一个res.length（作用域的问题)，第二种，也是我认为非常有可能的，就是你还没搞懂js的异步，前端在发请求的时候需要你提供一个“回调函数”，只有在这个回调函数里你才能获得res。比如

let res;
$.get(url, data => res = data);
console.log(res);  // undefined
res.length  // cannot read property 'length' of undefined

$.get(url, data => {
  console.log(data);
  console.log(data.length);
  // do something here
});

Answer 3

1）首先是楼主理解错误了，后端没有必要返回length长度这个变量
2）js直接获取一下数据的长度就解决了