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