There is no length attribute when thinkphp5 returns json?

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) ;
    }

returned json:

[{
    "id": 1,
    "name": "sys",
    "title": "",
    "type": 1,
    "status": 1,
    "condition": "",
    "pid": 0,
    "level": 0,
    "sort": 7,
    "data": [{
        "id": 11,
        "name": "conf\/lst",
        "title": "",
        "type": 1,
        "status": 1,
        "condition": "",
        "pid": 1,
        "level": 1,
        "sort": 50,
        "data": [{
            "id": 12,
            "name": "conf\/add",
            "title": "",
            "type": 1,
            "status": 1,
            "condition": "",
            "pid": 11,
            "level": 2,
            "sort": 50
        }, {
            "id": 13,
            "name": "conf\/del",
            "title": "",
            "type": 1,
            "status": 1,
            "condition": "",
            "pid": 11,
            "level": 2,
            "sort": 50
        }, {
            "id": 14,
            "name": "conf\/edit",
            "title": "",
            "type": 1,
            "status": 1,
            "condition": "",
            "pid": 11,
            "level": 2,
            "sort": 50
        }]
    }, {
        "id": 9,
        "name": "conf\/conf",
        "title": "",
        "type": 1,
        "status": 1,
        "condition": "",
        "pid": 1,
        "level": 1,
        "sort": 50
    }]
}, {
    "id": 15,
    "name": "admin",
    "title": "",
    "type": 1,
    "status": 1,
    "condition": "",
    "pid": 0,
    "level": 0,
    "sort": 50,
    "data": [{
        "id": 16,
        "name": "admin\/lst",
        "title": "",
        "type": 1,
        "status": 1,
        "condition": "",
        "pid": 15,
        "level": 1,
        "sort": 50,
        "data": [{
            "id": 17,
            "name": "admin\/add",
            "title": "",
            "type": 1,
            "status": 1,
            "condition": "",
            "pid": 16,
            "level": 2,
            "sort": 50
        }, {
            "id": 18,
            "name": "admin\/del",
            "title": "",
            "type": 1,
            "status": 1,
            "condition": "",
            "pid": 16,
            "level": 2,
            "sort": 50
        }, {
            "id": 19,
            "name": "admin\/edit",
            "title": "",
            "type": 1,
            "status": 1,
            "condition": "",
            "pid": 16,
            "level": 2,
            "sort": 50
        }]
    }]
}]

but when you use js to get res.length, it prompts cannot read property "length" of undefined

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 });