Angular2 Typescript：解析JSON数组时出错

Question

我正在尝试使用Angular 2打字稿解析一个简单的JSON结构。

JSON结构：

[
    {
        "_id": {
            "$oid": "594156331900006500e5f0f7"
        },
        "username": "John Smith",
        "place": {
            "name": "vvio",
            "point": {
                "type": "Point",
                "coordinates": [
                    51.5044484,
                    -0.1056523
                ]
            }
        },
        "from": "2017-05-01",
        "to": "2017-05-30",
        "availabilities": [
            {
                "date": "2017-05-01",
                "timeFrom": "20:00",
                "timeTo": "21:00"
            },
            {
                "date": "2017-06-01",
                "timeFrom": "20:00",
                "timeTo": "21:00"
            },
            {
                "date": "2017-06-19",
                "timeFrom": "15:25",
                "timeTo": "15:25"
            },
            {
                "date": "2017-06-19",
                "timeFrom": "15:59",
                "timeTo": "15:59"
            }
        ],
        "sports": [
            "Sport5",
            "Sport2"
        ]
    }
]

我创建这个类来映射JSON结构：

export class Checkin {
     _id: Id;
    username: string;
    place :Place;
    from: string;
    to: string;
    availabilities: Availability[];
    sports: string[];
 }

 export class Id {
     $oid: string;
 }

 export class Place {
    name: string;
    point: Point;
 }

 export class Point {
    type: string;
    coordinates: number[]; // number?
 }

 export class Availability {
     date: string;
     timeFrom: string;
     timeTo: string;
 }

这是服务：

getCheckins(userName : string): Promise<Checkin[]>  {

        var options = new RequestOptions({
             headers : new Headers({
            'Accept': 'application/json;q=0.9,*/*;q=0.8',
           'Content-Type':'application/json',
            })
        });

        console.log("checkin ");
         const url = `${this.checkinUrl}${userName}`;
        return this.http.get(url)
        .toPromise()
        .then(response => { 
            let result = response.json();
            console.log("response "+JSON.stringify(result));
            return result as Checkin[]})
        .catch(this.handleError);
    }

以下是我致电服务的方式：

getCheckins(): void {
        console.log("called get checkin ");
        this.userService.getCheckins("test@outlook.com").then(checkins => this.checkins = checkins);

            console.log("printing checkins "+JSON.stringify(this.checkins));
        for (let checkin of this.checkins) {
            console.log("checkin "+checkin.username);
        }
    }

这是我得到的错误（来自for循环）：

AppComponent.html:1 ERROR TypeError: Cannot read property 'length' of undefined
    at CheckinComponent.getCheckins (checkin.component.ts:46)
    at CheckinComponent.ngOnInit (checkin.component.ts:34)
    at checkAndUpdateDirectiveInline (provider.ts:275)
    at checkAndUpdateNodeInline (view.ts:456)
    at checkAndUpdateNode (view.ts:417)
    at debugCheckAndUpdateNode (services.ts:235)
    at debugCheckDirectivesFn (services.ts:294)
    at Object.View_AppComponent_0.co [as updateDirectives] (AppComponent.html:3)
    at Object.debugUpdateDirectives [as updateDirectives] (services.ts:273)
    at checkAndUpdateView (view.ts:345)

似乎我无法将json响应解析为json类..

任何帮助表示赞赏。