错误：未捕获的TypeError：无法读取属性＆＃39; groupBy＆＃39;未定义的

Question

当我尝试创建如下所示的JSON结构时，我遇到了一个问题：

所以为此我做的如下：

    class Aggregate{
    average: Number;
    count: String;
    max: Number;
    min: Number;
    total: Number;
    constructor(average,count,max,min,total) {
        this.average= average;
        this.count=count;
        this.max=max;
        this.min= min;
        this.total=total;
    }
}

var average = new Aggregate(43.833333333333336, 6, 90, 10, 263);
var aggregate = new Aggregate(0, undefined, 0, 0, 0);
var startDate=new Date("Tue Jul 05 2016 05:30:00 GMT+0530 (India Standard Time)");
var endDate=new Date("Tue Jul 05 2016 05:30:00 GMT+0530 (India Standard Time)");
interface date{
    aggregate: Aggregate;
    endDate: Date;
    groupBy: String;
    metricType: String;
    quarters: Array<DateMetric>;
    startDate: Date;
    type: String;
    quarter?: Number;
}
class DateMetric{
    aggregate: Aggregate;
    endDate: Date;
    groupBy: String;
    metricType: String;
    quarters: Array<DateMetric>;
    startDate: Date;
    type: String;
    quarter?: Number;

    constructor();
    constructor(obj: date);
    constructor(obj?: any) {
        this.aggregate= aggregate;
        this.endDate=endDate;
        this.groupBy=obj.groupBy;
        this.metricType= obj.metricType;
        this.quarters = obj.quarters;
        this.startDate = startDate;
        this.type = obj.type;
        this.quarter =obj.quarter;

    }    
}
var b = new DateMetric();
b.aggregate = aggregate;
b.quarter=4
b.startDate = startDate;


var a = new DateMetric({ aggregate: average, endDate: endDate, groupBy: "datetime", metricType: "distance_metric", quarters: [b], startDate: startDate, type: "person" })

但在这里我收到一条错误消息：Cannot read property 'groupBy' of undefined

我不知道为什么我收到错误，因为构造函数中的obj应包含创建对象时传递的对象a

任何帮助都将不胜感激。

Answer 1

像这样定义构造函数以消除错误。

constructor(obj: any | Date = {}) { }

这样，如果你创建一个没有在构造函数中传递任何数据的实例，基本上就是你正在访问。

{}.groupBy; // ({} aka 'obj' is now defined)

使用现有代码，您也可以继续使用if，如下所示：

constructor(obj: any | Date) { 
    if(!!obj) {
        // do stuff
    }
}

但是对于上面的例子，我认为第一种解决方案最好：）