javascript - 错误类型错误 : Cannot read property X of undefined

Question

我有以下接口(interface)用作 JSON 文件的类型:

export interface IIndustrySectors {
  IndustrySector: string;
  isSelected: string;
  dataSubjectCategories:string[];
  dataTypeCategories:string[];
  SubIndustries:[{
    IndustrySector: string;
    isSelected: string;
    dataSubjectCategories:string[];
    dataTypeCategories:string[];
    SubIndustries:[{}]
  }]
}

以及两项服务:

 
 
 // Read all the Data from the JSON file
 getMainIndustrySectors(): Observable<IIndustrySectors[]> {
    return this.http.get<IIndustrySectors[]>(this.industrySectorsURL).pipe(
      tap(((data) => console.log('All: ' + data) )),
      catchError(this.handleError)
    );
  }

//Get Specific object 
  getBySector(sector): Observable<IIndustrySectors|  undefined> {
    return this.getMainIndustrySectors().pipe(
      map((products: IIndustrySectors[]) => products.find(p => p.IndustrySector === sector)));

  }

这是 JSON 文件的一部分

 [
   {
      "IndustrySector":"Accommodation and Food Service Activities",
      "isSelected": "false",
      "dataSubjectCategories":["DS.Employees","DS.Collaborators"],
      "dataTypeCategories":"Personal Data",
      "SubIndustries": [
        {
          "IndustrySector":"Food and Beverage Service Activities",
          "isSelected": "false",
          "dataSubjectCategories":[],
          "dataTypeCategories":[],
          "SubIndustries":[]
        },
        {
          "IndustrySector":"Accommodation",
          "isSelected": "false",
          "dataSubjectCategories":["DS.Minor","DS.Parent","DS.Legal Person","DS.Natural Person","DS.Disable"],
          "dataTypeCategories":[],
          "SubIndustries":[]
        }
      ]
    }
  ]

问题是当我通过以下代码调用服务“getBySector”时:

this.readjsonService.getBySector(sector).subscribe(response=>{

      if(response.dataSubjectCategories.length>0)
      {
        for(i=0; i<response.dataSubjectCategories.length;i++ ){
          this.DSofSectores.push(response.dataSubjectCategories[i])
        }
      }
      
      })

当我看到响应的类型时，它是对象!

它抛出一个错误:

TypeError: Cannot read property 'dataSubjectCategories' of undefined "

不过它会打印值。

这是为什么呢？

要做的是，根据该部门，我获取与其相关的其他数据“响应”，并填充绑定(bind)到下拉列表的字符串类型数组。它工作正常，但下图显示了选择子扇区后会发生什么:

请帮助我，我是新手，我对此感到非常厌倦:((谢谢。

编辑:当我说

 if (response == undefined) {
          throw new Error("sector not found");
        }
        else { .....

它通过了条件，这意味着它不是未定义的，但它说“无法读取未定义”

Answer 1

过滤方法未找到匹配项。所以可观察的结果是未定义。

getBySector(sector): Observable<IIndustrySectors|  undefined> {
     return this.getMainIndustrySectors().pipe(
        map((products: IIndustrySectors[]) => products.find(p => p.IndustrySector === sector)));
                                                     // ^^^ matches none
 }