Unable to display object array of an object in Angular 2 template

问题

I am trying to display an object values in angular 2 templates and specifically when I try to get an object array using ngFor gives me an error saying

EXCEPTION: Uncaught (in promise): Error: Error in ./SgDetailComponent class SgDetailComponent - inline template:15:7 caused by: Cannot read property 'tags' of undefined I can get string/number property but not object property.

Here's my component.ts

@Component({
  selector: 'app-sg-detail',
  templateUrl: './sg-detail.component.html',
  styleUrls: ['./sg-detail.component.scss', '../../app.component.scss']
})
export class SgDetailComponent implements OnInit {

  errorMessage: string;
  sgDetail: Sg;
  groupId: string;
  ipPermissions: IpPermissions[];
  userIdGroupPairs: UserIdGroupPairs[];
  opPermissionsEgress: IpPermissionsEgress[];
  tags: Tags[];

  constructor(private activatedRoute: ActivatedRoute, private sgService: SgService) { }

  ngOnInit() {
    this.activatedRoute.params.subscribe((params: Params) => {
      let groupId = params['groupId'];
      this.groupId = groupId;
    });
    this.getSgsByGroupId(this.groupId);
  }
  getSgsByGroupId(groupId: String) {
    this.sgService.getSgByGroupId(groupId)
    .subscribe(
      sgDetail => this.sgDetail = sgDetail,
      error => {
        this.errorMessage = error.getMessage();
        console.error(error.getDescription());
      });
  }
}

here's my component.html

 <div class="container app_container">
    <ul class="app_list">
        <li *ngIf="sgDetail" class="app_list-item">
            <p>GroupId: {{sgDetail.groupId}}</p>
            <p>GroupName: {{sgDetail.groupName}}</p>
        </li>
        <li *ngFor="let tag of sgDetail.tags" class="app_list-item">
             //this loop sgDetail.tags errors out 
        </li>
    </ul>
 </div>

and here's my model.ts

export interface Sg {
 ownerId: number;
 groupName: string;
 groupId: string;
 description: string;
 ipPermissions: IpPermissions[];
 ipPermissionsEgress: IpPermissionsEgress[];
 vpcId: string;
 tags: Tags[];
}

export interface IpPermissions {
 ipProtocol: string;
 fromPort: number;
 toPort: number;
 userIdGroupPairs: UserIdGroupPairs[];
 ipRanges: Array<number>;
 prefixListIds: Array<number>;
}

export interface UserIdGroupPairs {
 userId: number;
 groupName: string;
 groupId: string;
 vpcId: string;
 vpcPeeringConnectionId: string;
 peeringStatus: string;
}

export interface IpPermissionsEgress {
 ipProtocol: string;
 fromPort: number;
 toPort: number;
 userIdGroupPairs: UserIdGroupPairs[];
 ipRanges: Array<number>;
 prefixListIds: Array<number>;
}

export interface Tags {
 key: string;
 value: string;
}

回答1:

The error is caused by Asynchorouse call --> getSgsByGroupId, which means the result ofgetSgsByGroupId hasn't come when angular renders the view.

try the safe way angular have provided:

<li *ngFor="let tag of sgDetail?.tags" class="app_list-item">
</li>

回答2:

Try this:-

<ul *ngIf="let tag of sgDetail">
  <li>tag.groupId</li>
<ul>

But before looping or using *ngIf you should print sgDetail as a json like this:-
  {{sgDetail | json}} 

来源：https://stackoverflow.com/questions/43577795/unable-to-display-object-array-of-an-object-in-angular-2-template