未捕获（承诺）：错误：无法读取未定义的属性

Question

组件使用参数从服务中获取用户

@Component({
    selector: 'users',
    providers: [UserService],
    template: `
    <p>{{user.id}}</p>
`
})
export class UserPageComponent implements OnInit  {
    constructor(
        private userService: UserService,
        private route: ActivatedRoute
    ) {};

    ngOnInit(): void {
        this.route.params.forEach((params: Params) => {
            let id = +params['id'];
            this.userService.getUser(id)
                .then(user => {console.log(user.id);this.user = user})
        });
    }

服务：

@Injectable()
export class UserService {
    private postUrl = 'http://127.0.0.1:8000/user-detail/';  // URL to web api
    constructor(private http: Http) {
    }

    getUser(id: number): Promise<User> {

        return this.http.get(this.postUrl+id)
            .toPromise()
            .then(response => response.json() as User)
            .catch(this.handleError);

    };

我收到错误Uncaught (in promise): Error: Error in ./UserPageComponent class UserPageComponent - inline template:1:7 caused by: Cannot read property 'id' of undefined

看起来好像服务没有发送承诺，尽管它应该发送。 如何解决这个问题？

Answer 1

看起来你的组件没有用户的默认值，所以当组件渲染尝试添加默认值时它是未定义的

@Component({
    selector: 'users',
    providers: [UserService],
    template: `
    <p>{{user.id}}</p>
`
})
export class UserPageComponent implements OnInit  {
    user = {} // default value for user

    constructor(
        private userService: UserService,
        private route: ActivatedRoute
    ) {};

    ngOnInit(): void {
        this.route.params.forEach((params: Params) => {
            let id = +params['id'];
            this.userService.getUser(id)
                .then(user => {console.log(user.id);this.user = user})
        });
    }

其实这里加一些条件逻辑会更好

<p *ngIf="user">{{user.id}}</p>

它应该可以工作