在Angular2中将可观察对象与http服务一起使用时出现错误：无法读取未定义的属性'[property]'

Question

I'm slowly building an angular2 site, but I'm very inexperienced with this platform still. Right now I have two components that use the same service: stories and storyDetails. The stories component just gets a list of all stories from my web api. The storyDetails component will list more detailed information about a single story. The stories component works fine, but I am getting an error when I try to load the storyDetails component.

Uncaught (in promise): Error: Error in /app/stories/details/storyDetails.component.template.html:0:22 caused by: Cannot read property 'title' of undefined

I've breakpointed on my service's extractData function and verified that json is being returned and has the following value: body = Object {storyId: 1, urlAffix: "a_story_title", title: "A Story Title", text: "This is the story", entityId: null…} However, it should be noted that I'm getting the error before this line is ever executed.

Here are the relevant files:

storyService.service.ts

import {Injectable} from '@angular/core';
import {Story} from '../story';
import { Http, Response } from '@angular/http';
import { Observable }     from 'rxjs/Observable';

@Injectable()
export class StoryService {
    private storyUrl = 'http://localhost:51332/api/storyapi';  // URL to web API
    constructor(private http: Http) { }
    getStories(): Observable<Story[]> {
        return this.http.get(this.storyUrl)
            .map(this.extractData)
            .catch(this.handleError);
    }
    getStoryDetails(storyId: Number): Observable<Story> {
        return this.http.get(`${this.storyUrl}/${storyId}`)
            .map(this.extractData)
            .catch(this.handleError);
    }
    private extractData(res: Response) {
        let body = res.json();
        return body;
    }
    private handleError(error: any) {
        let errMsg = (error.message) ? error.message :
            error.status ? `${error.status} - ${error.statusText}` : 'Server error';
        console.error(errMsg); // log to console instead
        return Observable.throw(errMsg);
    }
}

storyDetails.component.ts

import { Component } from '@angular/core';
import { OnInit } from '@angular/core';

import { Story } from '../story.ts';
import { StoryService } from '../services/storyService.service';

import { ActivatedRoute } from '@angular/router';

@Component({
    selector: 'storyDetails',
    templateUrl: '/app/stories/details/storyDetails.component.template.html',
    providers: [StoryService]
})

export class StoryDetails implements OnInit {
    errorMessage: string;
    story: Story;
    mode = 'Observable';
    storyId: Number;
    private sub: any;
    constructor(private storyService: StoryService, private route: ActivatedRoute) { }
    ngOnInit() {

        this.route.params.subscribe(params => {
            this.storyId = +params['id']; // (+) converts string 'id' to a number
            this.getStoryDetails(this.storyId);
        });
    }
    getStoryDetails(storyId : Number) {
        this.storyService.getStoryDetails(storyId)
            .subscribe(
            story => this.story = story,
            error => this.errorMessage = <any>error);
    }
    ngOnDestroy() {
        this.sub.unsubscribe();


  }
}

storyDetails.component.template.html

<h2>Story Details</h2>


{{story.title}}

story.ts

export class Story {
    public storyId: Number;
    public urlAffix: string
    public title: string;
}

I noticed that if I don't put anything in the template html file that references the story object, I don't get the error. I'm sure I'm making a rookie mistake but I just cannot find it.

Answer 1

The storyDetails component is trying to render {{story.title}} before it is defined. Try wrapping your component or just the title in an *ngIf. Once the asynchronous call is complete (and story is defined) the span will render along with the title.

<span *ngIf="story">
    {{story.title}}
</span>