Observable type error: cannot read property of undefined

Question

In my Angular 2 application, I get an error:

Cannot read property 'title' of undefined.

This is a very simple component, just trying to get a bare minimum to work here. It hits my API controller (curiously multiple times), and it appears to hit the callback after an object is returned. My console.log outputs the object I would expect. Here is the full error:

TypeError: Cannot read property 'title' of undefined
    at AbstractChangeDetector.ChangeDetector_About_0.detectChangesInRecordsInternal (eval at <anonymous> (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:10897:14), <anonymous>:31:26)
    at AbstractChangeDetector.detectChangesInRecords (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8824:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8807:12)
    at AbstractChangeDetector._detectChangesInViewChildren (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8877:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8811:12)
    at AbstractChangeDetector._detectChangesContentChildren (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8871:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8808:12)
    at AbstractChangeDetector._detectChangesInViewChildren (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8877:14)
    at AbstractChangeDetector.runDetectChanges (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8811:12)
    at AbstractChangeDetector.detectChanges (http://localhost:55707/lib/angular2/bundles/angular2.dev.js:8796:12)

The service (about.service.ts):

import {Http} from 'angular2/http';
import {Injectable} from 'angular2/core';
import {AboutModel} from './about.model';
import 'rxjs/add/operator/map';

@Injectable()
export class AboutService {
    constructor(private _http: Http) { }

    get() {
        return this._http.get('/api/about').map(res => {
            console.log(res.json()); // I get the error on the line above but this code is still hit.
            return <AboutModel>res.json();
        });
    }
}

The Component (about.component.ts):

import {Component, View, OnInit} from 'angular2/core';
import {AboutModel} from './about.model';
import {AboutService} from './about.service';
import {HTTP_PROVIDERS} from 'angular2/http';

@Component({
    selector: 'about',
    providers: [HTTP_PROVIDERS, AboutService],
    templateUrl: 'app/about/about.html'
})

export class About implements IAboutViewModel, OnInit {
    public about: AboutModel;

    constructor(private _aboutService: AboutService) {}

    ngOnInit() {    
        this._aboutService.get().subscribe((data: AboutModel) => {
            this.about = data;
        });
    }
}

export interface IAboutViewModel {
    about: AboutModel;
}

index.html

<script src="~/lib/systemjs/dist/system.src.js"></script>
<script src="~/lib/angular2/bundles/router.js"></script>
<script src="~/lib/angular2/bundles/http.js"></script>
<script src="~/lib/angular2/bundles/angular2-polyfills.js"></script>
<script src="~/lib/angular2/bundles/angular2.dev.js"></script>
<script src="~/lib/es6-shim/es6-shim.js"></script>
<script>
    System.config({
        packages: {
            app: {
                format: 'register',
                defaultExtension: 'js'
            },
            rxjs: {
                defaultExtension: 'js'
            }
        },
        map: {
            rxjs: "lib/rxjs"
        }
    });
    System.import('app/boot')
            .then(null, console.error.bind(console));
</script>

Answer 1

Please include your view and model next time (app/about/about.html and about.model).

If you are returning an array, you can use the asyncPipe, which "subscribes to an Observable or Promise and returns the latest value it has emitted. When a new value is emitted, the async pipe marks the component to be checked for changes" hence the view will be updated with the new value.

If you are returning a primitive type (string, number, boolean) you can also use the asyncPipe.

If you are returning an object, I'm not aware of any way to use asyncPipe, we could use the async pipe, in conjunction with the safe navigation operator ?. as follows:

{{(objectData$ | async)?.name}}

But that looks a bit complicated, and we'd have to repeat that for each object property we wanted to display.

As @pixelbits mentioned in a comment, you can subscribe() to the observable in the controller and store the contained object into a component property. Then use the safe navigation operator or NgIf in the template:

service.ts

import {Injectable} from 'angular2/core';
import {Http} from 'angular2/http';
import 'rxjs/add/operator/map';  // we need to import this now

@Injectable()
export class MyService {
  constructor(private _http:Http) {}
  getArrayData() {
    return this._http.get('./data/array.json')
      .map(data => data.json());
  }
  getPrimitiveData() {
    return this._http.get('./data/primitive.txt')
      .map(data => data.text());   // note .text() here
  }
  getObjectData() {
    return this._http.get('./data/object.json')
      .map(data => data.json());
  }
}

app.ts

@Component({
  selector: 'my-app',
  template: `
    <div>array data using '| async':
      <div *ngFor="let item of arrayData$ | async">{{item}}</div>
    </div>
    <div>primitive data using '| async': {{primitiveData$ | async}}</div>
    <div>object data using .?: {{objectData?.name}}</div>
    <div *ngIf="objectData">object data using NgIf: {{objectData.name}}</div>`
  providers: [HTTP_PROVIDERS, MyService]
})
export class AppComponent {
  constructor(private _myService:MyService) {}
  ngOnInit() {
    this.arrayData$     = this._myService.getArrayData();
    this.primitiveData$ = this._myService.getPrimitiveData();
    this._myService.getObjectData()
      .subscribe(data => this.objectData = data);
  }
}

data/array.json

[ 1,2,3 ]

data/primitive.json

Greetings SO friends!

data/object.json

{ "name": "Mark" }

Output:

array data using '| async':
1
2
3
primitive data using '| async': Greetings SO friends!
object data using .?: Mark
object data using NgIf: Mark

Plunker