Angular "Unable to read property" subscribe "undefined"

Question

Before starting my question, I would like to inform you that I have already done a lot of research and I can not find a solution (explanation) why I get this error.

Also note that I'm brand new to Angular, and I just started to learn how it works.

So, the problem I have is what I put in the title of this question.

I am trying to create a login system using Firebase based on the course that I am buying on Udemy.

The code I use is as follows:

auth.service.ts

import {Injectable} from '@angular/core';
import * as firebase from 'firebase';

@Injectable ()
export class AuthService {
    token: string;

    // ...

    singInUser ( email: string, password: string ) {
        // login process here ...
    }

    // Responsible to retrieve the authenticated user token
    getToken () {   
        return firebase
            .auth ()
            .currentUser
            .getIdToken ();
    }
}

Data-storage.service.ts

// ... Dependencies here
@Injectable ()
export class DataStorageService {
    private recipeEndPoint: string = 'https://my-unique-id.firebaseio.com/recipes.json';
    private recipeSubscription: Observable<any> = new Observable();

    constructor ( private http: Http,
                  private recipes: RecipeService,
                  private authService: AuthService ) {}

    // other functionality ...

    getRecipes () {
        const token = this.authService.getToken ();

        token.then (
            ( token: string ) => {
                this.recipeSubscription = this.http.get ( this.recipeEndPoint + '?auth=' + token ).map (
                    ( data: Response ) => {
                        return data.json ();
                    }
                );

                // THIS PARTICULAR CODE WORKS AS EXPECTED
                // WITH NO ISSUES
                this.recipeSubscription.subscribe (
                    ( data: Response ) => {
                        console.log ( 'Data response: ', data );
                    },
                    ( error ) => {
                        console.log ( 'Error: ' + error );
                    }
                )
            }
        );

        // This is supposed to return an Observable to the caller
        return this.recipeSubscription;
    }
}

header.component.ts

// Dependencies here ...

@Component({
  selector: 'app-header',
  templateUrl: './header.component.html',
  styleUrls: ['./header.component.css']
})
export class HeaderComponent implements OnInit {
  constructor(private dataStorage: DataStorageService, private recipeService: RecipeService) { }

  // Other Code Here ...

  onFetchData() {
    let recipeSubscription = this.dataStorage.getRecipes();

    // THIS RETURNS TRUE
    console.log(recipeSubscription instanceof Observable);

    // THIS LINE THEN RETURNS THE MESSAGE:
    // ERROR TypeError: Cannot read property 'subscribe' of undefined
    recipeSubscription.subscribe();

    // IF I COMMENT OUT THE PREVIOUS LINE
    setTimeout(
      () => {
        // THIS RETURNS TRUE
        console.log(recipeSubscription instanceof Observable);
      },
      500
    );

    setTimeout(
      () => {
        // AS WELL THIS ONE RETURNS TRUE
        console.log(recipeSubscription instanceof Observable);
      },
      1000
    );

    setTimeout(
      () => {
        // AS WELL THIS ONE RETURNS TRUE
        console.log(recipeSubscription instanceof Observable);
      },
      1500
    );
  }
}

So, unfortunately, I do not see what could be wrong with this code. Can someone notice something that I did wrong?

. , . , , , .

# 1

header.component.html

<nav class="navbar navbar-default">
    <div class="container-fluid">
        <div class="navbar-header">Logo Here</div>

        <div class="navbar-default">
            <ul class="nav navbar-nav">
                <!-- Left Navigation Options -->
            </ul>
            <ul class="nav navbar-nav navbar-right">
                <!-- Right Navigation Options -->
                <li class="dropdown" appDropdown>
                    <a routerLink="/" class="dropdown-toggle" role="button">Manage <span class="caret"></span></a>
                    <ul class="dropdown-menu">
                        <li>
                            <a style="cursor: pointer;" (click)="onSaveData()">Save Data</a>
                        </li>
                        <li>
                            <!-- Here is where I call the onFetchData method -->
                            <a style="cursor: pointer;" (click)="onFetchData()">Fetch Data</a>
                        </li>
                    </ul>
                </li>
            </ul>
        </div>
    </div>
</nav>

Answer 1

, , getRecipes():

// Numbers indicate the execution order

getRecipes () {
    const token = this.authService.getToken ();

    // 1. You call a promise, which will take a while to execute...
    token.then (
        ( token: string ) => {
            // 3. Finally, this bit gets executed, but only when the promise resolves.
            this.recipeSubscription = ...
        }
    );

    // 2. Then, you return a variable that hasn't been assigned yet,
    // due to the async nature of the promise.
    return this.recipeSubscription;
}

, getRecipes () . Promise, Observable.

- :

getRecipes() {
    // Convert the initial promise into an observable
    // so can you use operators like map(), mergeMap()... to transform it.
    const tokenObs = Observable.fromPromise(this.authService.getToken());

    // Merge the token observable into an HTTP observable
    // and return the JSON data from the response.
    return tokenObs
      .mergeMap(token => this.http.get('XXX?auth=' + token))
      .map(resp => resp.json());
}

HeaderComponent :

const recipeObs = this.dataStorage.getRecipes();
recipesObs.subcribe(jsonData => {
  // Use the JSON data from the HTTP response
});

:

• RxJS, . , :

import 'rxjs/add/observable/fromPromise';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/mergeMap';

• , . getRecipes(). . , , ( HTTP- , , ...).
• recipeSubscription, Observable, Subscription. subscribe(). : const subscription = observable.subscribe().
• , Firebase SDK . , AngularFire?

Answer 2

EventEmitter:

@Output() change: EventEmitter<any>;

:

@Output() change: EventEmitter<any> = new EventEmitter<any>();

, .

Answer 3

, , @Output @Output ngOnInit().

export class MyClass implements OnInit {

    @Output()
    onChange : EventEmitter<void>;

    ngOnInit() {
        // DO NOT initialize @Output event here
        this.onChange = new EventEmitter<void>();    
    }
}

, .

export class MyClass implements OnInit {

    @Output()
    onChange : EventEmitter<void> = new EventEmitter<void>();

    ngOnInit() {
    }
}

, , ( , ngOnInit() ).

Answer 4

, Token().

, , , .

public recipeSubscription: Subject<any> = new Subject();

this.recipeSubscription = this.http.get....

let response = this.http.get....

, , :

response.subscribe((res) => {this.recipeSubscription.next(res)})

this.dataStorage.recipeSubscription.subscribe((res) => {
    // Do stuff.
});

this.dataStorage.getRecipes();

, , :)

Answer 5

4.

I had a method that subscribes to an observable, calling this method in ngOnInitcaused this error. Moving the method call to the constructor solved my problem.

Answer 6

As mentioned in other answers, you must return the observable in order to subscribe to it. In my case, it was a mistake, despite this. I just restarted my corner app with

ng serve 

and then it all worked