Subscribe to observable is returning undefined

13
votes

So I am trying to subscribe to a simple service that return data from a local JSON file.

I have managed to get the service working, I can log it out in the function, but when I subscribe to the service in the angular 2 component, it is always undefined. I'm not sure why? Any help would be much appreciated.

API service

export class ApiService {
   public data: any;

   constructor(private _http: Http) {
   }

   getData(): any {
       return this._http.get('api.json').map((response: Response) => { 
       console.log('in response', response.json()); //This logs the Object
       this.data = response.json();
       return this.data;
   })
   .catch(this.handleError);
   }
}

Component

export class AppComponent {
   public data: any
   public informationData;

   constructor(private _api: ApiService) {}

   public ngOnInit(): void {
      console.log(this.getDataFromService()); // This return undefined
   }

   public getDataFromService() {
      this._api.getData().subscribe(response => {
          this.informationData = response;
          return this.informationData;
      });
   }
}
7
javascriptangulartypescriptobservableangular2-services
getDataFromService doesn't itself return anything. Change .subscribe to .map and return the resulting observable, then use | async to resolve it in the template. - jonrsharpe
A subscribe function doesnt return the data. It returns a Subscription object. i.e if you return the function call itself. - Suraj Rao
@SurajRao Yes, I noticed this, how do I get it to return the response data? - Sam Kelham
where do you want to return it to? - Suraj Rao
you can save it in class variable within subscribe if that is what you want - Suraj Rao

7 Answers

59
votes

Maybe some pictures help?

The numbers here indicate the order of operations.

Send the Http Request enter image description here

  1. Component is initialized and calls the getMovies method of the movieService.
  2. The movieService getMovies method returns an Observable. NOT the data at this point.
  3. The component calls subscribe on the returned Observable.
  4. The get request is submitted to the server for processing.
  5. The ngOnInit method is complete.

Any code here after the subscribe cannot access the movies property since the data has not yet been returned.

Receive the Http Response enter image description here

At some LATER point in time ...

  1. The movies are returned to the service.
  2. If the process was successful, the first callback function is executed.
  3. The local movies property is assigned to the movies returned from the service. It is only here that the movies property is finally set.

Attempting to access the movies property prior to step #8 results in an error.

Can we access the value here? NO enter image description here

To fix it: enter image description here

0
votes
objResponse;
this.service.getData().subscribe((result: any)=> { 
this.objResponse=result;
}

Returning something won't required

0
votes

you can do it like this:

In your app-component:

public getDataFromService() {
  this._api.getData(this);
}


public setData(data: any){
 this.data=data;
}

In your service/api.ts:

public getData(obj: appComponentModel){
    this.http.get(url).subscribe(res => obj.setData(res));
}
-1
votes

Try with:

getData(): any {
       return this._http.get('api.json');
}

or

   getData(): any {
           return this._http.get('api.json').map((response: Response) => { 
    response.json();
})
-1
votes

You've got a problem between sync and async function. You'r issue is: getDateFromService is syncronous and the content inside is async. So when the ngOnInit function call getDataFromService, you'r code don't wait the async task. you'r getDataFromService need to return an observer or need to implement the return of your API (you need to choose).

public ngOnInit(): void {
  console.log(this.getDataFromService().subscribe(data => console.log(data)); // This return undefined
}

public getDataFromService() {
  return this._api.getData();
}
-1
votes

Instead of logging at the ngOnInit() method as you did

 public ngOnInit(): void {
      console.log(this.getDataFromService()); // This return undefined    }

log inside the subscribe() method as

export class AppComponent {
  public data: any
  public informationData;

  constructor(private _api: ApiService) {}

  public ngOnInit(): void {
    this.getDataFromService(); //don't log here, logging here will return undefined
  }

  public getDataFromService() {
    this._api.getData().subscribe(response => {
      this.informationData = response;
      console.log(this.informationData); //log here, like this
      return this.informationData;
    });
  }
}
-2
votes

Imagine 'subscribe' as a separate thread running, write everything that is needed inside an anonymous function inside 'subscribe'. Whenever the 'data' is available, it will be available inside the subscribe method.

Hope this helps.