angular - Angular2 - Use value of Observable returning method in another Observable

Question

The problem

I dont know how to use the value of the currently returned Observable of getUserHeaders() in my http.get.

Current error

Type 'Observable<void>' is not assignable to type 'Observable<Participant[]>'

Expected result

Being able to use the value of my Observable returning method getUserHeaders() as a headers attribute within the http call. While only returning the http call's Observable.

Previous code

(which worked with harcoded Headers returned by getUserHeaders() (so not an Observable or Promise).

getAllParticipants(): Observable<Array<Participant>> {
    return this.http.get('someUrl',
                {headers: this.getUserHeaders()})
           .map(response => {
               return response.json();
        });
    });
   }

Current code

going by Chaining RxJS Observables from http data in Angular2 with TypeScript 's answer I came to the flatMap method. (note this code currently throws the 'current error')

getUserHeaders(): Observable<Headers> {
        let headers: Headers = new Headers();

        return NativeStorage.getItem("user").map(
            data => {
                headers.append('API_KEY', data.json().apiKey);
                headers.append('DEVICE_ID', data.json().deviceId);
                return headers
            }).catch( error => {
                console.log("error");
                headers.append('API_KEY', 'TEST');
                headers.append('DEVICE_ID', 'TEST');
                return headers;
            }
        );
    }

/** retrieves ALL the participants from the database */
    getAllParticipants(): Observable<Array<Participant>> {

         return this.getUserHeaders().flatMap( data => {
            return this.http.get('someUrl', {headers: data}).map(response => {
                return response.json();
            });
        });
   }

plunkr (Promise instead of Observable)

http://plnkr.co/edit/A0tEB9EUodWQS6AnqrtH?p=info

(note: Observable.fromPromise() didn't work here so I have created the two methods returning promises -- now I want to use the value of getUserHeaders() inside the promise of getParticipants() and still return the promise/observable of getParticipants() and nothing from getUserHeaders()

Answer 1

Change your getUserHeaders() to look like:

getUserHeaders(): Observable<any> { return Observable.of(NativeStorage.getItem("user"); }

Then construct your headers object within getParticipants(). This way you can keep flatMap