How to return Observable after some Promise get re

Question

I am trying to return an observable after a successful completion of my Promise, but that function is not returning Observable. To be specific to code, i want to fetch auth token from storage (returns promise) and after that data got fetched then generate a Post request to Api (returns Observable). By doing so, sublime text gives an error on function that "a function whose declared type is neither 'void' nor 'any' must return a value" below is my code,

logout() : Observable<any>{
  this.userData.getAuthToken().then((token)=>{
    this.token = token;
    this.headers = new Headers ({
      "X-USER-TOKEN": token
    });
    this.options = new RequestOptions ({
      headers: this.headers
    });
    var logout_url = "Api logout method";
    return this.http.post(logout_url,{},this.options)
      .map (res => res.json())
  });
}

if i simply do a post request then it returns fine like this

return this.http.post(logout_url,{},this.options)
  .map (res => res.json())

but when i try to fetch data it do not return value from this post request. Any help will be much Appreciated! Thanks in advance

Answer 1

You would be returning a Promise (if you hadn't missed the return part in front of it) which would return an Observable on success. Consider using Observables only, rather than mixing them both.

You can also encapsulate it into a new Observable: new Observable(observer =>
https://angular-2-training-book.rangle.io/handout/observables/using_observables.html

Answer 2

Use fromPromise to convert the promise into an observable and use mergeMap to emit the HTTP response into the composed observable:

import { Observable } from 'rxjs/Observable/';
import 'rxjs/add/observable/fromPromise';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/mergeMap';

logout(): Observable<any>{
  return Observable.fromPromise(this.userData.getAuthToken()).mergeMap(token => {
    this.token = token;
    this.headers = new Headers({
      "X-USER-TOKEN": token
    });
    this.options = new RequestOptions({
      headers: this.headers
    });
    var logout_url = "Api logout method";
    return this.http.post(logout_url, {}, this.options).map(res => res.json());
  });
}