Angular 2 how to return from map function

Question

Angular 2 how to return from map if it has subscription inside

getImage(imageId: string): Observable<ImageSource> {

      return this.http.get(`${ImagesUrl}/${imageId}`, { 
        responseType: 'blob', observe: 'response' }).map((response) => {
        // here I am converting blob to base64
            let blob = response; // lets say response is blob

            let reader = new FileReader();
            reader.readAsDataURL(blob);

            reader.onloadend = (e) => {
              // how can I return from here?????
            };
      }
    }

is there a way to return from map only after onloadend event?

Answer 1

I do not know FileReader class, but you can try to chain all the things you have to do (utilize streams).

function getImage(imageId: string): Observable<ImageSource> {
  return this.http.get(`${ImagesUrl}/${imageId}`, {responseType: 'blob', observe: 'response'})
      .switchMap((response) => {
        const blob = response;
        const reader = new FileReader();
        reader.readAsDataURL(blob);

        return Observable.fromPromise(new Promise((resolve, reject) => {
          reader.onloadend = resolve;
        }));
      });
}

reader.onloadend = resolve; is a shorthand for

reader.onloadend = (e) => { resolve(e); }

At the end of stream you will get event published by onloadend. If you need anything else, just pass another argument to resolve.

You should also handle case when something fails with reject.