Change responseType in angular (5) after receiving response

Question

Is there a way to change response type in post method (angular 5) AFTER receiving response?
The issue: when the response is okay I need responseType to be blob . If not - I need the json responseType.
I've done some googling around but was unable to find the answer that fully suit my situation.
Code sample (briefly):

// just simple method in service

export class MyService {

  constructor(private http: HttpClient) {}

  doSomething(data, fileName): Observable<any> {
    return this.http.post('url', data, {
      headers: new HttpHeaders().set('Content-Type', 'application/x-www-form-urlencoded'),
      params: new HttpParams().set('fileName', fileName),
      responseType: 'blob'
    })
  }
}

// just classic method in component

export class MyComponent {

  constructor(private myService: MyService) {}

  this.myService.doSomething(this.data, this.file).subscribe(() => {
    // here doing something useful
  }, (error: HttpErrorResponse) => {
    // handling the error
  })
}

So, one more time, in that case everytime I get the response in blob and it's great if all is fine. But if I've got error I need response to be in json. And vice versa.
How can I set correct responseType in both situations? Thanks in advance.

Answer 1

I believe you can do this:

 this.http.post('url', data, 
  { 
    observe: 'response',
    responseType: 'arraybuffer' ,
    headers: new HttpHeaders().set('Accept', 'application/octet-stream; application/json'), 
    params: new HttpParams().set('fileName', '') 
  })
  .pipe(
    map(res => {
      if (res.ok) {
        const blob:Blob = new Blob([res.body], {type: 'application/octet-stream'});
        return blob;
      }
      else {
        var decodedString = String.fromCharCode.apply(null, new Uint8Array(res.body));
        var obj = JSON.parse(decodedString);

        return obj;
      }
    })
  );